[0001] This application claims the benefit of U.S. Provisional Application No. 61/789,204, filed March 15, 2013; the entire contents of which are herein incorporated by reference.

[0002] The content of the electronically submitted sequence listing, filed herewith, is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY-SPONSORED

RESEARCH AND DEVELOPMENT

[0003] This invention was made with Government support under Agreement DE-AR0000006 awarded by the United States Department of Energy. The Government has certain rights in this invention.

FIELD OF THE INVENTION

[0004] The present invention relates to the field of industrial microbiology and dihydroxy- acid dehydratase variants for production pathways, including isobutanol biosynthetic pathways, in microorganisms. The invention also provides methods for screening for dihydroxy-acid dehydratase variants with improved characteristics. For example, dihydroxy- acid dehydratase variants are disclosed with increased activity compared to a parental dihydroxy-acid dehydratase.

BACKGROUND

[0005] Dihydroxy-acid dehydratase (DHAD), also called acetohydroxy acid dehydratase, catalyzes the conversion of 2,3-dihydroxyisovalerate to a-ketoisovalerate and of 2,3- dihydroxymethylvalerate to a-ketomethylvalerate. The DHAD enzyme, classified by the Enzyme Commission (EC) number 4.2.1.9, is part of the naturally occurring biosynthetic pathways that produce valine, isoleucine, leucine, and pantothenic acid (vitamin B5). DHAD-catalyzed conversion of 2,3-dihydroxyisovalerate to a-ketoisovalerate is also a common step in the multiple isobutanol biosynthetic pathways that are disclosed, for example, in U.S. Patent No. 7,851, 188. Disclosed therein is engineering of recombinant microorganisms for production of isobutanol. Isobutanol is useful as a fuel additive, and the availability of biologically-produced isobutanol can reduce the demand for petrochemical fuels. SUMMARY OF THE INVENTION

[0006] The present invention provides, for example, isolated polypeptides and fragments thereof having dihydroxy-acid dehydratase (DHAD) activity.

[0007] One aspect of the invention is directed to an isolated polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof comprises one or more amino acid substitutions selected from: (a) aspartic acid at a position corresponding to position 33 oi Streptococcus mutans DHAD; (b) glutamic acid at a position corresponding to position 62 of Streptococcus mutans DHAD; (c) valine at a position corresponding to position 115 oi Streptococcus mutans DHAD; (d) glutamic acid at a position corresponding to position 116 oi Streptococcus mutans DHAD; (e) serine at a position corresponding to position 119 of Streptococcus mutans DHAD; (f) arginine at a position corresponding to position 158 oi Streptococcus mutans DHAD; (g) glutamine at a position corresponding to position 176 of Streptococcus mutans DHAD; (h) leucine at a position corresponding to position 179 of Streptococcus mutans DHAD; (i) arginine at a position corresponding to position 322 of Streptococcus mutans DHAD; (j) serine at a position corresponding to position 425 of Streptococcus mutans DHAD; (k) glycine at a position corresponding to position 524 of Streptococcus mutans DHAD; (1) valine or leucine at a position corresponding to position 562 of Streptococcus mutans DHAD; (m) arginine, cysteine, or glycine at a position corresponding to position 563 of Streptococcus mutans DHAD; (n) glutamic acid at a position corresponding to position 564 of Streptococcus mutans DHAD; and (o) aspartic acid at a position corresponding to position 567 of Streptococcus mutans DHAD.

[0008] In an embodiment of the invention, the polypeptide or fragment thereof comprises a substitution of glutamic acid at a position corresponding to position 564 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of glutamic acid at a position corresponding to position 62 of Streptococcus mutans DHAD, and a substitution of valine at a position corresponding to position 562 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of aspartic acid at a position corresponding to position 33 of Streptococcus mutans DHAD, and a substitution of arginine at a position corresponding to position 563 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of valine at a position corresponding to position 562 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of arginine at a position corresponding to position 563 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of cysteine at a position corresponding to position 563 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of glycine at a position corresponding to position 563 of Streptococcus mutans DHAD. In yet another embodiment, the polypeptide or fragment thereof comprises a substitution of glycine at a position corresponding to position 524 of Streptococcus mutans DHAD, and a substitution of glycine at a position corresponding to position 563 of Streptococcus mutans DHAD.

9] In an embodiment of the invention, the polypeptide or fragment thereof comprises a substitution of valine at a position corresponding to position 115 of Streptococcus mutans DHAD, a substitution of arginine at a position corresponding to position 158 of Streptococcus mutans DHAD, and a substitution of aspartic acid at a position corresponding to position 567 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of glutamic acid at a position corresponding to position 116 of Streptococcus mutans DHAD, and a substitution of serine at a position corresponding to position 119 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of aspartic acid at a position corresponding to position 33 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of glutamic acid at a position corresponding to position 62 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of leucine at a position corresponding to position 562 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of glutamine at a position corresponding to position 176 of Streptococcus mutans DHAD, a substitution of leucine at a position corresponding to position 179 of Streptococcus mutans DHAD, a substitution of arginine at a position corresponding to position 322 of Streptococcus mutans DHAD, and a substitution of arginine at a position corresponding to position 563 of Streptococcus mutans DHAD. In yet another embodiment, the polypeptide or fragment thereof comprises a substitution of serine at a position corresponding to position 425 of Streptococcus mutans DHAD, and a substitution of arginine at a position corresponding to position 563 of Streptococcus mutans DHAD.

[0010] In another aspect, the invention is directed to an isolated polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof comprises one or more amino acid substitutions selected from: (a) glycine to aspartic acid at a position corresponding to position 33 of Streptococcus mutans DHAD; (b) aspartic acid to glutamic acid at a position corresponding to position 62 of Streptococcus mutans DHAD; (c) methionine to valine at a position corresponding to position 1 15 of Streptococcus mutans DHAD; (d) glycine to glutamic acid at a position corresponding to position 1 16 of Streptococcus mutans DHAD; (e) asparagine to serine at a position corresponding to position 1 19 of Streptococcus mutans DHAD; (f) glycine to arginine at a position corresponding to position 158 of Streptococcus mutans DHAD; (g) histidine to glutamine at a position corresponding to position 176 of Streptococcus mutans DHAD; (h) histidine to leucine at a position corresponding to position 179 of Streptococcus mutans DHAD; (i) glutamine to arginine at a position corresponding to position 322 of Streptococcus mutans DHAD; (j) alanine to serine at a position corresponding to position 425 of Streptococcus mutans DHAD; (k) glutamic acid to glycine at a position corresponding to position 524 of Streptococcus mutans DHAD; (1) phenylalanine to valine or leucine at a position corresponding to position 562 of Streptococcus mutans DHAD; (m) tryptophan to arginine, cysteine, or glycine at a position corresponding to position 563 of Streptococcus mutans DHAD; (n) lysine to glutamic acid at a position corresponding to position 564 of Streptococcus mutans DHAD; and (o) glutamic acid to aspartic acid at a position corresponding to position 567 of Streptococcus mutans DHAD.

[0011] In another aspect, the invention is directed to an isolated polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof comprises one or more amino acid substitutions selected from: (a) glycine to aspartic acid at position 33; (b) aspartic acid to glutamic acid at position 62; (c) methionine to valine at position 115; (d) glycine to glutamic acid at position 1 16; (e) asparagine to serine at position 119; (f) glycine to arginine at position 158; (g) histidine to glutamine at position 176; (h) histidine to leucine at position 179; (i) glutamine to arginine at position 322; (j) alanine to serine at position 425; (k) glutamic acid to glycine at position 524; (1) phenylalanine to valine or leucine at position 562; (m) tryptophan to arginine, cysteine, or glycine at position 563; (n) lysine to glutamic acid at position 564; and (o) glutamic acid to aspartic acid at position 567.

[0012] In certain embodiments, the isolated polypeptide or fragment thereof having DHAD activity is a [2Fe-2S] ²⁺ DHAD. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity is a [4Fe-4S] ²⁺ DHAD. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity catalyzes the conversion of 2,3- dihydroxyisovalerate to a-ketoisovalerate or catalyzes the conversion of 2,3- dihydroxymethylvalerate to a-ketomethylvalerate.

[0013] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity has an amino acid sequence that matches the Profile Hidden Markov Model (HMM) of Table 6 with an E value of < 10 ^~5 . In another embodiment, the isolated polypeptide or fragment thereof having DHAD activity comprises three conserved cysteines corresponding to positions 56, 129, and 201 of Streptococcus mutans DHAD.

[0014] In other embodiments, the polypeptide or fragment thereof having DHAD activity is from a prokaryotic organism. In certain embodiments, the polypeptide or fragment thereof having DHAD activity is from bacteria, fungi, or plant. In a particular embodiment, the polypeptide or fragment thereof having DHAD activity is from Streptococcus mutans.

[0015] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:528 and has a glutamic acid at position 564. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:528 and has a glutamic acid at position 564. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:528.

[0016] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:532 and has a glutamic acid at position 62 and a valine at position 562. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:532 and has a glutamic acid at position 62 and a valine at position 562. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:532. [0017] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:534 and has an aspartic acid at position 33 and an arginine at position 563. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:534 and has an aspartic acid at position 33 and an arginine at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:534.

[0018] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:537 and has a valine at position 562. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:537 and has a valine at position 562. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:537.

[0019] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:540 and has an arginine at position 563. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:540 and has an arginine at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO: 540.

[0020] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:545 and has a cysteine at position 563. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:545 and has a cysteine at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO: 545.

[0021] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:572 and has a glycine at position 563. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:572 and has a glycine at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:572.

[0022] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:548 and has a glycine at position 524 and a glycine at position 563. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:548 and has a glycine at position 524 and a glycine at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:548.

[0023] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:552 and has a valine at position 1 15, an arginine at position 158, and an aspartic acid at position 567. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:552 and has a valine at position 1 15, an arginine at position 158, and an aspartic acid at position 567. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:552.

[0024] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:555 and has a glutamic acid at position 1 16 and a serine at position 1 19. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:555 and has a glutamic acid at position 1 16 and a serine at position 119. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:555.

[0025] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:557 and has an aspartic acid at position 33. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:557 and has an aspartic acid at position 33. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:557.

[0026] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO: 561 and has a glutamic acid at position 62. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:561 and has a glutamic acid at position 62. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:561.

[0027] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:563 and has a leucine at position 562. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:563 and has a leucine at position 562. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:563.

[0028] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:566 and has a glutamine at position 176, a leucine at position 179, an arginine at position 322, and an arginine at position 563. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:566 and has a glutamine at position 176, a leucine at position 179, an arginine at position 322, and an arginine at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:566.

[0029] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:569 and has a serine at position 425 and an arginine at position 563. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:569 and has a serine at position 425 and an arginine at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:569.

[0030] In other embodiments, the isolated polypeptide or fragment thereof has DHAD activity that is increased relative to the DHAD activity of the polypeptide or fragment thereof without substitutions.

[0031] Another aspect of the invention is directed to an isolated polynucleotide molecule comprising a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof comprises one or more amino acid substitutions selected from: (a) glycine to aspartic acid at a position corresponding to position 33 of Streptococcus mutans DHAD; (b) aspartic acid to glutamic acid at a position corresponding to position 62 of Streptococcus mutans DHAD; (c) methionine to valine at a position corresponding to position 115 of Streptococcus mutans DHAD; (d) glycine to glutamic acid at a position corresponding to position 1 16 of Streptococcus mutans DHAD; (e) asparagine to serine at a position corresponding to position 1 19 of Streptococcus mutans DHAD; (f) glycine to arginine at a position corresponding to position 158 of Streptococcus mutans DHAD; (g) histidine to glutamine at a position corresponding to position 176 of Streptococcus mutans DHAD; (h) histidine to leucine at a position corresponding to position 179 of Streptococcus mutans DHAD; (i) glutamine to arginine at a position corresponding to position 322 of Streptococcus mutans DHAD; (j) alanine to serine at a position corresponding to position 425 of Streptococcus mutans DHAD; (k) glutamic acid to glycine at a position corresponding to position 524 of Streptococcus mutans DHAD; (1) phenylalanine to valine or leucine at a position corresponding to position 562 of Streptococcus mutans DHAD; (m) tryptophan to arginine, cysteine, or glycine at a position corresponding to position 563 of Streptococcus mutans DHAD; (n) lysine to glutamic acid at a position corresponding to position 564 of Streptococcus mutans DHAD; and (o) glutamic acid to aspartic acid at a position corresponding to position 567 of Streptococcus mutans DHAD.

[0032] In an embodiment of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of glutamic acid at a position corresponding to position 564 of Streptococcus mutans DHAD. In another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of glutamic acid at a position corresponding to position 62 of Streptococcus mutans DHAD, and a substitution of valine at a position corresponding to position 562 of Streptococcus mutans DHAD. In another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of aspartic acid at a position corresponding to position 33 of Streptococcus mutans DHAD, and a substitution of arginine at a position corresponding to position 563 of Streptococcus mutans DHAD. In another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of valine at a position corresponding to position 562 of Streptococcus mutans DHAD. In another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of arginine at a position corresponding to position 563 of Streptococcus mutans DHAD. In another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of cysteine at a position corresponding to position 563 of Streptococcus mutans DHAD. In another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of glycine at a position corresponding to position 563 of Streptococcus mutans DHAD. In another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of glycine at a position corresponding to position 524 of Streptococcus mutans DHAD, and a substitution of glycine at a position corresponding to position 563 of Streptococcus mutans DHAD.

3] In another embodiment of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of valine at a position corresponding to position 115 of Streptococcus mutans DHAD, a substitution of arginine at a position corresponding to position 158 of Streptococcus mutans DHAD, and a substitution of aspartic acid at a position corresponding to position 567 of Streptococcus mutans DHAD. In another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of glutamic acid at a position corresponding to position 116 of Streptococcus mutans DHAD, and a substitution of serine at a position corresponding to position 119 of Streptococcus mutans DHAD. In another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of aspartic acid at a position corresponding to position 33 of Streptococcus mutans DHAD. In another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of glutamic acid at a position corresponding to position 62 of Streptococcus mutans DHAD. In another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of leucine at a position corresponding to position 562 of Streptococcus mutans DHAD. In another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of glutamine at a position corresponding to position 176 of Streptococcus mutans DHAD, a substitution of leucine at a position corresponding to position 179 of Streptococcus mutans DHAD, a substitution of arginine at a position corresponding to position 322 of Streptococcus mutans DHAD, and a substitution of arginine at a position corresponding to position 563 of Streptococcus mutans DHAD. In yet another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof comprising a substitution of serine at a position corresponding to position 425 of Streptococcus mutans DHAD, and a substitution of arginine at a position corresponding to position 563 of Streptococcus mutans DHAD.

[0034] In another aspect, the invention is directed to an isolated polynucleotide molecule comprising a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof comprises one or more amino acid substitutions selected from: (a) glycine to aspartic acid at position 33; (b) aspartic acid to glutamic acid at position 62; (c) methionine to valine at position 115; (d) glycine to glutamic acid at position 1 16; (e) asparagine to serine at position 119; (f) glycine to arginine at position 158; (g) histidine to glutamine at position 176; (h) histidine to leucine at position 179; (i) glutamine to arginine at position 322; (j) alanine to serine at position 425; (k) glutamic acid to glycine at position 524; (1) phenylalanine to valine or leucine at position 562; (m) tryptophan to arginine, cysteine, or glycine at position 563; (n) lysine to glutamic acid at position 564; and (o) glutamic acid to aspartic acid at position 567.

[0035] In certain embodiments, the isolated polynucleotide molecule comprising a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity comprises a sequence selected from the group consisting of: SEQ ID NO:527, SEQ ID NO:529, SEQ ID NO:530, SEQ ID NO:531, SEQ ID NO:533, SEQ ID NO:535, SEQ ID NO:536, SEQ ID NO:538, SEQ ID NO:539, SEQ ID NO:606, SEQ ID NO:541, SEQ ID NO:542, SEQ ID NO:543, SEQ ID NO:544, SEQ ID NO:546, SEQ ID NO:547, SEQ ID NO:549, SEQ ID NO:550, SEQ ID NO:551, SEQ ID NO:553, SEQ ID NO:554, SEQ ID NO:556, SEQ ID NO:558, SEQ ID NO:559, SEQ ID NO:560, SEQ ID NO:562, SEQ ID NO:564, SEQ ID NO:565, SEQ ID NO:567, SEQ ID NO:568, SEQ ID NO:570, and SEQ ID NO:571.

[0036] In other embodiments, the invention is directed to an isolated polynucleotide molecule comprising a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity is a [2Fe-2S] ²⁺ DHAD. In other embodiments, the invention is directed to an isolated polynucleotide molecule comprising a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is a [4Fe-4S] ²⁺ DHAD. In other embodiments, the invention is directed to an isolated polynucleotide molecule comprising a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity catalyzes the conversion of 2,3- dihydroxyisovalerate to a-ketoisovalerate or catalyzes the conversion of 2,3- dihydroxymethylvalerate to a-ketomethylvalerate.

[0037] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof has an amino acid sequence that matches the Profile Hidden Markov Model (HMM) of Table 6 with an E value of < 10 ^~5 . In another embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof comprises three conserved cysteines corresponding to positions 56, 129, and 201 of Streptococcus mutans DHAD.

[0038] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is from a prokaryotic organism. In certain embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity from bacteria, fungi, or plant. In a particular embodiment, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity from Streptococcus mutatis.

[0039] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO: 528 and has a glutamic acid at position 564. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:528 and has a glutamic acid at position 564. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:528.

[0040] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:532 and has a glutamic acid at position 62 and a valine at position 562. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:532 and has a glutamic acid at position 62 and a valine at position 562. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:532.

[0041] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:534 and has an aspartic acid at position 33 and an arginine at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:534 and has an aspartic acid at position 33 and an arginine at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:534.

[0042] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:537 and has a valine at position 562. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:537 and has a valine at position 562. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:537.

[0043] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:540 and has an arginine at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:540 and has an arginine at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:540.

[0044] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:545 and has a cysteine at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:545 and has a cysteine at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:545.

[0045] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:572 and has a glycine at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:572 and has a glycine at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:572.

[0046] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:548 and has a glycine at position 524 and a glycine at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:548 and has a glycine at position 524 and a glycine at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:548.

[0047] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:552 and has a valine at position 115, an arginine at position 158, and an aspartic acid at position 567. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:552 and has a valine at position 115, an arginine at position 158, and an aspartic acid at position 567. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:552.

[0048] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:555 and has a glutamic acid at position 1 16 and a serine at position 1 19. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:555 and has a glutamic acid at position 1 16 and a serine at position 1 19. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:555. [0049] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:557 and has an aspartic acid at position 33. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:557 and has an aspartic acid at position 33. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:557.

[0050] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO: 561 and has a glutamic acid at position 62. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:561 and has a glutamic acid at position 62. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:561.

[0051] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:563 and has a leucine at position 562. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:563 and has a leucine at position 562. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:563.

[0052] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:566 and has a glutamine at position 176, a leucine at position 179, an arginine at position 322, and an arginine at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:566 and has a glutamine at position 176, a leucine at position 179, an arginine at position 322, and an arginine at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:566.

[0053] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:569 and has a serine at position 425 and an arginine at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:569 and has a serine at position 425 and an arginine at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:569.

[0054] In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment has DHAD activity that is increased relative to the DHAD activity of the polypeptide or fragment thereof without substitutions.

[0055] In certain embodiments, the isolated polynucleotide molecule is operatively linked to a promoter sequence. In other embodiments, the isolated polynucleotide molecule is comprised within a vector.

[0056] The invention also provides polypeptides encoded by the isolated nucleic acid molecules described herein.

[0057] Another aspect of the invention is directed to a recombinant host cell comprising the isolated nucleic acid molecules of the invention or a vector of the invention. In certain embodiments, the DHAD encoded by the isolated nucleic acid molecule is heterologous to the recombinant host cell. In other embodiments, the DHAD encoded by the isolated nucleic acid molecule is over-expressed in the recombinant host cell.

[0058] In still other embodiments, the recombinant host cell of the invention is a bacterial cell or a yeast cell. In some embodiments, the recombinant host cell of the invention is a bacterial cell, and the bacterial cell is a member of a genus of bacteria selected from Clostridium, Zymomonas, Escherichia, Salmonella, Rhodococcus, Pseudomonas, Bacillus, Lactobacillus, Enterococcus, Pediococcus, Alcaligenes, Klebsiella, Paenibacillus, Arthrobacter, Corynebacterium, Brevibacterium, Lactococcus, Leuconostoc, Oenococcus, Pediococcus, and Streptococcus. In other embodiments, the recombinant host cell of the invention is a yeast cell, and the yeast cell is a member of a genus of yeast selected from Saccharomyces, Schizosaccharomyces, Hansenula, Kluyveromyces, Candida, Pichia, and Yarrowia. In other embodiments, the recombinant host cell of the invention is Saccharomyces cerevisiae, Schizosaccharomyces pombe, Kluyveromyces lactis, Kluyveromyces thermotolerans, Candida glabrata, Candida albicans, Pichia stipitis, or Yarrowia lipolytica. In another embodiment, the recombinant host cell of the invention is Saccharomyces cerevisiae.

[0059] In some embodiments, the recombinant host cell of the invention is a yeast cell and the yeast cell further comprises a disruption in an endogenous ILV3 gene that encodes mitochondrial DHAD. In other embodiments, the yeast cell further comprises a disruption in one or more endogenous genes affecting iron-sulfur cluster biosynthesis selected from FRA2, GRX3, and GRX4. In yet other embodiments, the yeast cell has been further genetically engineered to upregulate the activity of at least one gene selected from AFT1 and AFT2. [0060] In some embodiments, the recombinant host cell of the invention is a bacterial cell, and the bacterial cell is a Lactobacillus. In other embodiments, the Lactobacillus further comprises at least one recombinant genetic expression element encoding iron-sulfur (Fe-S) cluster forming proteins. In yet other embodiments, the recombinant genetic expression element encoding iron-sulfur cluster forming proteins contains coding regions of an operon selected from Isc, Suf, and Nif operons. In some embodiments, the Suf operon comprises at least one coding region selected from SufC, SufD, SufS, SufU, SufB, SufA, and yseH. In some embodiments, the Suf operon is derived from Lactococcus lactis or Lactobacillus plantarum. In some embodiments, the Isc operon comprises at least one coding region selected from IscS, IscU, IscA, IscX, HscA, HscB, and Fdx. In some embodiments, the Isc operon is derived from Escherichia coli. In some embodiments, the Nif operon comprises at least one coding region selected from NifS and NifU. In some embodiments, the Nif operon is derived from Wolinella succinogenes.

