CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent Application Serial No. 61/441,939, filed February 11, 2011. BACKGROUND

Isobutyric acid (also referred to herein as "isobutyrate") is used in the production of fibers, resins, plastics, and dyestuffs, and is used as an intermediate in the manufacture of pharmaceuticals, cosmetics, and food additives. Isobutyrate also can be further converted to methacrylate (i.e., methacrylic acid - MAA) which is a commodity chemical.

MAA is often esterified to MMA (methyl methacrylate), a major commodity used in the production of plastics. MMA is often used to produce polymethyl methacrylate plastics, but also is used to produce, for example, ethylene methacrylate (EMA), butyl methacrylate (BMA), acrylic acid dope, adhesives, ion exchange resin, leather treatment chemicals, lubrication additives, and crosslinking agent. There are many routes to making MAA via traditional chemical synthesis techniques. Most routes begin with either natural gas or crude oil as the feedstock.

There is a need for new methods of producing commodity chemicals from renewable feedstocks. Producing commodity chemicals from renewable materials reduces the likelihood of economic impact from exhauting non-renewable feedstock materials and can spur economic development providing the renewable feedstocks.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a recombinant microbial cell modified to exhibit increased biosynthesis of isobutyic acid compared to a wild type control. In some cases, the recombinant microbial cell is a fungal cell such as, for example, a member of the

Saccharomycetaceae family such as, for example, Saccharomyces cerevisiae. In other cases, the recombinant cell can be a bacterial cell such as, for example, a member of the phylum Protobacteria such as, for example, a member of the Enterobacteriaceae family (e.g., Escherichia coli) or a member of the Pseudomonaceae family (e.g., Pseudomonas putidd). In other cases, the recombinant cell can be a bacterial cell such as, for example, a member of the phylum Firmicutes such as, for example, a member of the Bacillaceae family (e.g., Bacillus subtilis) or a member of the Streptococcaceae family (e.g., Lactococcus lactis).

In some embodiments, the recombinant microbial cell comprises at least one heterologous DNA molecule that encodes a polypeptide that catalyzes conversion of isobutyraldehyde to isobutyrate.

In some cases, the polypeptide that catalyzes conversion of isobutyraldehyde to isobutyrate comprises an aldehyde dehydrogenase such as, for example, E. coli

phenylacetaldehyde dehydrogenase (PadA), E. coli acetaldehyde dehydrogenase (AldB), E. coli 3-hydroxypropionaldehyde dehydrogenase (AldH), E. coli succinate semialdehyde dehydrogenase (GabD), E. coli γ-aminobutyraldehyde dehydrogenase (YdcW), B. ambifaria a-ketoglutaric semialdehyde dehydrogenase (KDH _ba ), or P. putida a-ketoglutaric

semialdehyde dehydrogenase (KDH _PP ).

In some embodiments, the polypeptide that catalyzes conversion of isobutyraldehyde to isobutyrate comprises a polypeptide having at least 80% amino acid sequence similarity to the amino acid sequence of any one of SEQ ID NO:l through SEQ ID NO: 106. In other embodiments, the polypeptide that catalyzes conversion of isobutyraldehyde to isobutyrate comprises a polypeptide having at least 80% amino acid sequence identity to the amino acid sequence of any one of SEQ ID NO:l through SEQ ID NO:106.

In some embodiments, the heterologous DNA molecule comprises a DNA molecule that encodes a polypeptide having at least 80% amino acid sequence similarity to the amino acid sequence of any one of SEQ ID NO:l through SEQ ID NO: 106. In other embodiments, the heterologous DNA molecule comprises a DNA molecule that encodes a polypeptide having at least 80% amino acid sequence identitity to the amino acid sequence of any one of SEQ ID NO: 1 through SEQ ID NO: 106.

In still other embodiments, the recombinant cell can include at least one heterologous DNA molecule that encodes a polypeptide that is a member of a pathway that catalyzes conversion of 2-ketovaline to isobutyrate. In of these embodiments, the polypeptide that is a member of a pathway that catalyzes conversion of 2-ketovaline to isobutyrate comprises a branched-chain keto acid dehydrogenase. In some embodiments, the polypeptide that is a member of a pathway that catalyzes conversion of 2-ketovaline to isobutyrate comprises a thioesterase. In some of these embodiments, the thioesterase can include TesA or TesB. In another aspect, the invention provides a genetically modified cell comprising at least one endogenous enzyme modified to increase its ability to convert isobutyraldehyde to isobutyrate. In some cases, the modified enzyme catalyzes the conversion of isobutyraldehyde to isobutyrate. In other cases, the modified enzyme increases the ability of the cell to tolerate an environment comprising a high level of isobutyrate.

In some embodiments of the recombinant microbial cell or genetically modified microbial cell, the cell further comprises a genetically modified version of a polypeptide that catalyzes the conversion of isobutyraldehyde to isobutanol, wherein the genetically modified version polypeptide exhibits a decrease in catalytic activity compared to the wild type polypeptide. In some cases, the genetically modified polypeptide comprises an alcohol dehydrogenase such as, for example, a polypeptide encoded by a genetically modified adhE or a genetically modified adhP. In other cases, the genetically modified polypeptide comprises an ethanolamine utilization protein such as, for example, a polypeptide encoded by a genetically modified eutG. In still other cases, the genetically modified polypeptide comprises a polypeptide encoded by a genetically modified yia Y, a genetically modified yqhD, or a genetically modified yigB.

In some embodiments of the recombinant microbial cell or genetically modified microbial cell, the cell further comprises a genetically modified version of a polypeptide that catalyzes the conversion of pyruvate to any one or more of lactate, formate, and acetate, wherein the genetically modified version polypeptide exhibits a decrease in catalytic activity compared to the wild type polypeptide. In some cases, the genetically modified polypeptide comprises a lactate dehydrogenase such as, for example, a polypeptide encoded by a genetically modified IdhA. In other cases, the genetically modified polypeptide comprises a pyruvate formate lyase I such as, for example, a polypeptide encoded by a genetically modified pflB. In other cases, the genetically modified polypeptide comprises a pyruvate oxidase such as, for example, a polypeptide encoded by a genetically modified poxB.

In some embodiments of the recombinant microbial cell or genetically modified microbial cell, the cell further comprises a genetically modified version of a polypeptide that catalyzes the conversion of acetyl-CoA to ethanol or acetyl-P, wherein the genetically modified version polypeptide exhibits a decrease in catalytic activity compared to the wild type polypeptide. In some cases, the genetically modified polypeptide comprises an alcohol dehydrogenase such as, for example, a polypeptide encoded by a genetically modified adhE. In other cases, the genetically modified polypeptide comprises a phosphate acetyltransferase such as, for example a polypeptide encoded by a genetically modified pta.

In some embodiments of the recombinant microbial cell or genetically modified microbial cell, the cell further comprises a polypeptide that catalyzes conversion of 2- ketoisovalerate to isobutyraldehyde such as, for example, a 2-ketoacid decarboxylase.

In some embodiments of the recombinant microbial cell or genetically modified microbial cell, the cell further comprises a plurality of polypeptides that sequentially catalyze conversion of pyruvate to 2-ketoisovalerate such as, for example, a dihydroxyacid

dehydratase