[0061] In some embodiments, the recombinant host cell of the invention produces butanol, for example, isobutanol. In other embodiments, the recombinant host cell of the invention comprises an isobutanol biosynthetic pathway. In some embodiments, the isobutanol biosynthetic pathway comprises genes encoding acetolactate synthase, acetohydroxy acid isomeroreductase, DHAD, a-keto acid decarboxylase, and alcohol dehydrogenase. In another embodiment, the isobutanol biosynthetic pathway comprises the following substrate to product conversions: (i) pyruvate to acetolactate; (ii) acetolactate to 2,3-dihydroxyisovalerate; (iii) 2,3-dihydroxyisovalerate to a-ketoisovalerate; (iv) a-ketoisovalerate to isobutyraldehyde; and (v) isobutyraldehyde to isobutanol.

[0062] The substrate to product conversion of pyruvate to acetolactate can be catalyzed in some embodiments by an acetolactate synthase. The substrate to product conversion of acetolactate to 2,3-dihydroxyisovalerate can be catalyzed in some embodiments by a ketol- acid reductoisomerase. The substrate to product conversion of 2,3-dihydroxyisovalerate to a- ketoisovalerate can be catalyzed in some embodiments by a DHAD. The substrate to product conversion of α-ketoisovalerate to isobutyraldehyde can be catalyzed in some embodiments by an a-keto acid decarboxylase. The substrate to product conversion of isobutyraldehyde to isobutanol can be catalyzed in some embodiments by an alcohol dehydrogenase.

[0063] In some embodiments, two or more of: acetolactate synthase, ketol-acid reductoisomerase, and a-keto acid decarboxylase are heterologous to the recombinant host cell. In other embodiments, two or more of: acetolactate synthase, ketol-acid reductoisomerase, and a-keto acid decarboxylase are over-expressed in the recombinant host cell.

[0064] In some embodiments, the recombinant host cell comprising an isolated nucleic acid molecule of the invention produces an isobutanol titer that is increased as compared to a recombinant host cell that does not contain a polypeptide or fragment thereof having DHAD activity comprising one or more amino acid substitutions. In some embodiments, the recombinant host cell comprising the isolated nucleic acid molecule produces isobutanol at a rate that is increased by from about 10% to about 300% as compared to a recombinant host cell that does not contain a polypeptide or fragment thereof having DHAD activity comprising one or more amino acid substitutions. In some embodiments, the recombinant host cell comprising the isolated nucleic acid molecules produces isobutanol at a rate that is increased by at least about 10%, at least about 20%, at least about 30%, at least about 40%, or at least about 50% as compared to a recombinant host cell that does not contain a polypeptide or fragment thereof having DHAD activity comprising one or more amino acid substitutions. In other embodiments, the polypeptide or fragment thereof having DHAD activity is expressed in the cytosol, or the polypeptide or fragment thereof having DHAD activity and the ketol-acid reductoisomerase are expressed in the cytosol.

[0065] Another aspect of the invention is directed to a method for the production of butanol, for example, isobutanol, comprising providing a recombinant host cell comprising an isolated nucleic acid molecule of the invention; culturing the recombinant host cell in a fermentation medium under suitable conditions to produce isobutanol from pyruvate; and recovering the isobutanol. In some embodiments, the isobutanol is produced at a titer that is increased as compared to a recombinant host cell that does not contain a polypeptide or fragment thereof having DHAD activity comprising one or more amino acid substitutions. In other embodiments, the isobutanol is produced at a rate that is increased by from about 10% to about 300% as compared to a recombinant host cell that does not contain a polypeptide or fragment thereof having DHAD activity comprising one or more amino acid substitutions. In other embodiments, the isobutanol is produced at a rate that is increased by at least about 10%, at least about 20%, at least about 30%, at least about 40%, or at least about 50% as compared to a recombinant host cell that does not contain a polypeptide or fragment thereof having DHAD activity comprising one or more amino acid substitutions. In another embodiment, the concentration of isobutanol in the fermentation medium is greater than or equal to about 20 mM. In another embodiment, the concentration of isobutanol in the fermentation medium is from about 30 mM to about 50 mM.

[0066] Another aspect of the invention is directed to a method of converting 2,3- dihydroxyisovalerate to a-ketoisovalerate or 2,3-dihydroxymethylvalerate to a- ketomethylvalerate, comprising providing an isolated polypeptide or fragment thereof of the invention, wherein the isolated polypeptide or fragment thereof catalyzes the conversion of 2,3-dihydroxyisovalerate to a-ketoisovalerate or 2,3-dihydroxymethylvalerate to a- ketomethylvalerate. In some embodiments of the method to convert 2,3-dihydroxyisovalerate to α-ketoisovalerate or 2,3-dihydroxymethylvalerate to a-ketomethylvalerate, the isolated polypeptide or fragment thereof is comprised within a recombinant host cell.

[0067] In some embodiments of the method to convert 2,3-dihydroxyisovalerate to a- ketoisovalerate or 2,3-dihydroxymethylvalerate to α-ketomethylvalerate, the recombinant host cell is cultured in a fermentation medium under suitable conditions to produce isobutanol from pyruvate, and the isobutanol is recovered. In some embodiments, the isobutanol is recovered by distillation, liquid-liquid extraction, adsorption, decantation, pervaporation, or combinations thereof. In some embodiments, solids are removed from the fermentation medium. In some embodiments, solids are removed from the fermentation medium by centrifugation, filtration, decantation, or combinations thereof. In other embodiments, the solids are removed before the isobutanol is recovered. In other embodiments, the conversion of 2,3-dihydroxyisovalerate to α-ketoisovalerate or 2,3- dihydroxymethylvalerate to α-ketomethylvalerate is improved as compared to a control conversion under the same conditions with a control polypeptide having DHAD activity which does not comprise an amino acid substitution.

[0068] Another aspect of the invention is directed to a composition comprising one or more recombinant host cells of the invention, and a fermentable carbon substrate.

[0069] Another aspect of the invention is directed to a composition comprising one or more recombinant host cells of the invention, and isobutanol. In other embodiments, the composition further comprises an extractant.

[0070] In some embodiments of the method of screening DHAD protein variants, the weak promoter is a truncated Leu2 promoter. In certain embodiments, the truncated Leu2 promoter is SEQ ID NO:575. In other embodiments of the method of screening DHAD protein variants, the weak promoter is a truncated FBA promoter. In certain embodiments, the truncated FBA promoter is SEQ ID NO:576.

[0071] In some embodiments of the method of screening DHAD protein variants, the low copy number plasmid has a copy number of one or two in yeast. In certain embodiments, the low copy number plasmid is pRS413.

[0072] In other embodiments of the method of screening DHAD protein variants, the growth of the strain is under oxygen limiting conditions. In yet other embodiments, the yeast strain is further transformed with genes encoding acetolactate synthase, acetohydroxy acid isomeroreductase, a-keto acid decarboxylase, and alcohol dehydrogenase. In certain embodiments of the method of screening DHAD protein variants, the method further comprises determining the rate of isobutanol production of the transformants.

[0073] Another aspect of the invention is directed to isolated polynucleotides comprising a nucleic acid sequence encoding a DHAD variant obtained by the methods of screening DHAD protein variants described herein. The invention is also directed to isolated DHAD variant polypeptides encoded by these nucleic acid sequences.

[0074] Another aspect of the invention is directed to recombinant host cells transformed with an isolated nucleic acid molecule comprising the nucleic acid sequence of SEQ ID NO:573. Another aspect of the invention is directed to recombinant host cells transformed with an isolated nucleic acid molecule comprising the nucleic acid sequence of SEQ ID NO:574.

DESCRIPTION OF THE DRAWINGS

[0075] The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.

[0076] FIG. 1 is a schematic diagram of plasmid pRS423 Leu2p(75)-IlvD (sm) GPMp-ADH (SEQ ID NO:549).

[0077] FIG. 2 shows biosynthetic pathways for isobutanol production.

[0078] FIG. 3 is a schematic diagram of the PNY2204 locus (pdclA::ilvD::pUC19- kan: :FBA-alsS::TRXl). DESCRIPTION OF THE INVENTION

[0079] For improved production of compounds synthesized in pathways including dihydroxy-acid dehydratase (DHAD), it is desirable to express a heterologous DHAD enzyme that provides this enzymatic activity in the production host of interest. However, there exists a need for alternative DHAD enzymes and DHAD variants that display increased activity as compared to a parental DHAD enzyme in heterologous organisms and for screening methods to identify such enzymes and variants. Such enzymes and variants can be employed for production of compounds from DHAD-requiring biosynthetic pathways.

[0080] The present invention satisfies these and other needs, and provides further related advantages, as will be made apparent by the description of the embodiments that follow.

[0081] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present application including the definitions will control. Also, unless otherwise required by context, singular terms shall include pluralities and plural terms shall include the singular. All publications, patents and other references mentioned herein are incorporated by reference in their entireties for all purposes.

Definitions

[0082] In order to further define this invention, the following terms and definitions are herein provided.

[0083] As used herein, the terms "comprises," "comprising," "includes," "including," "has," "having," "contains," or "containing," or any other variation thereof, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. For example, a composition, a mixture, a process, a method, an article, or an apparatus that comprises a list of elements is not necessarily limited to only those elements but can include other elements not expressly listed or inherent to such composition, mixture, process, method, article, or apparatus. Further, unless expressly stated to the contrary, "or" refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). [0084] As used herein, the term "consists of," or variations such as "consist of or "consisting of," as used throughout the specification and claims, indicate the inclusion of any recited integer or group of integers, but that no additional integer or group of integers can be added to the specified method, structure, or composition.

[0085] As used herein, the term "consists essentially of," or variations such as "consist essentially of or "consisting essentially of," as used throughout the specification and claims, indicate the inclusion of any recited integer or group of integers, and the optional inclusion of any recited integer or group of integers that do not materially change the basic or novel properties of the specified method, structure or composition.

[0086] Also, the indefinite articles "a" and "an" preceding an element or component of the invention are intended to be nonrestrictive regarding the number of instances, that is, occurrences of the element or component. Therefore, "a" or "an" should be read to include one or at least one, and the singular word form of the element or component also includes the plural unless the number is obviously meant to be singular.

[0087] The terms "invention" or "present invention" as used herein are non-limiting terms and are not intended to refer to any single embodiment of the particular invention but encompass all possible embodiments as described in the application.

[0088] As used herein, the term "about" modifying the quantity of an ingredient or reactant of the invention employed refers to variation in the numerical quantity that can occur, for example, through typical measuring and liquid handling procedures used for making concentrates or solutions in the real world; through inadvertent error in these procedures; through differences in the manufacture, source, or purity of the ingredients employed to make the compositions or to carry out the methods; and the like. The term "about" also encompasses amounts that differ due to different equilibrium conditions for a composition resulting from a particular initial mixture. Whether or not modified by the term "about", the claims include equivalents to the quantities. In one embodiment, the term "about" means within 10% of the reported numerical value; in another embodiment, within 5% of the reported numerical value.

[0089] The term "alcohol" as used herein refers to any of a series of hydroxyl compounds, the simplest of which are derived from saturated hydrocarbons, having the general formula C _n H2n+10H. Examples of alcohol include ethanol and butanol. [0090] The term "butanol" as used herein refers to n-butanol, 2-butanol, isobutanol, tert-butyl alcohol, individually or any mixtures thereof. Butanol can be from a biological source (i.e., biobutanol), for example.

[0091] The term "[2Fe-2S] ²⁺ DHAD" refers to DHAD enzymes having a bound [2Fe-2S] ²⁺ iron-sulfur cluster.

[0092] The term "[4Fe-4S] ²⁺ DHAD" refers to DHAD enzymes having a bound [4Fe-4S] ²⁺ iron-sulfur cluster.

[0093] The term "acetohydroxy acid dehydratase" and "dihydroxy-acid dehydratase" ("DHAD") refers to a polypeptide having enzyme activity that catalyzes the conversion of 2,3-dihydroxyisovalerate to a-ketoisovalerate or the conversion of 2,3- dihydroxymethylvalerate to a-ketomethylvalerate. Example dihydroxy-acid dehydratases are known by the EC number 4.2.1.9. Such enzymes are available from a vast array of microorganisms including, but not limited to, Escherichia coli (GenBank Nos: YP_026248, NC_000913), Saccharomyces cerevisiae (GenBank Nos: NP_012550, NC 001 142), Methanococcus maripaludis (GenBank Nos: CAF29874, BX957219), Bacillus subtilis (GenBank Nos: CAB14105, Z99115), Lactobacillus lactis, and Neurospora crassa. U.S. Patent Application Publication No. 2010/0081 154 and U.S. Patent No. 7,851, 188, both of which are incorporated herein by reference, describe dihydroxy-acid dehydratases including a dihydroxy-acid dehydratase from Streptococcus mutans (nucleic acid: SEQ ID NO: 167; amino acid: SEQ ID NO: 168). Dihydroxy-acid dehydratases also include, for example, the variant dihydroxy-acid dehydratases described herein.

[0094] "Increased" or "improved" properties of the DHAD variants of the invention is assessed in comparison to other DHAD enzymes, for example, a wild type DHAD, a parent DHAD, a non-substituted DHAD, or other reference DHAD. Such assessments include, but are not limited to, enzyme stability, solubility, activity, expression level, substrate to product conversion and/or isobutanol production. Methods for making these assessments are known and described in the present application.

[0095] The term "isobutanol biosynthetic pathway" as used herein refers to an enzyme pathway to produce isobutanol from pyruvate.

[0096] The terms "acetohydroxyacid synthase," "acetolactate synthase," and "acetolactate synthetase" (abbreviated "ALS") are used interchangeably herein to refer to a polypeptide having enzyme activity that catalyzes the conversion of pyruvate to acetolactate and CO ₂ . Example acetolactate synthases are known by the EC number 2.2.1.6 (Enzyme Nomenclature 1992, Academic Press, San Diego). These enzymes are available from a number of sources including, but not limited to, Bacillus subtilis (GenBank Nos. CAB07802.1, CAB15618, and Z99122, NCBI (National Center for Biotechnology Information) amino acid sequence, NCBI nucleotide sequence, respectively), Klebsiella pneumoniae (GenBank Nos. AAA25079 and M73842), and Lactococcus lactis (GenBank Nos. AAA25161 and L16975).

[0097] The terms "ketol-acid reductoisomerase" ("KARI"), "acetohydroxy acid reductoisomerase," and "acetohydroxy acid isomeroreductase" are used interchangeably herein to refer a polypeptide having enzyme activity that catalyzes the reaction of (S)- acetolactate to 2,3-dihydroxyisovalerate. Example KARI enzymes are classified as EC number 1.1.1.86 (Enzyme Nomenclature 1992, Academic Press, San Diego), and are available from a vast array of microorganisms including, but not limited to, Escherichia coli (GenBank Nos. NP_418222 and NC_000913), Saccharomyces cerevisiae (GenBank Nos. NP_013459 and NC_001 144), Methanococcus maripaludis (GenBank Nos. CAF30210 and BX957220), and Bacillus subtilis (GenBank Nos. CAB14789 and Z991 18). KARIs include, for example, Anaerostipes caccae KARI variants "K9G9," "K9D3," and "K9JB4P" (SEQ ID NO:569). k9jb4pKARI enzymes are also described in U.S. Patent Nos. 7,910,342 and 8, 129,162, U.S. Patent Application Publication No. 2010/0197519, and PCT Application Publication Nos. WO201 1/041415 and WO2012/129555, all of which are incorporated herein by reference. Examples of KARIs disclosed therein include those from Lactococcus lactis, Vibrio cholera, Pseudomonas aeruginosa PAOl, Pseudomonas fluorescens PF5, and Anaerostipes caccae. In some embodiments, the KARI may utilize NADH (reduced nicotinamide adenine dinucleotide). In some embodiments, the KARI may utilize NADPH (reduced nicotinamide adenine dinucleotide phosphate).

[0098] The terms "branched-chain a-keto acid decarboxylase," "a-ketoacid decarboxylase," "a-ketoisovalerate decarboxylase," and "2-ketoisovalerate decarboxylase" ("KTVD") are used interchangeably herein to refer to a polypeptide having enzyme activity that catalyzes the conversion of a-ketoisovalerate to isobutyraldehyde and CO ₂ . Example branched-chain a- keto acid decarboxylases are known by the EC number 4.1.1.72 and are available from a number of sources including, but not limited to, Lactococcus lactis (GenBank Nos. AAS49166, AY548760, CAG34226, and AJ746364), Salmonella typhimurium (GenBank Nos. NP_461346 and NC_003197), Clostridium acetobutylicum (GenBank Nos. NPJ49189 and NC_001988), Macrococcus caseolyticus, and Listeria grayi.

[0100] The terms "branched-chain alcohol dehydrogenase" and "alcohol dehydrogenase" ("ADH") are used interchangeably herein to refer to a polypeptide having enzyme activity that catalyzes the conversion of isobutyraldehyde to isobutanol. Example branched-chain alcohol dehydrogenases are known by the EC number 1.1.1.265, but can also be classified under other alcohol dehydrogenases (e.g., EC numbers 1.1.1.1 or 1.1.1.2). Alcohol dehydrogenases can be, for example, NADPH dependent or NADH dependent. Such enzymes are available from a number of sources including, but not limited to, Saccharomyces cerevisiae (GenBank Nos. NP_010656, NC_001136, NP_014051, and NC_001 145), Escherichia coli (GenBank Nos. NP_417484 and NC_000913), and Clostridium acetobutylicum (GenBank Nos. NP_349892, NC_003030, NPJ49891, and NC_003030). U.S. Patent No. 8, 188,250 (incorporated herein by reference) describes SadB, an alcohol dehydrogenase (ADH) from Achromobacter xylosoxidans. Alcohol dehydrogenases also include horse liver ADH and Beijerinkia indica ADH (as described in U.S. Patent Application Publication No. 2011/0269199, which is incorporated herein by reference).

[0101] The terms "carbon substrate" and "fermentable carbon substrate" are used interchangeably herein to refer to a carbon source capable of being metabolized by host organisms of the present invention and particularly carbon sources selected from the group consisting of monosaccharides, oligosaccharides, polysaccharides, and one-carbon substrates or mixtures thereof. Carbon substrates can include six-carbon (C6) and five-carbon (C5) sugars and mixtures thereof, such as, for example, glucose, sucrose, or xylose.

[0102] The term "polynucleotide" as used herein encompasses a singular nucleic acid as well as plural nucleic acids, and refers to a nucleic acid molecule or construct, for example, messenger RNA (mRNA) or plasmid DNA (pDNA). A polynucleotide can contain the nucleotide sequence of the full-length cDNA sequence, or a fragment thereof, including the untranslated 5' and 3' sequences and the coding sequences. The polynucleotide can be composed of any polyribonucleotide or polydeoxyribonucleotide, which can be unmodified RNA or DNA or modified RNA or DNA. For example, polynucleotides can be composed of single- and double-stranded DNA, DNA that is a mixture of single- and double-stranded regions, single- and double-stranded RNA, and RNA that is mixture of single- and double- stranded regions, hybrid molecules comprising DNA and RNA that can be single-stranded or, more typically, double-stranded or a mixture of single- and double-stranded regions. "Polynucleotide" embraces chemically, enzymatically, or metabolically modified forms.

[0103] The term "gene" refers to a nucleic acid fragment that is capable of being expressed as a specific protein, optionally including regulatory sequences preceding (5' non-coding sequences) and following (3' non-coding sequences) the coding sequence. "Native gene" refers to a gene as found in nature with its own regulatory sequences. "Chimeric gene" refers to any gene that is not a native gene, comprising regulatory and coding sequences that are not found together in nature. Accordingly, a chimeric gene can comprise regulatory sequences and coding sequences that are derived from different sources, or regulatory sequences and coding sequences derived from the same source, but arranged in a manner different than that found in nature. "Endogenous gene" refers to a native gene in its natural location in the genome of an organism. A "foreign gene" or "heterologous gene" refers to a gene not normally found in the host organism, but that is introduced into the host organism by gene transfer. Foreign genes can comprise native genes inserted into a non-native organism, or chimeric genes. A "transgene" is a gene that has been introduced into the genome by a transformation procedure.

[0104] As used herein, the term "coding region" refers to a DNA sequence that codes for a specific amino acid sequence. "Suitable regulatory sequences" refers to nucleotide sequences located upstream (5' non-coding sequences), within, or downstream (3' non-coding sequences) of a coding sequence, and which influence the transcription, RNA processing or stability, or translation of the associated coding sequence. Regulatory sequences can include promoters, translation leader sequences, introns, polyadenylation recognition sequences, RNA processing site, effector binding site and stem-loop structure.

[0105] "Regulatory sequences" refers to nucleotide sequences located upstream (5' non- coding sequences), within, or downstream (3' non-coding sequences) of a coding sequence, and which influence the transcription, RNA processing or stability, or translation of the associated coding sequence. Regulatory sequences can include promoters, enhancers, operators, repressors, transcription termination signals, translation leader sequences, introns, polyadenylation recognition sequences, RNA processing site, effector binding site and stem- loop structure.

[0106] The term "promoter" refers to a DNA sequence capable of controlling the expression of a coding sequence or functional RNA. In general, a coding sequence is located 3' to a promoter sequence. Promoters can be derived in their entirety from a native gene, or composed of different elements derived from different promoters found in nature, or even comprise synthetic DNA segments. It is understood by those skilled in the art that different promoters can direct the expression of a gene in different tissues or cell types, or at different stages of development, or in response to different environmental or physiological conditions. Promoters which cause a gene to be expressed in most cell types at most times are commonly referred to as "constitutive promoters." It is further recognized that since in most cases the exact boundaries of regulatory sequences have not been completely defined, DNA fragments of different lengths can have identical promoter activity.

[0107] The term "operably linked" refers to the association of nucleic acid sequences on a single nucleic acid fragment so that the function of one is affected by the other. For example, a promoter is operably linked with a coding sequence when it is capable of effecting the expression of that coding sequence (i.e., that the coding sequence is under the transcriptional control of the promoter). Coding sequences can be operably linked to regulatory sequences in sense or antisense orientation.

[0108] The term "expression" as used herein refers to the transcription and stable accumulation of sense (mRNA) or antisense RNA derived from the nucleic acid fragment of the invention. Expression can also refer to translation of mRNA into a polypeptide.

[0109] The term "over-expression" as used herein refers to expression that is higher than endogenous expression of the same or related polynucleotide or gene. A heterologous polynucleotide or gene is also over-expressed if its expression is higher than that of a comparable endogenous gene, or if its expression is higher than that of the same polynucleotide or gene introduced by a means that does not over-express the polynucleotide or gene. For example, a polynucleotide can be expressed in a host cell from a low copy number plasmid, which is present in only limited or few copies, and the same polynucleotide can be over-expressed in a host cell from a high copy number plasmid or a plasmid with a copy number that can be regulated, which is present in multiple copies. Any means can be used to over-express a polynucleotide, so long as it increases the copies of the polynucleotide in the host cell. In addition to using a high copy number plasmid or a plasmid with a copy number that can be regulated, a polynucleotide can be over-expressed by multiple chromosomal integrations. [0110] Expression or over-expression of a polypeptide of the invention in a recombinant host cell can be quantified according to any number of methods known to the skilled artisan and can be represented, for example, by a percent of total cell protein. The percent of total protein can be an amount selected from greater than about 0.001% of total cell protein; greater than about 0.01% of total cell protein; greater than about 0.1% of total cell protein; greater than about 0.5% of total cell protein; greater than about 1.0% of total cell protein; greater than about 2.0% of total cell protein; greater than about 3.0% of total cell protein; greater than about 4.0% of total cell protein; greater than about 5.0% of total cell protein; greater than about 6.0% of total cell protein; greater than about 7.0% of total cell protein; greater than about 8.0% of total cell protein; greater than about 9.0% of total cell protein; greater than about 10% of total cell protein; or greater than about 20% of total cell protein. In one embodiment, the amount of polypeptide expressed is greater than about 0.5% of total cell protein. In another embodiment, the amount of polypeptide expressed is greater than about 1.0% of total cell protein or greater than about 2.0% of total cell protein.

[0111] As used herein, the term "transformation" refers to the transfer of a nucleic acid fragment into a host organism, resulting in genetically stable inheritance. Host organisms containing the transformed nucleic acid fragments are referred to as "transgenic" or "recombinant" or "transformed" organisms.

[0112] The terms "plasmid" and "vector" as used herein refer to an extra chromosomal element often carrying genes which are not part of the central metabolism of the cell, and usually in the form of circular double-stranded DNA molecules. Such elements can be autonomously replicating sequences, genome integrating sequences, phage or nucleotide sequences, linear or circular, of a single- or double-stranded DNA or RNA, derived from any source, in which a number of nucleotide sequences have been joined or recombined into a unique construction which is capable of introducing a promoter fragment and DNA sequence for a selected gene product along with appropriate 3' untranslated sequence into a cell.

[0113] A "recombinant host cell" is defined as a host cell that has been genetically manipulated to express a biosynthetic production pathway, wherein the host cell either produces a biosynthetic product in greater quantities relative to an unmodified host cell or produces a biosynthetic product that is not ordinarily produced by an unmodified host cell.

[0114] The term "engineered" as applied to a isobutanol biosynthetic pathway refers to the isobutanol biosynthetic pathway that is manipulated, such that the carbon flux from pyruvate through the engineered isobutanol biosynthetic pathway is maximized, thereby producing an increased amount of isobutanol directly from the fermentable carbon substrate. Such engineering includes expression of heterologous polynucleotides or polypeptides, over- expression of endogenous polynucleotides or polypeptides, cytosolic localization of proteins that do not naturally localize to cytosol, increased cofactor availability, decreased activity of competitive pathways, etc.

[0115] The term "codon optimized" as it refers to genes or coding regions of nucleic acid molecules for transformation of various hosts, refers to the alteration of codons in the gene or coding regions of the nucleic acid molecules to reflect the typical codon usage of the host organism without altering the polypeptide encoded by the DNA. Such optimization includes replacing at least one, or more than one, or a significant number, of codons with one or more codons that are more frequently used in the genes of that organism.

[0116] Deviations in the nucleotide sequence that comprise the codons encoding the amino acids of any polypeptide chain allow for variations in the sequence coding for the gene. Since each codon consists of three nucleotides, and the nucleotides comprising DNA are restricted to four specific bases, there are 64 possible combinations of nucleotides, 61 of which encode amino acids (the remaining three codons encode signals ending translation). The "genetic code" which shows which codons encode which amino acids is reproduced herein as Table 1. As a result, many amino acids are designated by more than one codon. For example, the amino acids alanine and proline are coded for by four triplets, serine and arginine by six, whereas tryptophan and methionine are coded by just one triplet. This degeneracy allows for DNA base composition to vary over a wide range without altering the amino acid sequence of the proteins encoded by the DNA.

Table 1 : The Standard Genetic Code

[0117] Many organisms display a bias for use of particular codons to code for insertion of a particular amino acid in a growing peptide chain. Codon preference, or codon bias, differences in codon usage between organisms, is afforded by degeneracy of the genetic code, and is well documented among many organisms. Codon bias often correlates with the efficiency of translation of messenger RNA (mRNA), which is in turn believed to be dependent on, inter alia, the properties of the codons being translated and the availability of particular transfer RNA (tRNA) molecules. The predominance of selected tRNAs in a cell is generally a reflection of the codons used most frequently in peptide synthesis. Accordingly, genes can be tailored for optimal gene expression in a given organism based on codon optimization.

[0118] As used herein, an "isolated nucleic acid fragment" or "isolated nucleic acid molecule" are used interchangeably herein and mean a polymer of RNA or DNA that is single- or double-stranded, optionally containing synthetic, non-natural, or altered nucleotide bases. An isolated nucleic acid fragment in the form of a polymer of DNA can be comprised of one or more segments of cDNA, genomic DNA, or synthetic DNA. [0119] A nucleic acid fragment is "hybridizable" to another nucleic acid fragment, such as a cDNA, genomic DNA, or RNA molecule, when a single-stranded form of the nucleic acid fragment can anneal to the other nucleic acid fragment under the appropriate conditions of temperature and solution ionic strength. Hybridization and washing conditions are well known and exemplified, for example, in Sambrook, J., Fritsch, E. F. and Maniatis, T. Molecular Cloning: A Laboratory Manual, 2nd ed., Cold Spring Harbor Laboratory: Cold Spring Harbor, N.Y. (1989), particularly Chapter 1 1 and Table 1 1.1 therein (incorporated herein by reference in its entirety). The conditions of temperature and ionic strength determine the "stringency" of the hybridization. Stringency conditions can be adjusted to screen for moderately similar fragments (such as homologous sequences from distantly related organisms), to highly similar fragments (such as genes that duplicate functional enzymes from closely related organisms). Post-hybridization washes determine stringency conditions. One set of preferred conditions uses a series of washes starting with 6xSSC, 0.5% SDS at room temperature for 15 min, then repeated with 2xSSC, 0.5% SDS at 45°C for 30 min, and then repeated twice with 0.2xSSC, 0.5% SDS at 50°C for 30 min. A more preferred set of stringent conditions uses higher temperatures in which the washes are identical to those above except for the temperature of the final two 30 min washes in 0.2xSSC, 0.5% SDS was increased to 60°C. Another preferred set of highly stringent conditions uses two final washes in O. lxSSC, 0.1% SDS at 65°C. An additional set of stringent conditions include hybridization at O. lxSSC, 0.1% SDS, 65°C and washes with 2xSSC, 0.1% SDS followed by O. lxSSC, 0.1% SDS, for example.

[0120] Hybridization requires that the two nucleic acids contain complementary sequences, although depending on the stringency of the hybridization, mismatches between bases are possible. The appropriate stringency for hybridizing nucleic acids depends on the length of the nucleic acids and the degree of complementation, variables well known in the art. The greater the degree of similarity or homology between two nucleotide sequences, the greater the value of Tm for hybrids of nucleic acids having those sequences. The relative stability (corresponding to higher Tm) of nucleic acid hybridizations decreases in the following order: RNA: RNA, DNA:RNA, DNA:DNA. For hybrids of greater than 100 nucleotides in length, equations for calculating Tm have been derived (see Sambrook, et al, supra, 9.50-9.51). For hybridizations with shorter nucleic acids, for example, oligonucleotides, the position of mismatches becomes more important, and the length of the oligonucleotide determines its specificity (see Sambrook, et al, supra, 1 1.7-1 1.8). In one embodiment, the length for a hybridizable nucleic acid is at least about 10 nucleotides. Preferably, a minimum length for a hybridizable nucleic acid is at least about 15 nucleotides; more preferably at least about 20 nucleotides; and most preferably the length is at least about 30 nucleotides. Furthermore, the skilled artisan will recognize that the temperature and wash solution salt concentration can be adjusted as necessary according to factors such as length of the probe.

[0121] As used herein, the term "polypeptide" is intended to encompass a singular "polypeptide" as well as plural "polypeptides," and refers to a molecule composed of monomers (amino acids) linearly linked by amide bonds (also known as peptide bonds). The term "polypeptide" refers to any chain or chains of two or more amino acids, and does not refer to a specific length of the product. Thus, peptides, dipeptides, tripeptides, oligopeptides, "protein," "amino acid chain," or any other term used to refer to a chain or chains of two or more amino acids, are included within the definition of "polypeptide," and the term "polypeptide" can be used instead of, or interchangeably with any of these terms. A polypeptide can be derived from a natural biological source or produced by recombinant technology, but is not necessarily translated from a designated nucleic acid sequence. It can be generated in any manner, including by chemical synthesis.

[0122] By an "isolated" polypeptide or a fragment, variant, or derivative thereof is intended a polypeptide that is not in its natural milieu. No particular level of purification is required. For example, an isolated polypeptide can be removed from its native or natural environment. Recombinantly produced polypeptides and proteins expressed in host cells are considered isolated for purposed of the invention, as are native or recombinant polypeptides which have been separated, fractionated, or partially or substantially purified by any suitable technique.

[0123] As used herein, the term "variant" refers to a polypeptide differing from a specifically recited polypeptide of the invention, such as DHAD, by amino acid insertions, deletions, mutations, and substitutions, created using, for example, recombinant DNA techniques, such as mutagenesis. A variant also includes "silent" substitutions or "silent" mutations whereby a substitution of one or more nucleotide bases in a polynucleotide does not change the resulting amino acid sequence, but results in improved properties of the resulting polypeptide. Guidance in determining which amino acid residues can be replaced, added, or deleted without abolishing activities of interest, can be found by comparing the sequence of the particular polypeptide with that of homologous polypeptides, for example, yeast or bacterial, and minimizing the number of amino acid sequence changes made in regions of high homology (conserved regions) or by replacing amino acids with consensus sequences.

[0124] Alternatively, recombinant polynucleotide variants encoding these same or similar polypeptides can be synthesized or selected by making use of the "redundancy" in the genetic code. Various codon substitutions, such as silent changes which produce various restriction sites, can be introduced to optimize cloning into a plasmid or viral vector for expression. Mutations in the polynucleotide sequence can be reflected in the polypeptide or domains of other peptides added to the polypeptide to modify the properties of any part of the polypeptide.

[0125] Amino acid "substitutions" can be the result of replacing one amino acid with another amino acid having similar structural and/or chemical properties, for example, conservative amino acid replacements, or they can be the result of replacing one amino acid with an amino acid having different structural and/or chemical properties, for example, non-conservative amino acid replacements. "Conservative" amino acid substitutions can be made on the basis of similarity in polarity, charge, solubility, hydrophobicity, hydrophilicity, or the amphipathic nature of the residues involved. For example, nonpolar (hydrophobic) amino acids include alanine, leucine, isoleucine, valine, proline, phenylalanine, tryptophan, and methionine; polar neutral amino acids include glycine, serine, threonine, cysteine, tyrosine, asparagine, and glutamine; positively charged (basic) amino acids include arginine, lysine, and histidine; and negatively charged (acidic) amino acids include aspartic acid and glutamic acid. Alternatively, "non-conservative" amino acid substitutions can be made by selecting the differences in polarity, charge, solubility, hydrophobicity, hydrophilicity, or the amphipathic nature of any of these amino acids. "Insertions" or "deletions" can be within the range of variation as structurally or functionally tolerated by the recombinant proteins. The variation allowed can be experimentally determined by systematically making insertions, deletions, or substitutions of amino acids in a polypeptide molecule using recombinant DNA techniques and assaying the resulting recombinant variants for activity.

[0126] A "substantial portion" of an amino acid or nucleotide sequence is that portion comprising enough of the amino acid sequence of a polypeptide or the nucleotide sequence of a gene to putatively identify that polypeptide or gene, either by manual evaluation of the sequence by one skilled in the art, or by computer-automated sequence comparison and identification using algorithms such as BLAST (Altschul, et al, J. Mol. Biol, 215:403-410, 1993). In general, a sequence of ten or more contiguous amino acids or thirty or more nucleotides is necessary in order to putatively identify a polypeptide or nucleic acid sequence as homologous to a known protein or gene. Moreover, with respect to nucleotide sequences, gene specific oligonucleotide probes comprising 20-30 contiguous nucleotides can be used in sequence-dependent methods of gene identification (e.g., Southern hybridization) and isolation (e.g., in situ hybridization of bacterial colonies or bacteriophage plaques). In addition, short oligonucleotides of 12-15 bases can be used as amplification primers in PCR in order to obtain a particular nucleic acid fragment comprising the primers. Accordingly, a "substantial portion" of a nucleotide sequence comprises enough of the sequence to specifically identify and/or isolate a nucleic acid fragment comprising the sequence. The instant specification teaches the complete amino acid and nucleotide sequence encoding particular proteins. The skilled artisan, having the benefit of the sequences as reported herein, can now use all or a substantial portion of the disclosed sequences for purposes known to those skilled in this art. Accordingly, the instant invention comprises the complete sequences as reported in the accompanying Sequence Listing, as well as substantial portions of those sequences as defined herein.

[0127] The term "complementary" is used to describe the relationship between nucleotide bases that are capable of hybridizing to one another. For example, with respect to DNA, adenosine is complementary to thymine and cytosine is complementary to guanine.

[0128] The term "percent identity," as known in the art, is a relationship between two or more polypeptide sequences or two or more polynucleotide sequences, as determined by comparing the sequences. In the art, "identity" or "sequence identity" also means the degree of sequence relatedness between polypeptide or polynucleotide sequences, as the case may be, as determined by the match between strings of such sequences. "Identity" and "similarity" can be readily calculated by known methods, including but not limited to those described in:

(1) Computational Molecular Biology (Lesk, A. M., Ed.) Oxford University: NY (1988);

(2) Biocomputing: Informatics and Genome Projects (Smith, D. W., Ed.) Academic: NY (1993); (3) Computer Analysis of Sequence Data, Part I (Griffin, A. M., and Griffin, H. G., Eds.) Humania: NJ (1994); (4) Sequence Analysis in Molecular Biology (von Heinje, G., Ed.) Academic (1987); and (5) Sequence Analysis Primer (Gribskov, M. and Devereux, J., Eds.) Stockton: NY (1991). [0129] Preferred methods to determine identity are designed to give the best match between the sequences tested. Methods to determine identity and similarity are codified in publicly available computer programs. Sequence alignments and percent identity calculations can be performed using the MegAlign™ program of the LASERGENE bioinformatics computing suite (DNASTAR Inc., Madison, Wis.). Multiple alignment of the sequences is performed using the "Clustal method of alignment" which encompasses several varieties of the algorithm including the "Clustal V method of alignment" corresponding to the alignment method labeled Clustal V (described by Higgins and Sharp, CABIOS. 5: 151-153, 1989; Higgins, et al, Comput. Appl. Biosci. 8: 189-191, 1992) and found in the MegAlign™ program of the LASERGENE bioinformatics computing suite (DNASTAR Inc.). For multiple alignments, the default values correspond to GAP PENALTY=10 and GAP LENGTH PENALTY=10. Default parameters for pairwise alignments and calculation of percent identity of protein sequences using the Clustal method are KTUPLE=1, GAP PENALTY=3, WINDOW=5 and DIAGONALS SAVED=5. For nucleic acids, these parameters are KTUPLE=2, GAP PENALTY=5, WINDOW=4 and DIAGONALS SAVED=4. After alignment of the sequences using the Clustal V program, it is possible to obtain a "percent identity" by viewing the "sequence distances" table in the same program. Additionally, the "Clustal W method of alignment" is available and corresponds to the alignment method labeled Clustal W (described by Higgins and Sharp, CABIOS. 5: 151-153, 1989; Higgins, et al, Comput. Appl. Biosci. 8: 189-191, 1992; Thompson, et al, Nuc. Acid Res. 22: 4673 4680, 1994) and found in the MegAlign™ v6.1 program of the LASERGENE bioinformatics computing suite (DNASTAR Inc.). Default parameters for multiple alignment (GAP PENALTY=10, GAP LENGTH PENALTY=0.2, Delay Divergen Seqs (%)=30, DNA Transition Weight=0.5, Protein Weight Matrix=Gonnet Series, DNA Weight Matrix=IUB). After alignment of the sequences using the Clustal W program, it is possible to obtain a percent identity by viewing the sequence distances table in the same program.

[0130] It is well understood by one skilled in the art that many levels of sequence identity are useful in identifying polypeptides, from other species, wherein such polypeptides have the same or similar function or activity. Useful examples of percent identities include, but are not limited to: 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95%, or any integer percentage from 55% to 100% is useful in describing the present invention, such as 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99%. Suitable nucleic acid fragments not only have the above homologies but typically encode a polypeptide having at least 50 amino acids, preferably at least 100 amino acids, more preferably at least 150 amino acids, still more preferably at least 200 amino acids, and most preferably at least 250 amino acids.

[0131] The term "sequence analysis software" refers to any computer algorithm or software program that is useful for the analysis of nucleotide or amino acid sequences. Sequence analysis software can be commercially available or independently developed. Typical sequence analysis software will include, but is not limited to: (1) the GCG suite of programs (Wisconsin Package Version 9.0, Genetics Computer Group (GCG), Madison, Wis.);

(2) BLASTP, BLASTN, BLASTX (Altschul, et al, J. Mol. Biol. 215:403-410, 1990);

(3) DNASTAR (DNASTAR, Inc. Madison, Wis.); (4) Sequencher (Gene Codes Corporation, Ann Arbor, Mich.); and (5) the FASTA program incorporating the Smith-Waterman algorithm (W. R. Pearson, Comput. Methods Genome Res., [Proc. Int. Symp.] (1994), Meeting Date 1992, 11 1-20. Editor(s): Suhai, Sandor. Plenum: New York, N.Y.). Within the context of this application, it will be understood that where sequence analysis software is used for analysis, that the results of the analysis will be based on the "default values" of the program referenced, unless otherwise specified. As used herein, "default values" means any set of values or parameters that originally load with the software when first initialized.

[0132] Standard recombinant DNA and molecular cloning techniques used herein are well known in the art and are described by Sambrook, J., Fritsch, E. F. and Maniatis, T., Molecular Cloning: A Laboratory Manual, Second Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (1989) (hereinafter "Maniatis"); and by Silhavy, T. J., Bennan, M. L. and Enquist, L. W., Experiments with Gene Fusions, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (1984); and by Ausubel, F. M. et al, Current Protocols in Molecular Biology, published by Greene Publishing Assoc. and Wiley- Interscience (1987). Additional methods used herein are in Methods in Enzymology, Volume 194, Guide to Yeast Genetics and Molecular and Cell Biology (Part A, 2004, Christine Guthrie and Gerald R. Fink (Eds.), Elsevier Academic Press, San Diego, Calif).

[0133] "Fermentation medium" as used herein means a mixture of water, fermentable carbon substrates, dissolved solids, fermentation product and all other constituents of the material held in the fermentation vessel in which the fermentation product is being made by the reaction of fermentable carbon substrates to fermentation products, water and carbon dioxide (CO ₂ ) by the microorganisms present. From time to time, as used herein, the term "fermentation broth" and "fermentation mixture" can be used synonymously with "fermentation medium."

[0134] The term "aerobic conditions" as used herein means conditions in the presence of oxygen.

[0135] The term "oxygen limiting conditions" or "microaerobic conditions" as used herein means conditions with low levels of dissolved oxygen. For example, the oxygen level may be less than about 1% of air saturation.

[0136] The term "anaerobic conditions" as used herein means conditions in the absence of oxygen. It will be understood that in many fermentation processes, an initial amount of oxygen is present at the onset of the process, but such oxygen is depleted over the course of the fermentation such that the majority of the process takes place in the absence of detectable oxygen.

[0137] As used herein, the term "yield" refers to the amount of product in grams per amount of carbon source in grams (g/g). The yield can be exemplified, for example, for glucose as the carbon source. It is understood, unless otherwise noted, that yield is expressed as a percentage of the theoretical yield. In reference to a microorganism or metabolic pathway, "theoretical yield" is defined as the maximum amount of product that can be generated per total amount of substrate as dictated by the stoichiometry of the metabolic pathway used to make the product. For example, the theoretical yield for one typical conversion of glucose to isopropanol is 0.33 g/g. As such, a yield of isopropanol from glucose of 0.297 g/g would be expressed as 90% of theoretical or 90% theoretical yield. It is understood that while in the present disclosure the yield is exemplified for glucose as a carbon source, the invention can be applied to other carbon sources and the yield can vary depending on the carbon source used. One skilled in the art can calculate yields on various carbon sources.

[0138] The term "titer" as used herein refers to the total amount of butanol isomer produced by fermentation per liter of fermentation medium. The total amount of butanol isomer includes: (i) the amount of butanol in the fermentation medium; (ii) the amount of butanol isomer recovered from the organic extractant; and (iii) the amount of butanol isomer recovered from the gas phase, if gas stripping is used. DHAD Variants

[0139] As described herein, dihydroxy-acid dehydratase (DHAD), also called acetohydroxy acid dehydratase, catalyzes the conversion of 2,3-dihydroxyisovalerate to a-ketoisovalerate and of 2,3-dihydroxymethylvalerate to a-ketomethylvalerate. The DHAD enzyme is part of naturally occurring biosynthetic pathways producing valine, isoleucine, leucine and pantothenic acid (vitamin B5). DHAD catalyzed conversion of 2,3-dihydroxyisovalerate to a-ketoisovalerate is also a step in the multiple isobutanol biosynthetic pathways that are disclosed in commonly owned U.S. Patent No. 7,851,188 (incorporated herein by reference). For production of compounds synthesized in pathways including DHAD, it is desirable to express a heterologous DHAD enzyme that provides DHAD enzymatic activity in a host cell. A consideration for functional expression of dihydroxy-acid dehydratases in a heterologous host is the enzyme's requirement for an iron-sulfur (Fe-S) cluster, which involves availability and proper loading of the cluster into the DHAD apo-protein.

[0140] The present invention is based, in part, on the discovery that certain variants of DHAD have DHAD activity, and, in some embodiments, improved performance compared to the parental DHAD molecule. DHAD variants are desirable for production of products produced by DHAD containing biosynthetic pathways, particularly isobutanol.

[0141] The present invention includes DHAD variants comprising amino acid substitutions that result in improved DHAD activity as indicated by increased isobutanol production. For the purposes of the present invention, amino acid substitutions were made in the Streptococcus mutans DHAD enzyme (SEQ ID NO: 168), however, equivalent substitutions can be made in the homologous regions of DHAD enzymes from other organisms. A list of other DHAD enzymes that can be used to produce the DHAD variants of the invention is included below in Tables 3-5. Amino acids are described herein using either the full name of the amino acid or the 1 -letter or 3 -letter abbreviation of the amino acid, as indicated in Table 2. Table 2: Amino Acids and their Abbreviations

[0142] The amino acid changes that were made and/or contemplated by the present invention to produce alternative, active DHAD enzymes are described herein, for example, by a three character code that begins with the 1 -letter abbreviation of the native amino acid, followed by the amino acid position number, and followed by the 1 -letter abbreviation of the identity of the substituted amino acid. For example, "K564E" refers to a lysine to glutamic acid substitution of position 564 of the DHAD.

[0143] One aspect of the invention is directed to an isolated polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof comprises one or more amino acid substitutions (e.g., as compared to the native sequence other specifically identified sequence). In some embodiments, the isolated polypeptide or fragment thereof comprises one or more amino acid substitutions at an amino acid position selected from: (a) an amino acid corresponding to position 33 of Streptococcus mutans DHAD; (b) an amino acid corresponding to position 62 of Streptococcus mutans DHAD; (c) an amino acid corresponding to position 1 15 oi Streptococcus mutans DHAD; (d) an amino acid corresponding to position 1 16 oi Streptococcus mutans DHAD; (e) an amino acid corresponding to position 1 19 oi Streptococcus mutans DHAD; (f) an amino acid corresponding to position 158 of Streptococcus mutans DHAD; (g) an amino acid corresponding to position 176 of Streptococcus mutans DHAD; (h) an amino acid corresponding to position 179 of Streptococcus mutans DHAD; (i) an amino acid corresponding to position 322 of Streptococcus mutans DHAD; (j) an amino acid corresponding to position 425 of Streptococcus mutans DHAD; (k) an amino acid corresponding to position 524 of Streptococcus mutans DHAD; (1) an amino acid corresponding to position 562 of Streptococcus mutans DHAD; (m) an amino acid corresponding to position 563 of Streptococcus mutans DHAD; (n) an amino acid corresponding to position 564 of Streptococcus mutans DHAD; and (o) an amino acid corresponding to position 567 oi Streptococcus mutans DHAD.

4] In some embodiments, the invention is directed to an isolated polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof comprises one or more amino acid substitutions selected from: (a) aspartic acid or glutamic acid at a position corresponding to position 33 of Streptococcus mutans DHAD; (b) aspartic acid or glutamic acid at a position corresponding to position 62 of Streptococcus mutans DHAD; (c) glycine, alanine, valine, leucine, isoleucine, or proline at a position corresponding to position 1 15 of Streptococcus mutans DHAD; (d) aspartic acid or glutamic acid at a position corresponding to position 1 16 oi Streptococcus mutans DHAD; (e) serine, threonine, cysteine, methionine, asparagine, or glutamine at a position corresponding to position 119 oi Streptococcus mutans DHAD; (f) arginine, lysine, or histidine at a position corresponding to position 158 of Streptococcus mutans DHAD; (g) serine, threonine, cysteine, methionine, asparagine, or glutamine at a position corresponding to position 176 oi Streptococcus mutans DHAD; (h) glycine, alanine, valine, leucine, isoleucine, or proline at a position corresponding to position 179 of Streptococcus mutans DHAD; (i) arginine, lysine, or histidine at a position corresponding to position 322 oi Streptococcus mutans DHAD; (j) serine, threonine, cysteine, methionine, asparagine, or glutamine at a position corresponding to position 425 of Streptococcus mutans DHAD; (k) glycine, alanine, valine, leucine, isoleucine, or proline at a position corresponding to position 524 of Streptococcus mutans DHAD; (1) glycine, alanine, valine, leucine, isoleucine, or proline at a position corresponding to position 562 of Streptococcus mutans DHAD; (m) arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at a position corresponding to position 563 of Streptococcus mutans DHAD; (n) aspartic acid or glutamic acid at a position corresponding to position 564 of Streptococcus mutans DHAD; and (o) aspartic acid or glutamic acid at a position corresponding to position 567 of Streptococcus mutans DHAD.

[0145] In another embodiment, the invention is directed to an isolated polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof comprises one or more amino acid substitutions selected from: (a) aspartic acid at a position corresponding to position 33 of Streptococcus mutans DHAD; (b) glutamic acid at a position corresponding to position 62 of Streptococcus mutans DHAD; (c) valine at a position corresponding to position 1 15 of Streptococcus mutans DHAD; (d) glutamic acid at a position corresponding to position 1 16 of Streptococcus mutans DHAD; (e) serine at a position corresponding to position 119 of Streptococcus mutans DHAD; (f) arginine at a position corresponding to position 158 of Streptococcus mutans DHAD; (g) glutamine at a position corresponding to position 176 of Streptococcus mutans DHAD; (h) leucine at a position corresponding to position 179 of Streptococcus mutans DHAD; (i) arginine at a position corresponding to position 322 of Streptococcus mutans DHAD; (j) serine at a position corresponding to position 425 of Streptococcus mutans DHAD; (k) glycine at a position corresponding to position 524 of Streptococcus mutans DHAD; (1) valine or leucine at a position corresponding to position 562 of Streptococcus mutans DHAD; (m) arginine, cysteine, or glycine at a position corresponding to position 563 of Streptococcus mutans DHAD; (n) glutamic acid at a position corresponding to position 564 of Streptococcus mutans DHAD; and aspartic acid at a position corresponding to position 567 of Streptococcus mutans DHAD.

[0146] In an embodiment of the invention, the polypeptide or fragment thereof comprises a substitution of glutamic acid at a position corresponding to position 564 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of glutamic acid at a position corresponding to position 62 of Streptococcus mutans DHAD, and a substitution of valine at a position corresponding to position 562 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of aspartic acid at a position corresponding to position 33 of Streptococcus mutans DHAD, and a substitution of arginine at a position corresponding to position 563 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of valine at a position corresponding to position 562 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of arginine at a position corresponding to position 563 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of cysteine at a position corresponding to position 563 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of glycine at a position corresponding to position 563 of Streptococcus mutans DHAD. In yet another embodiment, the polypeptide or fragment thereof comprises a substitution of glycine at a position corresponding to position 524 of Streptococcus mutans DHAD, and a substitution of glycine at a position corresponding to position 563 of Streptococcus mutans DHAD.

7] In an embodiment of the invention, the polypeptide or fragment thereof comprises a substitution of valine at a position corresponding to position 115 of Streptococcus mutans DHAD, a substitution of arginine at a position corresponding to position 158 of Streptococcus mutans DHAD, and a substitution of aspartic acid at a position corresponding to position 567 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of glutamic acid at a position corresponding to position 116 of Streptococcus mutans DHAD, and a substitution of serine at a position corresponding to position 119 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of aspartic acid at a position corresponding to position 33 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of glutamic acid at a position corresponding to position 62 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of leucine at a position corresponding to position 562 of Streptococcus mutans DHAD. In another embodiment, the polypeptide or fragment thereof comprises a substitution of glutamine at a position corresponding to position 176 of Streptococcus mutans DHAD, a substitution of leucine at a position corresponding to position 179 of Streptococcus mutans DHAD, a substitution of arginine at a position corresponding to position 322 of Streptococcus mutans DHAD, and a substitution of arginine at a position corresponding to position 563 of Streptococcus mutans DHAD. In yet another embodiment, the polypeptide or fragment thereof comprise a substitution of serine at a position corresponding to position 425 of Streptococcus mutans DHAD, and a substitution of arginine at a position corresponding to position 563 of Streptococcus mutans DHAD.

8] The invention is also directed to an isolated polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof comprises one or more amino acid substitutions selected from: (a) glycine to aspartic acid or glutamic acid at a position corresponding to position 33 of Streptococcus mutans DHAD; (b) aspartic acid to glutamic acid at a position corresponding to position 62 of Streptococcus mutans DHAD; (c) methionine to valine, glycine, alanine, leucine, isoleucine, or proline at a position corresponding to position 115 of Streptococcus mutans DHAD; (d) glycine to glutamic acid or aspartic acid at a position corresponding to position 1 16 of Streptococcus mutans DHAD; (e) asparagine to serine, threonine, cysteine, methionine, asparagine, or glutamine at a position corresponding to position 1 19 of Streptococcus mutans DHAD; (f) glycine to arginine, histidine, or lysine at a position corresponding to position 158 of Streptococcus mutans DHAD; (g) histidine to glutamine, asparagine, methionine, cysteine, threonine, or serine at a position corresponding to position 176 of Streptococcus mutans DHAD; (h) histidine to leucine, isoleucine, proline, glycine, alanine, or valine at a position corresponding to position 179 of Streptococcus mutans DHAD; (i) glutamine to arginine, histidine, or lysine at a position corresponding to position 322 of Streptococcus mutans DHAD; (j) alanine to serine, threonine, cysteine, methionine, asparagine, or glutamine at a position corresponding to position 425 of Streptococcus mutans DHAD; (k) glutamic acid to glycine, alanine, valine, leucine, isoleucine, or proline at a position corresponding to position 524 of Streptococcus mutans DHAD; (1) phenylalanine to glycine, alanine, valine, leucine, isoleucine, or proline at a position corresponding to position 562 of Streptococcus mutans DHAD; (m) tryptophan to arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at a position corresponding to position 563 of Streptococcus mutans DHAD; (n) lysine to glutamic acid or aspartic acid at a position corresponding to position 564 of Streptococcus mutans DHAD; and (o) glutamic acid to aspartic acid at a position corresponding to position 567 of Streptococcus mutans DHAD.

[0149] The invention is also directed to an isolated polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof comprises one or more amino acid substitutions selected from: (a) glycine to aspartic acid at a position corresponding to position 33 oi Streptococcus mutans DHAD; (b) aspartic acid to glutamic acid at a position corresponding to position 62 of Streptococcus mutans DHAD; (c) methionine to valine at a position corresponding to position 115 oi Streptococcus mutans DHAD; (d) glycine to glutamic acid at a position corresponding to position 116 of Streptococcus mutans DHAD; (e) asparagine to serine at a position corresponding to position 119 oi Streptococcus mutans DHAD; (f) glycine to arginine at a position corresponding to position 158 of Streptococcus mutans DHAD; (g) histidine to glutamine at a position corresponding to position 176 oi Streptococcus mutans DHAD; (h) histidine to leucine at a position corresponding to position 179 of Streptococcus mutans DHAD; (i) glutamine to arginine at a position corresponding to position 322 of Streptococcus mutans DHAD; (j) alanine to serine at a position corresponding to position 425 of Streptococcus mutans DHAD; (k) glutamic acid to glycine at a position corresponding to position 524 of Streptococcus mutans DHAD; (1) phenylalanine to valine or leucine at a position corresponding to position 562 of Streptococcus mutans DHAD; (m) tryptophan to arginine, cysteine, or glycine at a position corresponding to position 563 of Streptococcus mutans DHAD; (n) lysine to glutamic acid at a position corresponding to position 564 of Streptococcus mutans DHAD; and (o) glutamic acid to aspartic acid at a position corresponding to position 567 oi Streptococcus mutans DHAD.

[0150] In another aspect, the invention is directed to an isolated polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof comprises one or more amino acid substitutions selected from: (a) glycine to aspartic acid or glutamic acid at position 33 of Streptococcus mutans DHAD; (b) aspartic acid to glutamic acid at position 62 of Streptococcus mutans DHAD; (c) methionine to valine, glycine, alanine, leucine, isoleucine, or proline at position 115 of Streptococcus mutans DHAD; (d) glycine to glutamic acid or aspartic acid at position 116 oi Streptococcus mutans DHAD; (e) asparagine to serine, threonine, cysteine, methionine, asparagine, or glutamine at position 119 oi Streptococcus mutans DHAD; (f) glycine to arginine, histidine, or lysine at position 158 of Streptococcus mutans DHAD; (g) histidine to glutamine, asparagine, methionine, cysteine, threonine, or serine at position 176 of Streptococcus mutans DHAD; (h) histidine to leucine, isoleucine, proline, glycine, alanine, or valine at position 179 of Streptococcus mutans DHAD; (i) glutamine to arginine, histidine, or lysine at position 322 of Streptococcus mutans DHAD; (j) alanine to serine, threonine, cysteine, methionine, asparagine, or glutamine at position 425 of Streptococcus mutans DHAD; (k) glutamic acid to alanine, valine, leucine, isoleucine, proline, or glycine at position 524 of Streptococcus mutans DHAD; (1) phenylalanine to alanine, valine, leucine, isoleucine, proline, or glycine at position 562 of Streptococcus mutans DHAD; (m) tryptophan to arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563 of Streptococcus mutans DHAD; (n) lysine to glutamic acid or aspartic acid at position 564 of Streptococcus mutans DHAD; and (o) glutamic acid to aspartic acid at position 567 of Streptococcus mutans DHAD.

[0151] In another aspect, the invention is directed to an isolated polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof comprises one or more amino acid substitutions selected from: (a) glycine to aspartic acid at position 33 of Streptococcus mutans DHAD; (b) aspartic acid to glutamic acid at position 62 of Streptococcus mutans DHAD; (c) methionine to valine at position 1 15 of Streptococcus mutans DHAD; (d) glycine to glutamic acid at position 116 of Streptococcus mutans DHAD; (e) asparagine to serine at position 119 of Streptococcus mutans DHAD; (f) glycine to arginine at position 158 of Streptococcus mutans DHAD; (g) histidine to glutamine at position 176 of Streptococcus mutans DHAD; (h) histidine to leucine at position 179 of Streptococcus mutans DHAD; (i) glutamine to arginine at position 322 of Streptococcus mutans DHAD; (j) alanine to serine at position 425 of Streptococcus mutans DHAD; (k) glutamic acid to glycine at position 524 of Streptococcus mutans DHAD; (1) phenylalanine to valine or leucine at position 562 of Streptococcus mutans DHAD; (m) tryptophan to arginine, cysteine, or glycine at position 563 of Streptococcus mutans DHAD; (n) lysine to glutamic acid at position 564 of Streptococcus mutans DHAD; and (o) glutamic acid to aspartic acid at position 567 of Streptococcus mutans DHAD.

[0152] The amino acid substitutions described herein can be made in any polypeptide or fragment thereof having DHAD activity at any corresponding position in the sequence. Exemplary DHAD enzymes that can be substituted are listed in Tables 3-5, below. Sequence alignment software can be used to identify the amino acids in the DHAD enzyme of interest that corresponds to a recited amino acid in the Streptococcus mutans DHAD sequence (SEQ ID NO: 168). In some embodiments, the isolated polypeptide or fragment thereof having DHAD activity is a [2Fe-2S] ²⁺ DHAD. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity is a [4Fe-4S] ²⁺ DHAD. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity catalyzes the conversion of 2,3-dihydroxyisovalerate to a-ketoisovalerate or catalyzes the conversion of 2,3- dihydroxymethylvalerate to a-ketomethylvalerate.

[0099] In certain embodiments of the invention, the amino acid substitutions described herein can be made in an isolated polypeptide or fragment thereof having DHAD activity and having an amino acid sequence that matches the Profile Hidden Markov Model (HMM) of Table 6 with an E value of < 10 ^"5 . Table 6 is a table of the Profile Hidden Markov Model (HMM) for dihydroxy-acid dehydratases based on enzymes with assayed function. Table 6 may be found on pages 108-155.

[0153] In another embodiment, the isolated polypeptide or fragment thereof having DHAD activity comprises three conserved cysteines corresponding to positions 56, 129, and 201 of Streptococcus mutans DHAD.

[0154] Amino acid substitutions can be made in polypeptides or fragment thereof having DHAD activity from prokaryotic organisms or eukaryotic organisms. In certain embodiments, the polypeptide or fragment thereof having DHAD activity is from bacteria, fungi, or plant. In a particular embodiment, the polypeptide or fragment thereof having DHAD activity is from Streptococcus mutans.

[0155] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:528 and has a glutamic acid or aspartic acid at position 564. In certain embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:528 and has a glutamic acid or aspartic acid at position 564. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:528.

[0156] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:532 and has a glutamic acid at position 62 and a glycine, alanine, valine, leucine, isoleucine, or proline at position 562. In certain embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:532 and has a glutamic acid at position 62 and a glycine, alanine, valine, leucine, isoleucine, or proline at position 562. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:532.

[0157] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:534 and has an aspartic acid at position 33 and an arginine at position 563. In certain embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:534 and has an aspartic acid or glutamic acid at position 33 and an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:534.

[0158] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:537 and has a glycine, alanine, valine, leucine, isoleucine, or proline at position 562. In certain embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:537 and has a glycine, alanine, valine, leucine, isoleucine, or proline at position 562. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:537.

[0159] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:540 and has an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In certain embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:540 and has an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:540.

[0160] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:545 and has a arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In certain embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:545 and has a arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:545.

[0161] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:572 and has an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In certain embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:572 and has an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:572.

[0162] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:548 and has a glycine, alanine, valine, leucine, isoleucine, or proline at position 524 and an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In certain embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:548 and has a glycine, alanine, valine, leucine, isoleucine, or proline at position 524 and an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:548.

[0163] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO: 552 and has a valine, alanine, glycine, leucine, isoleucine, or proline at position 115, an arginine, lysine, or histidine at position 158, and an aspartic acid or glutamic acid at position 567. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:552 and has a valine, alanine, glycine, leucine, isoleucine, or proline at position 115, an arginine, lysine, or histidine at position 158, and an aspartic acid or glutamic acid at position 567. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:552.

[0164] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:555 and has a glutamic acid or aspartic acid at position 1 16 and a serine, threonine, cysteine, methionine, asparagine, or glutamine at position 1 19. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:555 and has a glutamic acid or aspartic acid at position 1 16 and a serine, threonine, cysteine, methionine, asparagine, or glutamine at position 1 19. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:555.

[0165] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:557 and has an aspartic acid or glutamic acid at position 33. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:557 and has an aspartic acid or glutamic acid at position 33. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:557.

[0166] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO: 561 and has a glutamic acid or aspartic acid at position 62. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:561 and has a glutamic acid or aspartic acid at position 62. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:561.

[0167] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:563 and has a leucine, glycine, alanine, valine, isoleucine, or proline at position

562. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:563 and has a leucine, glycine, alanine, valine, isoleucine, or proline at position 562. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:563.

[0168] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:566 and has a glutamine, asparagine, methionine, serine, threonine, or cysteine at position 176, a leucine, glycine, alanine, valine, isoleucine, or proline at position 179, an arginine, lysine, or histidine at position 322, and an arginine, lysine, or histidine at position

563. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:566 and has a glutamine, asparagine, methionine, serine, threonine, or cysteine at position 176, a leucine, glycine, alanine, valine, isoleucine, or proline at position 179, an arginine, lysine, or histidine at position 322, and an arginine, lysine, or histidine at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:566. [0169] In certain embodiments of the invention, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:569 and has a glutamine, asparagine, methionine, serine, threonine, or cysteine at position 425 and an arginine, lysine, or histidine at position 563. In other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:569 and has a glutamine, asparagine, methionine, serine, threonine, or cysteine at position 425 and an arginine, lysine, or histidine at position 563. In yet other embodiments, the isolated polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:569.

[0170] The isolated polypeptides or fragments thereof having DHAD activity of the invention display increased DHAD activity compared to DHAD proteins without the amino acid substitutions. The phrase "increased activity" refers to any alteration in the protein that results in improved growth of a strain relative to a control strain expressing the parental DHAD enzyme, or an improved yield of a product made from a DHAD requiring pathway, such as isobutanol. "Increased activity" can result from a number of alterations in the function of the polypeptide or fragment thereof having DHAD activity including, but not limited to: improved stability of the protein, faster catalytic activity, increased binding to substrate, increased stability of the mRNA leading to more translation into protein, more efficient translation of the mRNA, or improved binding to a [4Fe-4S] ²⁺ cluster or a [2Fe- 2S] ²⁺ cluster. In some embodiments, DHAD variant proteins expressed in yeast cytosol have a specific activity of greater than about 0.10 units/mg, greater than about 0.15 units/mg, greater than about 0.20 units/mg, greater than about 0.25 units/mg, greater than about 0.30 units/mg, greater than about 0.35 units/mg, or greater than about 0.40 units/mg. In some embodiments, DHAD variant proteins expressed in yeast cytosol have a specific activity of about 0.10 units/mg to about 0.40 units/mg, or any range of values therein, for example, about 0.10 units/mg to about 0.35 units/mg, about 0.10 units/mg to about 0.30 units/mg, about 0.10 units/mg to about 0.25 units/mg, about 0.10 units/mg to about 0.20 units/mg, about 0.10 units/mg to about 0.15 units/mg.

[0171] The invention is also directed to isolated polynucleotide molecules comprising a nucleic acid sequence that encodes the DHAD variant polypeptides described herein.

[0172] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:528 and has a glutamic acid or aspartic acid at position 564. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:528 and has a glutamic acid or aspartic acid at position 564. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:528. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:527 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:527. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:529 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:529. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:530 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:530.

3] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:532 and has a glutamic acid at position 62 and a glycine, alanine, valine, leucine, isoleucine, or proline at position 562. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:532 and has a glutamic acid at position 62 and a glycine, alanine, valine, leucine, isoleucine or proline at position 562. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:532. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:531 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:531.

4] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:534 and has an aspartic acid or glutamic acid at position 33 and an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:534 and has an aspartic acid or glutamic acid at position 33 and an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:534. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:533 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:533. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:535 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:535.

[0175] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:537 and has a glycine, alanine, valine, leucine, isoleucine, or proline at position 562. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:537 and has a glycine, alanine, valine, leucine, isoleucine, or proline at position 562. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:537. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:536 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:536. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:538 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:538.

[0176] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:540 and has an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:540 and has an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:540. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:539 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:539. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO: 606 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:606. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:541 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:541. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:542 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:542. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:543 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:543.

[0177] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:545 and has an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:545 and has an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:545. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:544 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:544. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO: 546 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO: 546.

[0178] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:572 and has an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:572 and has an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:572. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:571 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:571.

9] In certain embodiment of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:548 and has a glycine, alanine, valine, leucine, isoleucine, or proline at position 524 and an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:548 and has a glycine, alanine, valine, leucine, isoleucine, or proline at position 524 and an arginine, lysine, histidine, cysteine, serine, threonine, methionine, asparagine, glutamine, glycine, alanine, valine, leucine, isoleucine, or proline at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:548. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO: 547 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:547. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:549 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:549. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:550 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:550.

0] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:552 and has a valine, alanine, glycine, leucine, isoleucine, or proline at position 115, an arginine, lysine, or histidine at position 158, and an aspartic acid or glutamic acid at position 567. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:552 and has a valine, alanine, glycine, leucine, isoleucine, or proline at position 115, an arginine, lysine, or histidine at position 158, and an aspartic acid or glutamic acid at position 567. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:552. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:551 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:551. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:553 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:553.

[0181] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:555 and has a glutamic acid or aspartic acid at position 116 and a serine, threonine, cysteine, methionine, asparagine, or glutamine at position 119. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:555 and has a glutamic acid or aspartic acid at position 1 16 and a serine, threonine, cysteine, methionine, asparagine, or glutamine at position 1 19. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:555. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:554 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:554.

[0182] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:557 and has an aspartic acid or glutamic acid at position 33. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:557 and has an aspartic acid or glutamic acid at position 33. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:557. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:556 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:556. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:558 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:558. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:559 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:559.

83] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:561 and has a glutamic acid or aspartic acid at position 62. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:561 and has a glutamic acid or aspartic acid at position 62. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:561. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:560 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:560.

84] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:563 and has a leucine, glycine, alanine, valine, isoleucine, or proline at position 562. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:563 and has a leucine, glycine, alanine, valine, isoleucine, or proline at position 562. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:563. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:562 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:562. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:564 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:564.

[0185] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:566 and has a glutamine, asparagine, methionine, serine, threonine, or cysteine at position 176, a leucine, glycine, alanine, valine, isoleucine, or proline at position 179, an arginine, lysine, or histidine at position 322, and an arginine, lysine, or histidine at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:566 and has a glutamine, asparagine, methionine, serine, threonine, or cysteine at position 176, a leucine, glycine, alanine, valine, isoleucine, or proline at position 179, an arginine, lysine, or histidine at position 322, and an arginine, lysine, or histidine at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO:566. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO: 565 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:565. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:567 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:567.

[0186] In certain embodiments of the invention, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having dihydroxy-acid dehydratase (DHAD) activity, wherein the polypeptide or fragment thereof having DHAD activity comprises an amino acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to SEQ ID NO:569 and has a glutamine, asparagine, methionine, serine, threonine, or cysteine at position 425 and an arginine, lysine, or histidine at position 563. In other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity is at least 95% identical to SEQ ID NO:569 and has a glutamine, asparagine, methionine, serine, threonine, or cysteine at position 425 and an arginine, lysine, or histidine at position 563. In yet other embodiments, the isolated polynucleotide molecule comprises a nucleotide sequence that encodes a polypeptide or fragment thereof having DHAD activity, wherein the polypeptide or fragment thereof having DHAD activity comprises the amino acid sequence of SEQ ID NO: 569. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:568 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:568. In certain embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:570 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:570.

7] In other embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:573 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:573. In another embodiments, the polynucleotide sequence comprises a nucleic acid sequence that is at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of SEQ ID NO:574 and encodes a polypeptide or fragment thereof having DHAD activity. In a specific embodiment, the polynucleotide sequence comprises the nucleic acid sequence of SEQ ID NO:574. DHAD Proteins

8] Any DHAD protein can be used as a parental, or starting, molecule for creating a DHAD variant polypeptide of the invention. DHADs that can be used herein can be derived from bacterial, fungal, or plant sources. DHADs that can be used can have a [4Fe-4S] ²⁺ cluster or a [2Fe-2S] ²⁺ cluster bound by the apoprotein. Tables 3-5 list SEQ ID NOs for coding regions and proteins of representative DHADs that can be used in the present invention. Proteins with at least about 95% identity to those listed sequences have been omitted for simplification, but it is understood that omitted proteins with at least about 95% sequence identity to any of the proteins listed in Tables 3-5 and having DHAD activity can be used as disclosed herein. Additional DHAD proteins and their encoding sequences can be identified by BLAST searching of public databases, as well known to one skilled in the art. Typically BLAST (described herein) searching of publicly available databases with known DHAD sequences, such as those provided herein, is used to identify DHADs and their encoding sequences that can be expressed in the present cells. For example, DHAD proteins having amino acid sequence identities of at least about 80-85%, at least about 85-90%, at least about 90-95%, or at least about 98% sequence identity to any of the DHAD proteins disclosed herein can be expressed in the present cells. Identities are based on the Clustal W method of alignment using the default parameters of GAP PENALTY=10, GAP LENGTH PENALTY=0.1, and Gonnet 250 series of protein weight matrix.

Table 3 : SEQ ID NOs of Representative Bacterial [2Fe-2S] ²⁺ DHAD Proteins

and Encodin Se uences

Arthrobacter aurescens TC 1 29 30

Leifsonia xyli subsp. xyli str. CTCB07 31 32 marine actinobacterium PHSC20C1 33 34

Clavibacter michiganensis subsp. michiganensis

35 36 NCPPB 382

Saccharopolyspora erythraea NRRL 2338 37 38

Acidothermus cellulolyticus 11B 39 40

Corynebacterium efficiens YS-314 41 42

Brevibacterium linens BL2 43 44

Tropheryma whipplei TW08/27 45 46

Methylobacterium extorquens PA1 47 48

Methylobacterium nodulans ORS 2060 49 50

Rhodopseudomonas palustris BisB5 51 52

Rhodopseudomonas palustris BisB18 53 54

Bradyrhizobium sp. ORS278 55 56

Bradyrhizobium japonicum USDA 110 57 58

Fulvimarina pelagi HTCC2506 59 60

Aurantimonas sp. SI85-9A1 61 62

Hoeflea phototrophica DFL-43 63 64

Mesorhizobium loti MAFF303099 65 66

Mesorhizobium sp. BNC1 67 68

Parvibaculum lavamentivorans DS-1 69 70

Loktanella vestfoldensis SKA53 71 72

Roseobacter sp. CCS2 73 74

Dinoroseobacter shibae DFL 12 75 76

Roseovarius nubinhibens ISM 77 78

Sagittula stellata E-37 79 80

Roseobacter sp. AzwK-3b 81 82

Roseovarius sp. TM1035 83 84

Oceanicola batsensis HTCC2597 85 86

Oceanicola granulosus HTCC2516 87 88

Rhodobacterales bacterium HTCC2150 89 90

Paracoccus denitrificans PD1222 91 92

Oceanibulbus indolifex HEL-45 93 94

Sulfltobacter sp. EE-36 95 96

Roseobacter denitrificans OCh 114 97 98

Jannaschia sp. CCS1 99 100

Caulobacter sp. K31 101 102

Candidatus Pelagibacter ubique HTCC1062 103 104

Erythrobacter litoralis HTCC2594 105 106

Erythrobacter sp. NAP1 107 108

Comamonas testosterone KF- 1 109 110

Sphingomonas wittichii RW1 111 112

Burkholderia xenovorans LB400 113 114

Burkholderia phytoflrmans PsJN 115 116

Bordetella petrii DSM 12804 117 118

Bordetella bronchiseptica RB50 119 120 Bradyrhizobium sp. ORS278 121 122

Bradyrhizobium sp. BTAil 123 124

Brady rh izobium japon icum 125 126

Sphingomonas wittichii RW1 127 128

Rhodobacterales bacterium HTCC2654 129 130

Solibacter usitatus Ellin6076 131 132

Roseiflexus sp. RS- 1 133 134

Rubrobacter xylanophilus DSM 9941 135 136

Salinispora tropica CNB-440 137 138

Acidobacteria bacterium Ellin345 139 140

Thermus thermophilus HB27 141 142

Maricaulis maris MCS10 143 144

Parvularcula bermudensis HTCC2503 145 146

Oceanicaulis alexandrii HTCC2633 147 148

Plesiocystis pacifica SIR-1 149 150

Bacillus sp. NRRL B- 14911 151 152

Oceanobacillus iheyensis HTE831 153 154

Staphylococcus saprophyticus subsp. saprophyticus

155 156 ATCC 15305

Bacillus selenitireducens MLS 10 157 158

Streptococcus pneumoniae SP6-BS73 159 160

Streptococcus sanguinis SK36 161 162

Streptococcus thermophilus LMG 18311 163 164

Streptococcus suis 89/1591 165 166

Streptococcus mutans UA159 167 168

Leptospira borgpetersenii serovar Hardjo-bovis

169 170 L550

Candidatus Vesicomyosocius okutanii HA 171 172

Candidatus Ruthia magnifica str. Cm (Calyptogena

173 174 magnifica)

Methylococcus capsulatus str. Bath 175 176 uncultured marine bacterium EB80 02D08 177 178 uncultured marine gamma proteobacterium

179 180 EBAC31A08

uncultured marine gamma proteobacterium

181 182 EBAC20E09

uncultured gamma proteobacterium eBACHOT4E07 183 184

Alcanivorax borkumensis SK2 185 186

Chromohalobacter salexigens DSM 3043 187 188

Marinobacter algicola DG893 189 190

Marinobacter aquaeolei VT8 191 192

Marinobacter sp. ELB17 193 194

Pseudoalteromonas haloplanktis TAC125 195 196

Acinetobacter sp. ADP1 197 198

Opitutaceae bacterium TAV2 199 200

Flavobacterium sp. MED217 201 202

Cellulophaga sp. MED 134 203 204 Kordia algicida OT- 1 205 206

Flavobacteriales bacterium ALC- 1 207 208

Psychroflexus torquis ATCC 700755 209 210

Flavobacteriales bacterium HTCC2170 211 212 unidentified eubacterium SCB49 213 214

Gramella forsetii KT0803 215 216

Robiginitalea biformata HTCC2501 217 218

Tenacibaculum sp. MED 152 219 220

Polaribacter irgensii 23 -P 221 222

Pedobacter sp. BAL39 223 224

Flavobacteria bacterium BAL38 225 226

Flavobacterium psychrophilum JIP02/86 227 228

Flavobacteriumjohnsoniae UW101 229 230

Lactococcus lactis subsp. cremoris SKI 1 231 232

Psychromonas ingrahamii 37 233 234

Microscilla marina ATCC 23134 235 236

Cytophaga hutchinsonii ATCC 33406 237 238

Rhodopirellula baltica SH 1 239 240

Blastopirellula marina DSM 3645 241 242

Planctomyces maris DSM 8797 243 244

Algoriphagus sp. PR1 245 246

Candidatus Sulcia muelleri str. He (Homalodisca

247 248 coagulata)

Candidatus Carsonella ruddii PV 249 250

Synechococcus sp. RS9916 251 252

Synechococcus sp. WH 7803 253 254

Synechococcus sp. CC931 1 255 256

Synechococcus sp. CC9605 257 258

Synechococcus sp. WH 8102 259 260

Synechococcus sp. BL107 261 262

Synechococcus sp. RCC307 263 264

Synechococcus sp. RS9917 265 266

Synechococcus sp. WH 5701 267 268

Prochlorococcus marinus str. MIT 9313 269 270

Prochlorococcus marinus str. NATL2A 271 272

Prochlorococcus marinus str. MIT 9215 273 274

Prochlorococcus marinus str. AS9601 275 276

Prochlorococcus marinus str. MIT 9515 277 278

Prochlorococcus marinus subsp. pastoris str.

279 280 CCMP1986

Prochlorococcus marinus str. MIT 9211 281 282

Prochlorococcus marinus subsp. marinus str.

283 284 CCMP1375

Nodularia spumigena CCY9414 285 286

Nostoc punctiforme PCC 73102 287 288

Nostoc sp. PCC 7120 289 290

Trichodesmium erythraeum IMS 101 291 292 Acaryochloris marina MBIC11017 293 294

Lyngbya sp. PCC 8106 295 296

Synechocystis sp. PCC 6803 297 298

Cyanothece sp. CCY0110 299 300

Thermosynechococcus elongatus BP-1 301 302

Synechococcus sp. JA-2-3B'a(2-13) 303 304

Gloeobacter violaceus PCC 7421 305 306

Nitrosomonas eutropha C91 307 308

Nitrosomonas europaea ATCC 19718 309 310

Nitrosospira multiformis ATCC 25196 311 312

Chloroflexus aggregans DSM 9485 313 314

Leptospirillum sp. Group II UBA 315 316

Leptospirillum sp. Group II UBA 317 318

Halorhodospira haiophiia SL1 319 320

Nitrococcus mobilis Nb-231 321 322

Alkalilimnicola ehrlichei MLHE-1 323 324

Deinococcus geothermaiis DSM 11300 325 326

Polynucleobacter sp. QLW-PlDMWA-1 327 328

Polynucleobacter necessarius STIR1 329 330

Azoarcus sp. Eb l 331 332

Burkholderia phymatum STM815 333 334

Burkholderia xenovorans LB400 335 336

Burkholderia multivorans ATCC 17616 337 338

Burkholderia cenocepacia PC 184 339 340

Burkholderia mallei GB8 horse 4 341 342

Ralstonia eutropha JMP134 343 344

Ralstonia metallidurans CH34 345 346

Ralstonia solanacearum UW551 347 348

Ralstonia pickettii 12J 349 350

Limnobacter sp. MED 105 351 352

Herminiimonas arsenicoxydans 353 354

Bordetella parapertussis 355 356

Bordetella petrii DSM 12804 357 358

Polaromonas sp. JS666 359 360

Polaromonas naphthalenivorans CJ2 361 362

Rhodoferax ferrireducens Tl 18 363 364

Verminephrobacter eiseniae EF01-2 365 366

Acidovorax sp. JS42 367 368

Delftia acidovorans SPH- 1 369 370

Methylibium petroleiphilum PM1 371 372 gamma proteobacterium KT 71 373 374

Tremblaya princeps 375 376

Blastopirellula marina DSM 3645 377 378

Planctomyces maris DSM 8797 379 380

Microcystis aeruginosa PCC 7806 381 382

Salinibacter ruber DSM 13855 383 384

Methylobacterium chloromethanicum 385 386 Table 4: SEQ ID NOs of Representative Fungal and Plant [2Fe-2S] ²⁺ DHAD Proteins

Usti ago maydis 521 493 494

Table 5: SEQ ID NOs of Representative [4Fe-4S] ²⁺ DHAD Proteins

and Encoding Sequences

Organism SEQ ID NO: SEQ ID NO:

Nucleic acid Peptide

Escherichia coli str. K-12 substr. MG1655 495 496

Bacillus subtilis subsp. subtilis str. 168 497 498

Agrobacterium tumefaciens str. C58 499 500

Burkholderia cenocepacia MCO-3 501 502

Psychrobacter cryohalolentis K5 503 504

Psychromonas sp. CNPT3 505 506

Deinococcus radiodurans Rl 507 508

Wolinella succinogenes DSM 1740 509 510

Zymomonas mobilis subsp. mobilis ZM4 511 512

Clostridium acetobutylicum ATCC 824 513 514

Clostridium beijerinckii NCIMB 8052 515 516

Pseudomonas fluorescens Pf-5 517 518

Methanococcus maripaludis C7 519 520

Methanococcus aeolicus Nankai-3 521 522

Vibrio flscheri ATCC 700601 (ESI 14) 523 524

Shewanella oneidensis MR-1 ATCC 700550 525 526 9] Additional [2Fe-2S] ²⁺ DHADs can be identified using the analysis described in copending U.S. Patent Application Publication No. 2010/0081 154, which is herein incorporated by reference. The analysis is as follows: A Profile Hidden Markov Model (HMM) was prepared based on amino acid sequences of eight functionally verified DHADs. These DHADs are from Nitrosomonas europaea (DNA SEQ ID NO:309; protein SEQ ID NO:310), Synechocystis sp. PCC6803 (DNA SEQ ID:297; protein SEQ ID NO:298), Streptococcus mutans (DNA SEQ ID NO: 167; protein SEQ ID NO: 168), Streptococcus thermophilus (DNA SEQ ID NO: 163; protein SEQ ID NO: 164), Ralstonia metallidurans (DNA SEQ ID NO:345; protein SEQ ID NO:346), Ralstonia eutropha (DNA SEQ ID NO:343; protein SEQ ID NO:344), and Lactococcus lactis (DNA SEQ ID NO:231 ; protein SEQ ID NO:232). In addition, the DHAD from Flavobacterium johnsoniae (DNA SEQ ID NO:229; protein SEQ ID NO:230) was found to have DHAD activity when expressed in Escherichia coli and was used in making the Profile.

[0190] The Profile HMM was built as follows:

Step 1. Build a Sequence Alignment

The eight sequences for the functionally verified DHADs listed above were aligned using Clustal W with default parameters.

Step 2. Build a Profile HMM

The hmmbuild program was run on the set of aligned sequences using default parameters. The hmmbuild reads the multiple sequence alignment file, builds a new Profile HMM, and saves the Profile HMM to file. Using this program, an un-calibrated profile was generated from the multiple alignment for each set of subunit sequences described herein.

[0191] The following information based on the HMMER software user guide gives some description of the way that the hmmbuild program prepares a Profile HMM. A Profile HMM is capable of modeling gapped alignments, for example, insertions and deletions, which allows the software to describe a complete conserved domain (rather than just a small ungapped motif). Insertions and deletions are modeled using insertion (I) states and deletion (D) states. All columns that contain more than a certain fraction x of gap characters will be assigned as an insert column. By default, x is set to 0.5. Each match state has an I and a D state associated with it. HMMER calls a group of three states (M/D/I) at the same consensus position in the alignment a "node." These states are interconnected with arrows called state transition probabilities. M and I states are emitters, while D states are silent. The transitions are arranged so that at each node, either the M state is used (and a residue is aligned and scored) or the D state is used (and no residue is aligned, resulting in a deletion-gap character, '-'). Insertions occur between nodes, and I states have a self-transition, allowing one or more inserted residues to occur between consensus columns.

[0192] The scores of residues in a match state (i.e., match state emission scores) or in an insert state (i.e., insert state emission scores) are proportional to Log_2 (p_x)/(null_x). Where p_x is the probability of an amino acid residue, at a particular position in the alignment, according to the Profile HMM and null_x is the probability according to the Null model. The Null model is a simple one state probabilistic model with pre-calculated set of emission probabilities for each of the 20 amino acids derived from the distribution of amino acids in the SWISS-PROT release 24.

[0193] State transition scores are also calculated as log odds parameters and are proportional to Log_2 (t_x). Where t_x is the probability of transiting to an emitter or non-emitter state.

Step 3. Calibrate the Profile HMM

[0194] The Profile HMM was read using hmmcalibrate which scores a large number of synthesized random sequences with the Profile (the default number of synthetic sequences used is 5,000), fits an extreme value distribution (EVD) to the histogram of those scores, and re-saves the HMM file now including the EVD parameters. These EVD parameters (μ and λ) are used to calculate the E-values of bit scores when the profile is searched against a protein sequence database, hmmcalibrate writes two parameters into the HMM file on a line labeled "EVD": these parameters are the μ (location) and λ (scale) parameters of an extreme value distribution (EVD) that best fits a histogram of scores calculated on randomly generated sequences of about the same length and residue composition as SWISS-PROT. This calibration was done once for the Profile HMM.

[0195] The calibrated Profile HMM for the DHAD set of sequences is provided in Table 6 (found on pages 108-155). The Profile HMM is provided in a chart that gives the probability of each amino acid occurring at each position in the amino acid sequence. The highest probability is highlighted for each position. The first line for each position reports the match emission scores: probability for each amino acid to be in that state (highest score is highlighted). The second line reports the insert emission scores, and the third line reports on state transition scores: M→M, M→I, M→D; I→M, I→I; D→M, D→D; B→M; M→E.

[0196] For example, the DHAD Profile HMM shows that methionine has a 1757 probability of being in the first position, the highest probability which is highlighted. In the second position, glutamic acid has the highest probability, which is 1356. In the third position, lysine has the highest probability, which is 1569.

Step 4. Test the Specificity and Sensitivity of the Built Profile HMMs

[0197] The Profile HMM was evaluated using hmmsearch, which reads a Profile HMM from hmmfile and searches a sequence file for significantly similar sequence matches. The sequence file searched contained 976 sequences (see above). During the search, the size of the database (Z parameter) was set to 1 billion. This size setting ensures that significant E- values against the current database will remain significant in the foreseeable future. The E- value cutoff was set at 10.

[0198] A hmmer search with the Profile HMM generated from the alignment of the eight DHADs with experimentally verified function, matched all 976 sequences with an E value <10 ^~5 . This result indicates that members of the dehydratase superfamily share significant sequence similarity. A hmmer search with a cutoff of E value 10 ^~5 was used to separate DHAD related dehydratases from other more remote but related proteins, as described herein.

[0199] The Profile HMM is prepared using the HMMER software package (see Durbin, et al, Biological sequence analysis: probabilistic models of proteins and nucleic acids, Cambridge University Press, 1998; Krogh, et al, J. Mol. Biol. 235: 1501-1531, 1994), following the user guide which is available from HMMER (Janelia Farm Research Campus, Ashburn, VA). The output of the HMMER software program is a Profile Hidden Markov Model (HMM) that characterizes the input sequences. The Profile HMM prepared for the eight DHAD proteins is given in Table 6 (pages 108-155).

[0200] This Profile HMM for DHADs can be used to identify DHAD related proteins. Any protein that matches the Profile HMM with an E value of <10 ^"5 is a DHAD related protein, which includes [4Fe-4S] ²⁺ DHADs, [2Fe-2S] ²⁺ DHADs, aldonic acid dehydratases, and phosphogluconate dehydratases.

[0201] Sequences matching the Profile HMM given herein are then analyzed for the presence of the three conserved cysteines described herein. The exact positions of the three conserved cysteines can vary, and these can be identified in the context of the surrounding sequence using multiple sequence alignments performed with the Clustal W algorithm (Thompson, et al, Nuc. Acid Res. 22: 4673-4680, 1994) employing the following parameters: 1) for pairwise alignment parameters, a Gap opening=10; Gap extend=0.1; matrix is Gonnet 250; and mode— Slow-accurate, 2) for multiple alignment parameters, Gap opening=10; Gap extension=0.2; and matrix is Gonnet series. For example, the three conserved cysteines are located at amino acid positions 56, 129, and 201 in the Streptococcus mutans DHAD (SEQ ID NO: 168), and at amino acid positions 61, 135, and 207 in the Lactococcus lactis DHAD (SEQ ID NO:232). The exact positions of the three conserved cysteines in other protein sequences correspond to these positions in the Streptococcus mutans or the Lactococcus lactis amino acid sequence. One skilled in the art will readily be able to identify the presence or absence of each of the three conserved cysteines in the amino acid sequence of a DHAD protein using pairwise or multiple sequence alignments. In addition, other methods can be used to determine the presence of the three conserved cysteines, such as by visual analysis.

[0202] The DHAD Profile HMM matching proteins that have two, but not the third (position 56) conserved cysteine, include [4Fe-4S] ²⁺ DHADs and phosphogluconate dehydratases (EDDs). Proteins having the three conserved cysteines include arabonate dehydratases and [2Fe-2S] ²⁺ DHADs, and are members of a [2Fe-2S] ²⁺ DHAD/aldonic acid dehydratase group. The [2Fe-2S] ²⁺ DHADs can be distinguished from the aldonic acid dehydratases by analyzing for signature conserved amino acids found to be present in the [2Fe-2S] ²⁺ DHADs or in the aldonic acid dehydratases at positions corresponding to the following positions in the Streptococcus mutans DHAD amino acid sequence. These signature amino acids are in [2Fe- 2S] ²⁺ DHADs or in aldonic acid dehydratases, respectively, at the following positions (with greater than 90% occurrence): 88 asparagine vs. glutamic acid; 113 not conserved vs. glutamic acid; 142 arginine or asparagine vs. not conserved; 165 not conserved vs. glycine; 208 asparagine vs. not conserved; 454 leucine vs. not conserved; 477 phenylalanine or tyrosine vs. not conserved; and 487 glycine vs. not conserved.

[0203] The disclosed methods for identification of [2Fe-2S] ²⁺ DHAD enzymes can be carried out on a single sequence or on a group of sequences. In an embodiment, one or more sequence databases may be queried with a Profile HMM as described herein.

[0204] Additionally, the sequences of DHAD coding regions provided herein can be used to identify other homologs in nature. Such methods are well-known in the art, and various methods that can be used to isolate genes encoding homologous proteins are described in U.S. Patent Application Publication No. 2010/0081 154, which such methods are incorporated by reference herein.

[0205] DHAD variant polypeptides provided herein may be, for example, of a size of about 10 or more, about 20 or more, about 25 or more, about 50 or more, about 75 or more, about 100 or more, about 200 or more, about 500 or more, about 1,000 or more, or about 2,000 or more amino acids. Polypeptides can have a defined three-dimensional structure, although they do not necessarily have such structure. Polypeptides with a defined three-dimensional structure are referred to as folded, and polypeptides which do not possess a defined three- dimensional structure, but rather can adopt a large number of different conformations, and are referred to as unfolded.

[0206] Also provided are active fragments of the DHAD variant polypeptides. A "fragment" is a unique portion of a polypeptide or other enzyme used in the invention which is identical in sequence to but shorter in length than the parent full-length sequence. A fragment can comprise up to the entire length of the defined sequence, minus one amino acid residue. For example, a fragment can comprise from about 5 to about 1,000 contiguous amino acid residues. A fragment can be, for example, at least 5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 100, 150, 250, 500, 750, or 1,000 contiguous amino acid residues in length. Fragments can be preferentially selected from certain regions of a molecule. For example, a polypeptide fragment can comprise a certain length of contiguous amino acids selected from the first 100, 200, 300, 400, or 500 amino acids of a polypeptide as shown in a certain defined sequence. Alternatively, a polypeptide fragment can comprise a certain length of contiguous amino acids selected from the last 100, 200, 300, 400, or 500 amino acids of a polypeptide as shown in a certain defined sequence. Clearly these lengths are exemplary, and any length that is supported by the specification, including the Sequence Listing, tables, and figures, can be encompassed by the present embodiments. In certain embodiments, the DHAD variant polypeptide fragments have DHAD activity, and thus are capable of catalyzing the conversion of 2,3-dihydroxyisovalerate to a-ketoisovalerate.

DHAD Activity Assays

[0207] The presence of DHAD activity in a cell engineered to express a heterologous DHAD can be confirmed using methods known in the art and/or described herein. As one example, crude extracts from cells engineered to express a bacterial DHAD can be used in a DHAD assay as described in the Examples herein or as described by Flint and Emptage (J. Biol. Chem. 263(8): 3558-64, 1988) using dinitrophenylhydrazine. In another example, DHAD activity can be assayed by the methods disclosed in U.S. Patent Application Publication No. 2010/0081 154, incorporated herein by reference, in a yeast strain that lacks endogenous DHAD activity. If DHAD activity is present, the yeast strain will grow in the absence of branched-chain amino acids. DHAD activity can also be confirmed by more indirect methods, such as by assaying for a downstream product in a pathway requiring DHAD activity. Any product that has a-ketoisovalerate or a-ketomethylvalerate as a pathway intermediate can be measured in an assay for DHAD activity. A list of such products includes, but is not limited to, valine, isoleucine, leucine, pantothenic acid, 2-methyl-l- butanol, 3 -methyl- 1-butanol, and isobutanol.

Nucleic Acid Molecules

[0208] Provided herein are isolated nucleic acid molecules that encode the DHAD variant polypeptides described herein. The coding region of the isolated nucleic acid encoding the DHAD variant can be codon optimized for a particular target host cell, as is well known to one skilled in the art. The isolated nucleic acid molecules of the invention can be comprised in a vector. Vectors useful for the transformation of a variety of host cells are common and commercially available from companies such as Epicentre™ (Madison, WI), Invitrogen Corp. (Carlsbad, CA), Stratagene (La Jolla, CA), and New England Biolabs, Inc. (Beverly, MA). Typically, the vector contains a selectable marker and sequences allowing autonomous replication or chromosomal integration in the desired host. In addition, suitable vectors comprise a promoter region which harbors transcriptional initiation controls and a transcriptional termination control region, between which a coding region DNA fragment can be inserted, to provide expression of the inserted coding region. Both control regions can be derived from genes homologous to the transformed host cell, although it is to be understood that such control regions can also be, for example, derived from genes that are not native to the specific species chosen as a host.

[0209] Initiation control regions or promoters, which are useful to drive expression of bacterial DHAD variant coding regions in the desired bacterial host cell, are numerous and familiar to those skilled in the art. Virtually any promoter capable of driving these genetic elements is suitable for the present invention including, but not limited to, lac, ara, tet, trp, IPL, IPR, T7, tac, and trc promoters (useful for expression in Escherichia coli, Alcaligenes, and Pseudomonas); the amy, apr, and npr promoters, and various phage promoters useful for expression in Bacillus subtilis, Bacillus licheniformis, and Paenibacillus macerans; nisA (useful for expression Gram-positive bacteria, Eichenbaum, et al, Appl. Environ. Microbiol. 64(8):2763-2769, 1998); and the synthetic Pl l promoter (useful for expression in Lactobacillus plantarum, Rud, et al, Microbiology 152: 1011-1019, 2006). Termination control regions can also be derived from various genes native to the preferred hosts. Optionally, a termination site can be unnecessary; however, it is most preferred if included.

[0210] Certain vectors are capable of replicating in a broad range of host bacteria and can be transferred by conjugation. The complete and annotated sequence of pRK404 and three related vectors: pRK437, pRK442, and pRK442(H), are available. These derivatives have proven to be valuable tools for genetic manipulation in Gram-negative bacteria (Scott, et al, Plasmid 50(l):74-79, 2003). Several plasmid derivatives of broad-host-range Inc P4 plasmid RSF1010 are also available with promoters that can function in a range of Gram-negative bacteria. Plasmid pAYC36 and pAYC37 have active promoters along with multiple cloning sites to allow for heterologous gene expression in Gram-negative bacteria. Some vectors that are useful for transformation of Bacillus subtilis and Lactobacillus include pAMBl and derivatives thereof (Renault, et al., Gene 183: 175-182, 1996; and O'Sullivan, et al, Gene 137:227-231, 1993); pMBBl and pHW800, a derivative of pMBBl (Wyckoff, et al, Appl. Environ. Microbiol. 62: 1481-1486, 1996); pMGl, a conjugative plasmid (Tanimoto, et al, J. Bacteriol. 184:5800-5804, 2002); pNZ9520 (Kleerebezem, et al, Appl. Environ. Microbiol. 63 :4581-4584, 1997); pAM401 (Fujimoto, et al, Appl. Environ. Microbiol. 67: 1262-1267, 2001); and pAT392 (Arthur, et al, Antimicrob. Agents Chemother. 38: 1899-1903, 1994). Several plasmids from Lactobacillus plantarum have also been reported (van Kranenburg, et al, Appl. Environ. Microbiol. 71(3): 1223-1230, 2005).

[0211] Chromosomal gene replacement tools are also widely available. For example, a thermosensitive variant of the broad-host-range replicon pWVlOl has been modified to construct a plasmid pVE6002 which can be used to effect gene replacement in a range of Gram-positive bacteria (Maguin, et al, J. Bacteriol. 174(17):5633-5638, 1992). Additionally, in vitro transposomes are available from commercial sources such as Epicentre™ to create random mutations in a variety of genomes.

[0212] Vectors suitable for expression and propagation in yeast cells are also well known.

Methods for gene expression in yeast are known in the art (see, e.g., Methods in Enzymology, Volume 194, Guide to Yeast Genetics and Molecular and Cell Biology (Part A, 2004, Christine Guthrie and Gerald R. Fink (Eds.), Elsevier Academic Press, San Diego, Calif). Expression of genes in yeast typically requires a promoter, operably linked to a coding region of interest, and a transcriptional terminator. A number of yeast promoters can be used in constructing expression cassettes for genes in yeast including, but not limited to, promoters derived from the following genes: CYC1, HIS3, GAL1, GAL 10, ADH1, PGK, PH05, GAPDH, ADC1, TRP1, URA3, LEU2, ENO, TPI, CUP1, FBA, GPD, GPM, and AOX1. Suitable transcriptional terminators include, but are not limited to, FBAt, GPDt, GPMt, ERG lOt, GAL It, CYC1, and ADH1.

[0213] Suitable promoters, transcriptional terminators, and a DHAD variant coding regions can be cloned into Escherichia coli (E. co/z ^' )-yeast shuttle vectors, and transformed into yeast cells, for example. These vectors allow strain propagation in both E. coli and yeast strains. Typically, the vector used contains a selectable marker and sequences allowing autonomous replication or chromosomal integration in the desired host. Typically used plasmids in yeast are shuttle vectors pRS423, pRS424, pRS425, and pRS426 (American Type Culture Collection, Rockville, Md.), which contain an E. coli replication origin (e.g., pMBl), a yeast 2μ origin of replication, and a marker for nutritional selection. The selection markers for these four vectors are His3 (vector pRS423), Trpl (vector pRS424), Leu2 (vector pRS425), and Ura3 (vector pRS426). Construction of expression vectors with a chimeric gene encoding the described DHAD variants can be performed, for example, by either standard molecular cloning techniques in E. coli or by the gap repair recombination method in yeast.

[0214] The gap repair cloning approach takes advantage of the highly efficient homologous recombination in yeast. Typically, a yeast vector DNA is digested (e.g., in its multiple cloning site) to create a "gap" in its sequence. A number of insert DNAs of interest are generated that contain a 21 bp sequence at both the 5' and the 3' ends that sequentially overlap with each other, and with the 5' and 3' terminus of the vector DNA. For example, to construct a yeast expression vector for "Gene X", a yeast promoter and a yeast terminator are selected for the expression cassette. The promoter and terminator are amplified from the yeast genomic DNA, and Gene X is either PCR amplified from its source organism or obtained from a cloning vector comprising Gene X sequence. There is at least a 21 bp overlapping sequence between the 5' end of the linearized vector and the promoter sequence, between the promoter and Gene X, between Gene X and the terminator sequence, and between the terminator and the 3' end of the linearized vector. The "gapped" vector and the insert DNAs are then co-transformed into a yeast strain and plated on the medium containing the appropriate compound mixtures that allow complementation of the nutritional selection markers on the plasmids. The presence of correct insert combinations can be confirmed by PCR mapping using plasmid DNA prepared from the selected cells. The plasmid DNA isolated from yeast (usually low in concentration) can then be transformed into an E. coli strain, for example, TOP 10, followed by mini preps and restriction mapping to further verify the plasmid construct. Finally, the construct can be verified by sequence analysis.

[0215] Like the gap repair technique, integration into the yeast genome also takes advantage of the homologous recombination system in yeast. Typically, a cassette containing a coding region plus control elements (promoter and terminator) and auxotrophic marker is PCR- amplified with a high-fidelity DNA polymerase using primers that hybridize to the cassette and contain 40-70 base pairs of sequence homology to the regions 5' and 3' of the genomic area where insertion is desired. The PCR product is then transformed into yeast and plated on medium containing the appropriate compound mixtures that allow selection for the integrated auxotrophic marker. For example, to integrate "Gene X" into chromosomal location "Y," the promoter-coding region X-terminator construct is PCR amplified from a plasmid DNA construct and joined to an autotrophic marker (such as URA3) by either SOE PCR or by common restriction digests and cloning. The full cassette, containing the promoter-coding region X-terminator-URA3 region, is PCR amplified with primer sequences that contain 40-70 base pairs (bps) of homology to the regions 5' and 3' of location "Y" on the yeast chromosome. The PCR product is transformed into yeast and selected on growth media lacking uracil. Transformants can be verified either by colony PCR or by direct sequencing of chromosomal DNA.

Recombinant Host Cells

[0216] The isolated nucleic acid molecules and vectors of the invention can be transformed into a host cell for DHAD expression and activity. Suitable host cells include any cell capable of genetic manipulation, and include bacteria, cyanobacteria, filamentous fungi, and yeasts.

[0217] The microbial hosts selected for the production of isobutanol are preferably tolerant to isobutanol and should be able to convert carbohydrates to isobutanol. The criteria for selection of suitable microbial hosts include, for example, the following: intrinsic tolerance to isobutanol, high rate of glucose utilization, availability of genetic tools for gene manipulation, and the ability to generate stable chromosomal alterations. Yeast Cells

[0218] Yeast cells that can be hosts for expression of a DHAD variant of the invention are any yeast cells that are amenable to genetic manipulation and include, but are not limited to, Saccharomyces, Schizosaccharomyces, Hansenula, Candida, Kluyveromyces, Yarrowia, and Pichia. Suitable strains include, but are not limited to, Saccharomyces cerevisiae, Schizosaccharomyces pombe, Kluyveromyces lactis, Kluyveromyces thermotolerans, Candida glabrata, Candida albicans, Pichia stipitis, and Yarrowia lipolytica. In some embodiments, the yeast host is Saccharomyces cerevisiae. Saccharomyces cerevisiae yeast are known in the art and are available from a variety of sources including, but not limited to, American Type Culture Collection (Rockville, MD), Centraalbureau voor Schimmelcultures (CBS) Fungal Biodiversity Centre, LeSaffre, Gert Strand AB, Ferm Solutions, North American Bioproducts, Martrex, and Lallemand. Saccharomyces cerevisiae include, but are not limited to, BY4741, CEN.PK 1 13-7D, Ethanol Red® yeast, Ferm Pro™ yeast, Bio-Ferm® XR yeast, Gert Strand Prestige Batch Turbo alcohol yeast, Gert Strand Pot Distillers yeast, Gert Strand Distillers Turbo yeast, FerMax™ Green yeast, FerMax™ Gold yeast, Thermosacc® yeast, BG-1, PE-2, CAT-1, CBS7959, CBS7960, and CBS7961.

[0219] Expression is achieved by transforming the host cell with a gene comprising a sequence encoding any of the DHAD variants of the invention. The coding region for the DHAD to be expressed can be codon optimized for the yeast cell, as is well known to one skilled in the art.

[0220] In some embodiments, reducing production of an endogenous iron-sulfur (Fe-S) protein in a yeast host cell may result in an improvement in activity of an expressed heterologous Fe-S cluster protein, such as the variant DHAD enzymes of the invention. For example, in the yeast Saccharomyces cerevisiae, the native DHAD is encoded by ILV3, and is a mitochondrially-localized protein. Thus, in any of the yeast hosts described herein, an endogenous ILV3 gene can be inactivated to reduce endogenous Fe-S protein expression. ILV3 encodes mitochondrial DHAD that is involved in branched chain amino acid biosynthesis. Mitochondrial DHAD is encoded by a nuclear gene, and has a mitochondrial targeting signal sequence so that it is transported to and localized in the mitochondrion. Any IL V3 gene can be inactivated in a yeast host cell of this disclosure. Examples of yeast IL V3 inactivation target genes and their encoded proteins are those from Saccharomyces cerevisiae YJM78 (coding SEQ ID NO:389; protein SEQ ID NO:390), Schizosaccharomyces pombe (coding SEQ ID NO:387; protein SEQ ID NO:3884), Candida galbrata strain CBS 138 (coding SEQ ID NO:399; protein SEQ ID NO:400), Candida albicans SC5314 (coding SEQ ID NO:393; protein SEQ ID NO:394), Kluyveromyces lactis (coding SEQ ID NO:391; protein SEQ ID NO:392), Yarrowia lipolytica (coding SEQ ID NO:397; protein SEQ ID NO:398), and Pichia stipitis CBS 6054 (coding SEQ ID NO:395; protein SEQ ID NO:396).

[0221] In addition, in some embodiments, over-expression of the transcriptional activator genes AFT1 and/or AFT2 or homologs thereof in a recombinant yeast microorganism improves DHAD activity. Thus, the invention also provides recombinant yeast host cells comprising the isolated nucleic acid molecules of the invention, further genetically engineered to have increased heterologous or native expression of AFT 1 and/or AFT2 or homologs thereof. Grx3, Grx4, and Fra2 are proteins involved in iron-sulfur cluster biosynthesis in yeast. Grx3 and Grx4 are monothiol glutaredoxins that have been shown to be involved in cellular Fe content modulation and delivery in yeast. Glutaredoxins are glutathione-dependent thiol-disulfide oxidoreductases that function in maintaining the cellular redox homeostasis. Saccharomyces cerevisiae has two dithiol glutaredoxins (Grxl and Grx2) and three monothiol glutaredoxins (Grx3, Grx4, and Grx5). The monothiol glutaredoxins are believed to reduce mixed disulfides formed between a protein and glutathione in a process known as deglutathionylation. Thus, the invention is also directed to a recombinant host described herein (e.g., yeast) further genetically modified to disrupt a gene encoding an endogenous Fra2, Grx3, and/or Grx4 or a homolog thereof. In some embodiments, increases in DHAD activity may be observed in yeast cells with disruptions in FRA2, GRX3, and/or GRX4.

[0222] In some embodiments, the invention is also directed to a recombinant host described herein (e.g., yeast) further genetically modified to disrupt (e.g., delete) a gene encoding pyruvate decarboxylase (PDC). In some embodiments, the PDC is PDC1, PDC5, PDC6, or combinations thereof.

Bacterial Cells

[0223] In some embodiments, the recombinant host cell is a prokaryotic cell. In certain embodiments, the recombinant host cell is a bacterial cell. In other embodiments, the bacterial cell is a lactic acid bacterial (LAB) cell selected from the group consisting of Lactococcus, Lactobacillus, Leuconostoc, Oenococcus, Pediococcus, and Streptococcus. In still other embodiments, the bacterial host cell is the lactic acid bacteria Lactobacillus. In some embodiments, the bacterial host cell is Lactobacillus plantarum.

[0224] Bacterial cells that can be hosts for expression of a heterologous bacterial [2Fe-2S] ²⁺ DHAD include, but are not limited to, Clostridium, Zymomonas, Escherichia, Salmonella, Rhodococcus, Pseudomonas, Bacillus, Lactobacillus, Enterococcus, Pediococcus, Alcaligenes, Klebsiella, Paenibacillus, Arthrobacter, Corynebacterium, Brevibacterium, Lactococcus, Leuconostoc, Oenococcus, Pediococcus, and Streptococcus. Engineering expression of a heterologous DHAD variant can increase DHAD activity in a host bacterial cell that naturally expresses a [2Fe-2S] ²⁺ DHAD or a [4Fe-4S] ²⁺ DHAD. Such host cells can include, for example, Escherichia coli and Bacillus subtilis. Furthermore, engineering expression of a heterologous DHAD variant provides DHAD activity in a host bacterial cell that has no endogenous DHAD activity. Such host cells can include, for example, Lactobacillus, Enterococcus, Pediococcus, and Leuconostoc.

[0225] Specific hosts include: Escherichia coli, Alcaligenes eutrophus, Bacillus licheniformis, Paenibacillus macerans, Rhodococcus erythropolis, Pseudomonas putida, Lactobacillus plantarum, Enterococcus faecium, Enterococcus gallinarium, Enterococcus faecalis, and Bacillus subtilis. Bacterial cells can be genetically modified for expression of DHAD variants using methods well known to one skilled in the art. Expression of DHAD variants is generally achieved by transforming suitable bacterial host cells with a sequence encoding a DHAD variant protein. Typically, the coding sequence is part of a chimeric gene used for transformation, which includes a promoter operably linked to the coding sequence as well as a ribosome binding site and a termination control region. The coding region can be from the host cell for transformation and combined with regulatory sequences that are not native to the natural gene encoding the variant DHAD. Alternatively, the coding region can be from another host cell.

[0226] Initiation control regions or promoters, which are useful to drive expression of a DHAD variant coding region in bacteria, are familiar to those skilled in the art. Some examples include the amy, apr, and npr promoters; nisA promoter (useful for expression Gram-positive bacteria (Eichenbaum, et al, Appl. Environ. Microbiol. 64(8):2763-2769, 1998); and the synthetic P l l promoter (useful for expression in Lactobacillus plantarum, Rud, et al, Microbiology 152: 101 1-1019, 2006). In addition, the ldhLl and fabZl promoters of Lactobacillus plantarum are useful for expression of chimeric genes in bacteria. The fabZl promoter directs transcription of an operon with the first gene, fabZl, encoding (3R)- hydroxymyristoyl-[acyl carrier protein] dehydratase. Termination control regions can also be derived from various genes, typically from genes native to the preferred hosts. In other embodiments, a termination site is unnecessary.

[0227] Vectors can be introduced into lactic acid bacteria (LAB) host cells using methods known in the art, such as electroporation (Cruz-Rodz, et al, Molecular Genetics and Genomics 224: 1252-154, 1990; Bringel, et al, Appl. Microbiol. Biotechnol. 33:664-670, 1990; Alegre, et al, FEMS Microbiology Letters 241 :73-77, 2004), and conjugation (Shrago, et al, Appl. Environ. Microbiol. 52:574-576, 1986). A chimeric DHAD gene can also be integrated into the chromosome of LAB using integration vectors (Hols, et al, Appl. Environ. Microbiol. 60: 1401-1403, 1990; Jang, et al, Micro. Lett. 24: 191-195, 2003 ).

[0228] Lactic acid bacteria are well characterized and are used commercially in a number of industrial processes. Although it is known that some lactic acid bacteria possess iron-sulfur (Fe-S) cluster requiring enzymes (Liu, et al, J. Biol. Chem. 275(17) ; 12367-12373, 2000) and therefore possess the genetic machinery to produce Fe-S clusters, little is known about the ability of lactic acid bacteria to insert Fe-S clusters into heterologous enzymes, and little is known about the facility with which Fe-S cluster forming proteins can be expressed in lactic acid bacteria.

[0229] To obtain high levels of product in a lactic acid bacteria from a biosynthetic pathway including DHAD activity, high expression of DHAD activity is desired. The activity of the Fe-S requiring DHAD enzyme in a host cell can be limited, for example, by the availability of Fe-S clusters in the cell. Increasing the expression of Fe-S cluster forming proteins effectively increased the activity of DHAD in LAB cells. Thus, in certain embodiments, a lactic acid bacterial host cell is genetically engineered to express at least one recombinant genetic expression element encoding Fe-S cluster forming proteins. The genetic engineering of lactic acid bacteria to express iron-sulfur cluster forming proteins is described in U.S. Patent Application Publication No. 2010/0081 182, which is herein incorporated by reference.

[0230] Expression of any set of proteins for Fe-S cluster formation can be used to increase DHAD activity in LAB cells. There are three known groups of Fe-S cluster forming proteins. These proteins are encoded by three types of operons: the Suf operon, the Isc operon, and the Nif operon. U.S. Patent Application Publication No. 2010/0081182 discloses the Suf operons of Lactobacillus plantarum (L. plantarum) , Lactobacillus lactis (L. lactis), and Escherichia coli (E. coli); the Isc operon of E. coli; and the Nif operon of Wolinella succinogenes .

Culture Conditions for Butanol Production

[0231] The invention also provides a method for the production of butanol (e.g., isobutanol) comprising providing recombinant host cells comprising the isolated nucleic acid molecules of the invention; culturing the recombinant host cell in a fermentation medium under suitable conditions to produce isobutanol from pyruvate; and recovering the isobutanol. In certain embodiments, the isobutanol is produced at a titer that is increased as compared to a recombinant host cell that does not contain the amino acid substitutions. In other embodiments, the isobutanol is produced at a rate that is increased by at least about 5%, at least about 10%, at least about 15%, at least about 20%, at least about 25%, at least about 30%, at least about 35%, at least about 40%, at least about 45%, at least about 50%, at least about 60%, at least about 70%, at least about 80%, at least about 90%, at least about 100%, at least about 200%, or at least about 300% as compared to a recombinant host cell that does not contain the amino acid substitutions. In other aspects of the method to produce isobutanol, the concentration of isobutanol in the fermentation medium is greater than or equal to about 10 mM, greater than or equal to about 20 mM, greater than or equal to about 30 mM, greater than or equal to about 40 mM, greater than or equal to about 50 mM, greater than or equal to about 60 mM, greater than or equal to about 70 mM, greater than or equal to about 80 mM, greater than or equal to about 90 mM, or greater than or equal to about 100 mM.

[0232] Recombinant host cells disclosed herein are grown in media which contains suitable carbon substrates. Additional carbon substrates can include, but are not limited to, monosaccharides such as fructose; oligosaccharides such as lactose, maltose, galactose, or sucrose; polysaccharides such as starch or cellulose; or mixtures thereof and unpurified mixtures from renewable feedstocks such as cheese whey permeate, cornsteep liquor, sugar beet molasses, and barley malt. Other carbon substrates can include, but are not limited to, ethanol, lactate, succinate, and glycerol.

[0233] Additionally the carbon substrate can also be one carbon substrates such as carbon dioxide, or methanol for which metabolic conversion into key biochemical intermediates has been demonstrated. In addition to one and two carbon substrates, methylotrophic organisms are also known to utilize a number of other carbon containing compounds such as methylamine, glucosamine, and a variety of amino acids for metabolic activity. For example, methylotrophic yeasts are known to utilize the carbon from methylamine to form trehalose or glycerol (Bellion, et al, Microb. Growth CI Compd., [Int. Symp.], 7th (1993), 415 32, Editor(s): Murrell, J. Collin; Kelly, Don P. Publisher: Intercept, Andover, UK). Similarly, various species of Candida will metabolize alanine or oleic acid (Suiter, et al, Arch. Microbiol. 153 :485 489, 1990). Hence, it is contemplated that the source of carbon utilized in the present invention can encompass a wide variety of carbon containing substrates and will only be limited by the choice of organism.

[0234] Although it is contemplated that all of the above mentioned carbon substrates and mixtures thereof are suitable in the present invention, in some embodiments, the carbon substrates may be glucose, fructose, and sucrose, or mixtures of these with five-carbon (C5) sugars such as xylose and/or arabinose for yeasts cells modified to use C5 sugars. Sucrose can be derived from renewable sugar sources such as sugar cane, sugar beets, cassava, sweet sorghum, and mixtures thereof. Glucose and dextrose can be derived from renewable grain sources through saccharification of starch based feedstocks including grains such as corn, wheat, rye, barley, oats, and mixtures thereof. In addition, fermentable sugars can be derived from renewable cellulosic or lignocellulosic biomass through processes of pretreatment and saccharification, as described, for example, in U.S. Patent No. 7,932,063, which is herein incorporated by reference. Biomass refers to any cellulosic or lignocellulosic material and includes materials comprising cellulose, and optionally further comprising hemicellulose, lignin, starch, oligosaccharides, and/or monosaccharides. Biomass can also comprise additional components, such as protein and/or lipid. Biomass can be derived from a single source, or biomass can comprise a mixture derived from more than one source; for example, biomass can comprise a mixture of corn cobs and corn stover, or a mixture of grass and leaves. Biomass includes, but is not limited to, bioenergy crops, agricultural residues, municipal solid waste, industrial solid waste, sludge from paper manufacture, yard waste, wood and forestry waste. Examples of biomass include, but are not limited to, corn grain, corn cobs, crop residues such as corn husks, corn stover, grasses, wheat, wheat straw, barley, barley straw, hay, rice straw, switchgrass, waste paper, sugar cane bagasse, sorghum, soy, components obtained from milling of grains, trees, branches, roots, leaves, wood chips, sawdust, shrubs, bushes, vegetables, fruits, flowers, animal manure, and mixtures thereof.

[0235] In addition to an appropriate carbon source, fermentation media may contain suitable minerals, salts, cofactors, buffers, and other components, known to those skilled in the art, suitable for the growth of the cultures and promotion of an enzymatic pathway comprising a DHAD.

[0236] Typically, cells are grown at a temperature in the range of about 20°C to about 40°C in an appropriate medium. Suitable growth media for the present invention include, for example, common commercially prepared media such as Luria Bertani (LB) broth, Sabouraud Dextrose (SD) broth, Yeast Medium (YM) broth, or broth that includes yeast nitrogen base, ammonium sulfate, and dextrose (as the carbon/energy source) or Yeast Extract Peptone Dextrose (YPD) Medium, a blend of peptone, yeast extract, and dextrose in optimal proportions for growing most Saccharomyces cerevisiae strains. Other defined or synthetic growth media can also be used, and the appropriate medium for growth of the particular microorganism will be known by one skilled in the art of microbiology or fermentation science. The use of agents known to modulate catabolite repression directly or indirectly, for example, cyclic adenosine 2':3' monophosphate, can also be incorporated into the fermentation medium.

[0237] Suitable pH ranges for the fermentation are between about pH 5.0 to about pH 9.0. In one embodiment, about pH 6.0 to about pH 8.0 is used for the initial condition. Suitable pH ranges for the fermentation of yeast are typically between about pH 3.0 to about pH 9.0. In one embodiment, about pH 5.0 to about pH 8.0 is used for the initial condition. Suitable pH ranges for the fermentation of other microorganisms are between about pH 3.0 to about pH 7.5. In one embodiment, about pH 4.5 to about pH 6.5 is used for the initial condition.

[0238] Fermentations can be performed under aerobic or anaerobic conditions. In one embodiment, anaerobic or microaerobic conditions are used for fermentations.

Industrial Batch and Continuous Fermentations

[0239] Isobutanol, or other products, can be produced using a batch method of fermentation.

A classical batch fermentation is a closed system where the composition of the medium is set at the beginning of the fermentation and not subject to artificial alterations during the fermentation. A variation on the standard batch system is the fed batch system. Fed batch fermentation processes are also suitable in the present invention and comprise a typical batch system with the exception that the substrate is added in increments as the fermentation progresses. Fed batch systems are useful when catabolite repression is apt to inhibit the metabolism of the cells and where it is desirable to have limited amounts of substrate in the media. Batch and fed batch fermentations are common and well known in the art, examples of which are described by Thomas D. Brock in Biotechnology: A Textbook of Industrial Microbiology, Second Edition (1989) Sinauer Associates, Inc., Sunderland, M.A., or Deshpande, Mukund V., Appl. Biochem. Biotechnol, 36:227, (1992), herein incorporated by reference.

[0240] Isobutanol, or other products, can also be produced using continuous fermentation methods. Continuous fermentation is an open system where a defined fermentation medium is added continuously to a bioreactor and an equal amount of conditioned media is removed simultaneously for processing. Continuous fermentation generally maintains the cultures at a constant high density where cells are primarily in log phase growth. Continuous fermentation allows for the modulation of one factor or any number of factors that affect cell growth or end product concentration. Methods of modulating nutrients and growth factors for continuous fermentation processes as well as techniques for maximizing the rate of product formation are well known in the art of industrial microbiology and a variety of methods are detailed by Brock, supra.

[0241] It is contemplated that the production of isobutanol, or other products, can be practiced using batch, fed batch, or continuous processes and that any known mode of fermentation is suitable. Additionally, it is contemplated that cells can be immobilized on a substrate as whole cell catalysts and subjected to fermentation conditions for isobutanol production.

Biosynthetic Pathways

[0242] Expression of a DHAD variant in bacteria or yeast, as described herein, provides the transformed, recombinant host cell with dihydroxy-acid dehydratase (DHAD) activity for conversion of 2,3-dihydroxyisovalerate to a-ketoisovalerate or 2,3-dihydroxymethylvalerate to a-ketomethylvalerate. Any product that has a-ketoisovalerate or a-ketomethylvalerate as a pathway intermediate can be produced in a bacterial or yeast strain disclosed herein having the described heterologous DHAD variants. A list of such products includes, but is not limited to, valine, isoleucine, leucine, pantothenic acid, 2-methyl-l-butanol, 3 -methyl- 1- butanol, and isobutanol.

[0243] For example, yeast biosynthesis of valine includes steps of acetolactate conversion to 2,3-dihydroxy-isovalerate by acetohydroxyacid reductoisomerase (ILV5), conversion of 2,3- dihydroxy-isovalerate to α-ketoisovalerate (also called 2-keto-isovalerate) by dihydroxy-acid dehydratase, and conversion of α-ketoisovalerate to valine by branched-chain amino acid transaminase (BAT2) and branched-chain amino acid aminotransferase (BAT1). Biosynthesis of leucine includes the same steps to a-ketoisovalerate, followed by conversion of α-ketoisovalerate to a-isopropylmalate by a-isopropylmalate synthase (LEU9, LEU4), conversion of α-isopropylmalate to beta-isopropylmalate by isopropylmalate isomerase (LEU1), conversion of beta-isopropylmalate to a-ketoisocaproate by beta-IPM dehydrogenase (LEU2), and finally conversion of a-ketoisocaproate to leucine by branched- chain amino acid transaminase (BAT2) and branched-chain amino acid aminotransferase (BAT1). The bacterial pathway is similar, involving differently named proteins and genes. Increased conversion of 2,3-dihydroxy-isovalerate to α-ketoisovalerate will increase flow in these pathways, particularly if one or more additional enzymes of a pathway is over- expressed. Thus, it is desired for production of valine or leucine to use a strain disclosed herein.

[0244] Biosynthesis of pantothenic acid includes a step performed by DHAD, as well as steps performed by ketopantoate hydroxymethyltransferase and pantothenate synthase. Engineering of expression of these enzymes for enhanced production of pantothenic acid biosynthesis in microorganisms is described, for example, in U.S. Patent No. 6, 177,264, which is incorporated by reference herein.

[0245] The α-ketoisovalerate product of DHAD is an intermediate in isobutanol biosynthetic pathways disclosed, for example, in U.S. Patent No. 7,851, 188, which is incorporated by reference herein. A diagram of the disclosed isobutanol biosynthetic pathways is provided in FIG. 2. Production of isobutanol in a strain disclosed herein benefits from increased DHAD activity. As disclosed herein, DHAD activity is provided by expression of a variant DHAD in a bacterial or yeast cell. As described in U.S. Patent No. 7,851, 188, steps in an example isobutanol biosynthetic pathway include conversion of: pyruvate to acetolactate as catalyzed, for example, by acetolactate synthase, acetolactate to 2,3-dihydroxyisovalerate as catalyzed, for example, by acetohydroxy acid isomeroreductase; 2,3-dihydroxyisovalerate to a- ketoisovalerate as catalyzed, for example, by acetohydroxy acid dehydratase, also called dihydroxy-acid dehydratase (DHAD); α-ketoisovalerate to isobutyraldehyde as catalyzed, for example, by branched-chain a-keto acid decarboxylase; and isobutyraldehyde to isobutanol as catalyzed, for example, by branched-chain alcohol dehydrogenase. The substrate to product conversions, and enzymes involved in these reactions, are described, for example, in U.S. Patent No. 7,851, 188, which is incorporated by reference herein. [0246] Genes that can be used for expression of the pathway step enzymes named above other than the variant DHADs disclosed herein, as well as those for two additional isobutanol pathways, are described, for example, in U.S. Patent No. 7,851,188, which is incorporated by reference herein, and additional genes that can be used can be identified by one skilled in the art through bioinformatics or experimentally as described herein. Ketol-acid reductoisomerase (KARI) enzymes are also disclosed, for example, in U.S. Patent No. 7,910,342 and PCT Application Publication No. WO2012/129555, both incorporated by reference herein. Examples of KARIs disclosed therein include KARIs from Vibrio cholerae (DNA: SEQ ID NO:599; protein SEQ ID NO:600), Pseudomonas aeruginosa PAOl, (DNA: SEQ ID NO:601; protein SEQ ID NO:602), Pseudomonas fluoresces PF5 (DNA: SEQ ID NO:603; protein SEQ ID NO:604), and Anaerostipes caccae (protein SEQ ID NO:605).

[0247] Additionally described in U.S. Patent No. 7,851, 188 are construction of chimeric genes and genetic engineering of bacteria and yeast for isobutanol production using the disclosed biosynthetic pathways. In some embodiments, one or more components of the biosynthetic pathways described herein can be endogenous to the host cell of choice, or can be heterologous. Additionally, in other embodiments, one or more of the genes encoding the enzymes required in the biosynthetic pathways can be over-expressed in the host cell.

Methods for Butanol Isolation from the Fermentation Medium

[0248] Methods for butanol isolation from fermentation medium have been described. For example, bioproduced isobutanol can be isolated from the fermentation medium using methods known in the art for ABE fermentations (see, e.g., Durre, Appl. Microbiol. Biotechnol. 49:639-648, 1998; Groot, et al, Process. Biochem. 27:61-75, 1992, and references therein). For example, solids may be removed from the fermentation medium by centrifugation, filtration, decantation, or the like. Then, the isobutanol can be isolated from the fermentation medium using methods such as distillation, azeotropic distillation, liquid- liquid extraction, adsorption, gas stripping, membrane evaporation, pervaporation, or combinations thereof.

[0249] Because isobutanol forms a low boiling point, azeotropic mixture with water, distillation can be used to separate the mixture up to its azeotropic composition. Distillation may be used in combination with another separation method to obtain separation around the azeotrope. Methods that can be used in combination with distillation to isolate and purify butanol include, but are not limited to, decantation, liquid-liquid extraction, adsorption, and membrane-based techniques. Additionally, butanol can be isolated using azeotropic distillation using an entrainer (see, e.g., Doherty and Malone, Conceptual Design of Distillation Systems, McGraw Hill, New York, 2001).

[0250] The butanol-water mixture forms a heterogeneous azeotrope so that distillation can be used in combination with decantation to isolate and purify the isobutanol. In this method, the isobutanol containing fermentation broth is distilled to near the azeotropic composition. Then, the azeotropic mixture is condensed, and the isobutanol is separated from the fermentation medium by decantation. The decanted aqueous phase can be returned to the first distillation column as reflux. The isobutanol-rich decanted organic phase can be further purified by distillation in a second distillation column.

[0251] The isobutanol can also be isolated from the fermentation medium using liquid-liquid extraction in combination with distillation. In this method, the isobutanol is extracted from the fermentation broth using liquid-liquid extraction with a suitable solvent. The isobutanol- containing organic phase is then distilled to separate the butanol from the solvent.

[0252] Distillation in combination with adsorption can also be used to isolate isobutanol from the fermentation medium. In this method, the fermentation broth containing the isobutanol is distilled to near the azeotropic composition and then the remaining water is removed by use of an adsorbent, such as molecular sieves (Aden, et al, Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and Enzymatic Hydrolysis for Corn Stover, Report NREL/TP-510-32438, National Renewable Energy Laboratory, June 2002).

[0253] Additionally, distillation in combination with pervaporation may be used to isolate and purify the isobutanol from the fermentation medium. In this method, the fermentation broth containing the isobutanol is distilled to near the azeotropic composition, and then the remaining water is removed by pervaporation through a hydrophilic membrane (Guo, et al, J. Membr. Sci. 245, 199-210, 2004).

[0254] In situ product removal (ISPR) (also referred to as extractive fermentation) can be used to remove butanol (or other fermentative alcohol) from the fermentation vessel as it is produced, thereby allowing the microorganism to produce butanol at high yields. One method for ISPR for removing fermentative alcohol that has been described in the art is liquid-liquid extraction. In general, with regard to butanol fermentation, for example, the fermentation medium, which includes the microorganism, is contacted with an organic extractant at a time before the butanol concentration reaches a toxic level. The organic extractant and the fermentation medium form a biphasic mixture. The butanol partitions into the organic extractant phase, decreasing the concentration in the aqueous phase containing the microorganism, thereby limiting the exposure of the microorganism to the inhibitory butanol.

[0255] Liquid-liquid extraction can be performed, for example, according to the processes described in U.S. Patent Application Publication No. 2009/0305370, the disclosure of which is hereby incorporated in its entirety. U.S. Patent Application Publication No. 2009/0305370 describes methods for producing and recovering butanol from a fermentation broth using liquid-liquid extraction, the methods comprising the step of contacting the fermentation broth with a water immiscible extractant to form a two-phase mixture comprising an aqueous phase and an organic phase. Typically, the extractant can be an organic extractant selected from the group consisting of saturated, mono-unsaturated, poly-unsaturated (and mixtures thereof) C ₁₂ to C22 fatty alcohols, C ₁₂ to C22 fatty acids, esters of C ₁₂ to C22 fatty acids, C ₁₂ to C22 fatty aldehydes, and mixtures thereof. The extractant(s) for ISPR can be non-alcohol extractants. The ISPR extractant can be an exogenous organic extractant such as oleyl alcohol, behenyl alcohol, cetyl alcohol, lauryl alcohol, myristyl alcohol, stearyl alcohol, 1-undecanol, oleic acid, lauric acid, linoleic acid, linolenic acid, myristic acid, stearic acid, methyl myristate, methyl oleate, undecanal, lauric aldehyde, 20-methylundecanal, and mixtures thereof.

[0256] In some embodiments, the alcohol can be formed by contacting the alcohol in a fermentation medium with an organic acid (e.g., fatty acids) and a catalyst capable of esterfiying the alcohol with the organic acid. In such embodiments, the organic acid can serve as an ISPR extractant into which the alcohol esters partition. The organic acid can be supplied to the fermentation vessel and/or derived from the biomass supplying fermentable carbon fed to the fermentation vessel. Lipids present in the feedstock can be catalytically hydrolyzed to organic acid, and the same catalyst (e.g., enzymes) can esterify the organic acid with the alcohol. The catalyst can be supplied to the feedstock prior to fermentation, or can be supplied to the fermentation vessel before or contemporaneously with the supplying of the feedstock. When the catalyst is supplied to the fermentation vessel, alcohol esters can be obtained by hydrolysis of the lipids forming organic acid and substantially simultaneous esterification of the organic acid with butanol present in the fermentation vessel. Organic acid and/or native oil not derived from the feedstock can also be fed to the fermentation vessel, with the native oil being hydrolyzed into organic acid. Any organic acid not esterified with the alcohol can serve as part of the ISPR extractant. The extractant containing alcohol esters can be separated from the fermentation medium, and the alcohol can be recovered from the extractant. The extractant can be recycled to the fermentation vessel. Thus, in the case of butanol production, for example, the conversion of the butanol to an ester reduces the free butanol concentration in the fermentation medium, shielding the microorganism from the toxic effect of increasing butanol concentration. In addition, unfractionated grain can be used as feedstock without separation of lipids therein, since the lipids can be catalytically hydrolyzed to organic acid, thereby decreasing the rate of build-up of lipids in the ISPR extractant.

[0257] In situ product removal can be carried out in a batch mode or a continuous mode. In a continuous mode of in situ product removal, product is continually removed from the reactor. In a batchwise mode of in situ product removal, a volume of organic extractant is added to the fermentation vessel and the extractant is not removed during the process. For in situ product removal, the organic extractant can contact the fermentation medium at the start of the fermentation forming a biphasic fermentation medium. Alternatively, the organic extractant can contact the fermentation medium after the microorganism has achieved a desired amount of growth, which can be determined by measuring the optical density of the culture. Further, the organic extractant can contact the fermentation medium at a time at which the product alcohol level in the fermentation medium reaches a preselected level. In the case of butanol production according to some embodiments of the present invention, the organic acid extractant can contact the fermentation medium at a time before the butanol concentration reaches a toxic level, so as to esterify the butanol with the organic acid to produce butanol esters and consequently reduce the concentration of butanol in the fermentation vessel. The ester-containing organic phase can then be removed from the fermentation vessel (and separated from the fermentation broth which constitutes the aqueous phase) after a desired effective titer of the butanol esters is achieved. In some embodiments, the ester-containing organic phase is separated from the aqueous phase after fermentation of the available fermentable sugar in the fermentation vessel is substantially complete.

Methods of Screening for DHAD Variants

[0258] The invention also provides yeast strains and methods of using the yeast strains to screen for DHAD variants with increased DHAD activity as compared to a parental DHAD enzyme. The premise behind the screen is to reduce DHAD expression and/or activity in a yeast strain to artificially create a system where DHAD activity is rate-limiting for growth. Introducing a mutation into the DHAD gene resulting in a DHAD variant enzyme with increased activity will overcome the rate-limiting step, allowing the strain to grow or produce an increased amount of product, such as isobutanol. Therefore, yeast isolates expressing DHAD variants with increased DHAD activity can be identified and selected based on their growth differential as compared to a control strain, or based on an increased production of a product compared to a control strain.

[0259] In certain embodiments, the invention provides a yeast strain with a defect in the genetic pathway that converts pyruvate to ethanol, such that the yeast strain cannot grow or grows poorly in fermentation medium containing glucose as the primary carbon source. The defect in the genetic pathway that converts pyruvate to ethanol can comprise a deletion of a PDC gene or a mutation in a PDC gene that reduces PDC activity. In certain embodiments, the PDC gene is PDC1, PDC5, PDC6, or a combination thereof.

[0260] The "genetic pathway that converts pyruvate to ethanol" comprises at least the following genes in Saccharomyces cerevisiae: PDC1, PDC5, PDC6, and ADH1. Pyruvate is first converted to acetaldehyde by pyruvate decarboxylase. Subsequently, acetaldehyde is converted to ethanol by alcohol dehydrogenase. Any defect (e.g., an insertion, deletion, mutation, or substitution in one or more pathway genes) that disrupts the ability of the yeast strain to produce ethanol from glucose but leaves intact the ability to produce isobutanol from glucose is contemplated to be a defect in the genetic pathway that converts pyruvate to ethanol.

[0261] The growth rate of the yeast strain of the invention is such that it can be modulated by increasing or decreasing the amount of DHAD activity within the cell. For example, a yeast strain of the invention that expresses very low levels of a DHAD enzyme will grow poorly or not at all when grown in fermentation medium with glucose as the primary carbon source. Thus, a "low level" of DHAD activity is defined as an amount of DHAD enzyme activity that results in no growth or poor growth of the yeast strain of the invention. "Poor growth," for the purposes of the present invention, can be considered a growth rate that is slow enough that a detectable difference in growth rates can be observed when DHAD activity is restored. Conversely, a yeast strain of the invention that expresses high levels of DHAD enzyme will grow well when grown in fermentation medium with glucose as the primary carbon source. As such, a "high level" of DHAD activity is defined as an amount of DHAD enzyme activity that results in a growth rate that is detectably improved compared to the same strain expressing a low level of DHAD activity. The growth differential between a strain with low levels of DHAD activity and a strain with high levels of DHAD activity can be determined or detected by methods known to a skilled artisan, such as calculating doubling times, determining the density of cells in culture, or simply by visually assessing the size of individual colonies grown on solid media after a given amount of time.

[0262] Numerous methods can be used to modulate the expression levels of a DHAD enzyme in the yeast strain of the invention and are well known to skilled artisans. These methods include, but are not limited to, expressing a DHAD enzyme under a weak promoter, expressing a DHAD enzyme on a low copy number plasmid, expressing a DHAD enzyme under an inducible promoter and varying the amount of inducing agent, optimizing codon usage for the organism in which it is to be expressed (to increase expression) or adjusting codon usage to be sub-optimal for the organism in which it is to be expressed (to decrease expression).

[0263] Low copy number plasmids generally exist in a cell in less than about 100 copies/cell.

In certain embodiments, the low copy number plasmid exists in a cell in less than about 50, less than about 40, less than about 30, less than about 20, less than about 10, less than about 5, or less than about 2 copies/cell. In certain embodiments, the low copy number plasmid exists in a cell in about one copy per cell.

[0264] In methods to screen for DHAD variants, the yeast strain of the invention is transformed with a polynucleotide comprising a nucleic acid sequence encoding a parental DHAD enzyme under conditions wherein a low level of DHAD enzyme activity is achieved, and no growth or poor growth of the strain is seen in fermentation medium wherein glucose is the primary carbon source. This transformant is used as a control strain. A library of polynucleotides is prepared, each polynucleotide comprising a nucleic acid sequence encoding a DHAD variant. This library of DHAD variants is transformed into the yeast strain of the invention, under the same conditions as the control strain, and growth rates or product yield of individual isolates transformed with a variant is assessed. Variants that result in increased DHAD activity will grow more robustly and/or produce a higher yield of product than the control strain, and can be isolated for further analysis. [0265] Thus, an aspect of the invention is directed to a method of screening DHAD protein variants, comprising: (a) providing a yeast strain with a defect in a genetic pathway that converts pyruvate to ethanol, wherein the yeast strain cannot grow or grows poorly in fermentation medium containing glucose as the primary carbon source; (b) transforming the yeast strain with a library of polynucleotides, each polynucleotide comprising a nucleic acid sequence encoding a DHAD variant, wherein: (i) the nucleic acid sequence encoding the DHAD variant is operably linked to a weak promoter; or (ii) the nucleic acid sequence encoding the DHAD variant is comprised within a low copy number plasmid; wherein the yeast strain cannot grow or grows poorly when transformed with a control polynucleotide comprising a nucleic acid sequence encoding a wild type DHAD, operably linked to the weak promoter or transformed with a control low copy number plasmid comprising a nucleic acid sequence encoding a wild type DHAD; and (c) selecting transformants with improved growth compared to growth of a strain transformed with the control polynucleotide. In some embodiments, the defect in the genetic pathway that converts pyruvate to ethanol comprises a deletion of a pyruvate decarboxylase (PDC) gene. In other embodiments, the defect in the genetic pathway that converts pyruvate to ethanol comprises a mutation in a PDC gene that reduces PDC activity. In certain embodiments, the PDC gene is PDC1, PDC5, PDC6, or a combination thereof.

[0266] In some embodiments of the method of screening DHAD protein variants, the weak promoter is a truncated Leu2 promoter. In certain embodiments, the truncated Leu2 promoter is SEQ ID NO: 545. In other embodiments of the method of screening DHAD protein variants, the weak promoter is a truncated FBA promoter. In certain embodiments, the truncated FBA promoter is SEQ ID NO:546. Other weak promoters are known in the art, such as the Ste5 promoter, the Ura3 promoter, and the Cyc 1 promoter. Other promoters that are considered strong or moderate promoters in their full-length state can be made weak by truncation or other modifications. For the purposes of the present invention, a "weak promoter" is defined as a promoter that results in a level of expression of a parental DHAD enzyme in a strain of the invention that does not allow growth or allows only poor growth on fermentation medium with glucose as the primary carbon source.

[0267] In some embodiments of the method of screening DHAD protein variants, the low copy number plasmid has a copy number of one or two in yeast. In certain embodiments, the low copy number plasmid is pRS413. Low copy number plasmids for use in yeast include the yeast integrating plasmids (Yip) and yeast centromere plasmids (YCp), as well as the pRS series of plasmids. pRS plasmids were first described by Sikorski, et al. (Genetics, 122: 19- 27, 1989) and include, but are not limited to, pRS303, pRS304, pRS305, pRS306, pRS313, pRS314, pRS315, and pRS316.

[0268] In other embodiments of the method of screening DHAD protein variants, the growth of the strain is under oxygen limiting conditions. In yet other embodiments, the yeast strain is further transformed with genes encoding acetolactate synthase, acetohydroxy acid isomeroreductase, a-keto acid decarboxylase, and alcohol dehydrogenase. In certain embodiments of the method of screening DHAD protein variants, the method further comprises determining the rate of isobutanol production of the transformants.

[0269] Another aspect of the invention is directed to isolated polynucleotides comprising a nucleic acid sequence encoding a DHAD variant obtained by the method of screening DHAD protein variants as described herein. The invention is also directed to isolated DHAD variant polypeptides encoded by these nucleic acid sequences.

EXAMPLES

Example 1 : Construction of Yeast Strain PNY2204

[0270] The purpose of this example is to describe construction of a vector to enable integration of a gene encoding acetolactate synthase into the naturally occurring intergenic region between the PDCl and TR l coding sequences in Chromosome XII. Construction of yeast strain PNY2204 is also described, for example, in U.S. Application Publication No. 2012/0237988, which is incorporated herein by reference.

Construction of Integration Vector pUC19-kan::pdcl ::FBA-alsS::TRXl

[0271] The FBA-alsS-CYCt cassette was constructed by moving the 1.7 kb BbvCI PacI fragment from pRS426::GPD::alsS::CYC (U.S. Appl. Pub. No. 2007/0092957, incorporated by reference) to pRS426::FBA::ILV5::CYC (U.S. Application Publication No. 2007/0092957, incorporated by reference, previously digested with BbvCI/PacI to release the ILV5 gene). Ligation reactions were transformed into E. coli TOP 10 cells and transformants were screened by PCR using primers N98SeqFl (SEQ ID NO:580) and N99SeqR2 (SEQ ID NO:581). The FBA-alsS-CYCt cassette was isolated from the vector using Bglll and Notl for cloning into pUC19-URA3 ::ilvD-TRXl (as described in U.S. Application Publication No. 2012/0156735, incorporated herein by reference, clone "Β;" herein SEQ ID NO:582) at the Aflll site (Klenow fragment was used to make ends compatible for ligation). Transformants containing the alsS cassette in both orientations in the vector were obtained and confirmed by PCR using primers N98SeqF4 (SEQ ID NO:583) and Ni l 11 (SEQ ID NO:584) for configuration "A" and N98SeqF4 (SEQ ID NO:583) and Ni l 10 (SEQ ID NO:585) for configuration "B." A geneticin selectable version of the "A" configuration vector was then made by removing the URA3 gene (1.2 kb Notl Nael fragment) and adding a geneticin cassette (SEQ ID NO:586 herein; previously described in U.S. Application Publication No. 2012/0156735, incorporated herein by reference) maintained in a pUC19 vector (cloned at the Smal site). The kan gene was isolated from pUC19 by first digesting with Kpnl, removal of 3' overhanging DNA using Klenow fragment (New England BioLabs, Inc., Ipswich, MA; Cat. No. M212), digesting with Hindi, and then gel purifying the 1.8 kb gene fragment (Zymoclean™ Gel DNA Recovery Kit, Cat. No. D4001, Zymo Research, Orange, CA; SEQ ID NO:587). Klenow fragment was used to make all ends compatible for ligation, and trans formants were screened by PCR to select a clone with the geneticin resistance gene in the same orientation as the previous URA3 marker using primers BK468 (SEQ ID NO:588) and N160SeqF5 (SEQ ID NO:589). The resulting clone was called pUC19-kan::pdcl ::FBA- alsS::TRXl (clone A) (SEQ ID NO:590).

Construction of alsS Integrant Strains and Isobutanol-Producing Derivatives

[0272] The pUC19-kan::pdcl ::FBA-alsS integration vector described above was linearized with Pmel and transformed into PNY1507 (described, for example, in U.S. Application Publication No. 2012/0156735, incorporated herein by reference). Pmel cuts the vector within the cloned pdcl-TR l intergenic region and thus leads to targeted integration at that location (Rothstein, Methods in Enzymology, 1991, volume 194, pp. 281-301). Transformants were selected on YPE plus 50 μg/ml G418. Patched transformants were screened by PCR for the integration event using primers N160SeqF5 (SEQ ID NO:589) and oBP512 (SEQ ID NO:591). Two transformants were tested indirectly for acetolactate synthase function by evaluating the strains ability to make isobutanol. To do this, additional isobutanol pathway genes were supplied on E. co/z ^' -yeast shuttle vectors (pYZ090AalsS and pBP915, described below). One clone, strain MATa ura3A::loxP his3Apdc6ApdclA::P[PDCl]-DHAD|ilvD_Sm-PDClt-pUC19-loxP-kanMX -loxP-P [FBA1]- ALS|alsS_Bs-CYClt pdc5A::P[PDC5]-ADH|sadB_Ax-PDC5t gpd2A::loxP fra2AadhlA::UAS(PGKl)P[FBAl]-kivD_Ll(y)-ADHlt was designated as PNY2204. The plasmid-free parent strain was designated PNY2204. The PNY2204 locus (pdclA::ilvD::pUC19-kan::FBA-alsS::TRXl) is depicted in FIG. 3.

Isobutanol Pathway Plasmids (pYZ090AalsS and pBP915)

[0273] pYZ090 (SEQ ID NO:592) was digested with Spel and Notl to remove most of the CUP1 promoter and all of the alsS coding sequence and CYC terminator. The vector was then self-ligated after treatment with Klenow fragment and transformed into E. coli Stbl3 cells, selecting for ampicillin resistance. Removal of the DNA region was confirmed for two independent clones by DNA sequencing across the ligation junction by PCR using primer N191 (SEQ ID NO:593). The resulting plasmid was named pYZ090AalsS (SEQ ID NO:594).

[0274] pBP915 was constructed from pLH468 (SEQ ID NO:595) by deleting the kivD gene and 957 base pairs of the TDH3 promoter upstream of kivD. pLH468 was digested with Swal and the large fragment (12,896 bp) was purified on an agarose gel followed by a Gel Extraction kit (Qiagen, Valencia, CA). The isolated fragment of DNA was self-ligated with T4 DNA ligase and used to transform electrocompetent TOP 10 E. coli (Invitrogen; Carlsbad, CA). Plasmids from transformants were isolated and checked for the proper deletion by restriction analysis with the Swal restriction enzyme. Isolates were also sequenced across the deletion site with primers oBP556 (SEQ ID NO:596) and oBP561 (SEQ ID NO:597). A clone with the proper deletion was designated pBP915 (pLH468AkivD) (SEQ ID NO:598).

Example 2: Expression of DHAD with a Weak Promoter for Isobutanol Production in Yeast

[0275] The use of dihydroxy-acid dehydratase (DHAD) enzymes, such as IlvD from Streptococcus mutans, for isobutanol production in yeast has been previously described, for example, in U.S. Application Publication No. 2012/0237988, incorporated herein by reference. In this example, yeast strain PNY2204 [MATa ura3A::loxP his3A pdc6A pdclA::P[PDCl]-DHAD|ilvD_Sm-PDClt-pUC19-loxP-kanMX-loxP-P[FB Al]- ALS|alsS_Bs-CYClt pdc5A::P[PDC5]-ADH|sadB_Ax-PDC5t gpd2A::loxP fra2A adhlA::UAS(PGKl)P[FBAl]-kivD_Ll(y)-ADHlt, described above] was used as a starting point to determine the growth rate and isobutanol production rate of a strain with low levels of DHAD expression.

[0276] First, to create a control strain, yeast strain PNY2204 was transformed with plasmid pHR81 Ilv5p-K9G9 containing a KARI variant (SEQ ID NO:577). Transformants were selected on SE (-Ura) plates. The resulting strain was designated as PNY2204(K9G9). Next, plasmid pRS423 FBAp-IlvD(sm) GPMp-ADH (SEQ ID NO:578) was transformed into strain PNY2204 (K9G9) to complete the isobutanol pathway. On the plasmid pRS423 FBAp- IlvD(sm) GPMp-ADH, the DHAD gene is under the control of a strong FBA promoter. In addition, this plasmid also contained an additional ADH gene under the control of the GPM promoter to ensure a high level of activity for the downstream isobutanol pathway. Transformants were selected on agar plates with SE (-Ura -His) medium. This strain grew well on 2% glucose and produced significant amounts of isobutanol.

[0277] To create a strain with low levels of DHAD expression, strain PNY2204 (K9G9) was transformed with plasmid pRS423 Leu2p(75)-IlvD(Sm) GPMp-ADH (SEQ ID NO:579). This plasmid is identical to plasmid pRS423 FBAp-IlvD(sm) GPMp-ADH, described above, however the DHAD gene is under the control of a weak, truncated Leu2 promoter containing only 75 base pairs upstream of the ATG start site of the Leu2 coding region (SEQ ID NO:575). Transformants were selected on SE (-Ura -His) medium. The transformants obtained grew poorly on 2% glucose and made less isobutanol (lower titer) as compared to the strains containing plasmid pRS423 FBAp-IlvD(sm). This result indicated that a rate- limiting step for DHAD activity had been established using the truncated Leu2 promoter for expression.

Example 3 : Construction and Screening of DHAD Mutant Libraries

[0278] Typically, pyruvate decarboxylase (PDC) deletion strains have reduced growth in the presence of 2% glucose in growth medium, especially under oxygen-limiting conditions. However, the introduction of an isobutanol pathway often results in increased growth. As shown in Example 2, DHAD enzyme activity is a rate-limiting step in the isobutanol pathway in strain PNY2204(K9G9) or PNY2204(K9D3). One way to overcome this rate-limiting step is to improve the DHAD activity through mutagenesis of its gene ilvD. Better growth and increased isobutanol production can thus be used to screen for strains with higher DHAD activity.

[0279] Mutagenesis of the ilvD gene was carried out with the GeneMorph II Random Mutagenesis kit (Agilent Technologies, Santa Clara, CA). Randomly mutagenized PCR product was ligated into the pRS423 Leu2p(75) vector (SEQ ID NO:579) using the restriction sites Spel and Notl. The ligation mixture was transformed into E. coli. Transformants were spread onto large LB plates (22cm x 23cm) supplemented with 100 μg/ml of ampicillin. About 200,000 to 300,000 colonies were obtained per plate. Colonies were scraped from the plates, and aliquots of cell suspensions were taken for plasmid preparation. The library of randomly mutagenized pRS423 Leu2p(75) plasmids was transformed into PNY2204(K9G9) yeast strain. The transformation mixture was spread onto SE (-His, -Ura) plates to obtain about 7,000 colony forming units (CFUs) per plate. When colonies were visible following incubation, cells were scraped off the plates into SE broth. The yeast cells were allowed to grow in serum bottles containing SD liquid medium (2% glucose, -His, -Ura). After three days, 10 ml of the culture was transferred into new serum bottles with fresh SD medium to enrich the population that can grow on 2% glucose. Three passages later, an aliquot of culture was spread onto plates with YPDE medium (YPD with 0.1% ethanol) and were allowed to grow under anaerobic conditions to select individual colonies. Colonies that grew well were selected and patched onto YPDE plates. Plasmids from these strains were isolated with a yeast plasmid isolation kit (Zymoprep II Yeast Plasmid Mini Prep, Zymo Research, Orange, CA) and transformed into E. coli for plasmid isolation. The ilvD genes on the plasmids were sequenced to determine the sites of any mutations.

[0280] As described above, strains containing a wild type DHAD enzyme from Streptococcus mutans under the control of a truncated Leu2 promoter grew poorly in medium containing 2% glucose. It was expected that strains that grew well under anaerobic conditions would contain DHAD enzymes with mutations that result in increased activity. Sequencing of the mutant plasmids obtained from the above-described screen resulted in 15 DHAD variants, listed in Table 7:

Table 7: Streptococcus mutans DHAD Variants

[0281] Many of the variants were isolated multiple times from the screen. In addition to the mutations leading to the amino acid substitutions listed in Table 7, many of the isolates also contained silent mutations. DHAD variants, the number of isolates obtained of each variant, the locations of the silent mutations in each isolate, as well as the isobutanol titer obtained from each isolate are listed in Table 8: Table 8: Isolates of each Streptococcus mutans DHAD Variant

[0282] Common mutations were clustered around the amino acids at the 562, 563, 564 positions near the C-terminus. Numerous isolates with substitutions at Trp-563 were obtained. As shown in Table 8, these results suggest that the mutations obtained in these amino acids improve DHAD function and therefore, result in increased growth on 2% glucose and increased isobutanol production. When isobutanol production was measured, the titer in these strains doubled as compared to the strain with the wild type DHAD under the truncated Leu2 promoter ("Control" in Table 8). The results obtained here successfully demonstrated the utility of a screening method employed for identification of desirable mutations in the DHAD enzyme.

[0283] Two isolates were obtained that had increased growth on 2% glucose and increased isobutanol production, but did not contain an amino acid substitution. However, both of these isolates contained silent mutations. The nucleic acid sequence of the first of these isolates is represented by SEQ ID NO:573. This Streptococcus mutans ilvD isolate has a silent mutation at proline 228 (CCG to CCA), and resulted in an isobutanol titer of 46.8 mM. The nucleic acid sequence of the second of these isolates is represented by SEQ ID NO:574. This Streptococcus mutans ilvD isolate has a silent mutation at glycine 93 (GGA to GGT), isoleucine 155 (ATT to ATC), and aspartic acid 511 (GAC to GAT), and resulted in an isobutanol titer of 47.0 mM. [0284] While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

[0285] All publications, patents and patent applications mentioned in this specification are indicative of the level of skill of those skilled in the art to which this invention pertains, and are herein incorporated by reference to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference.

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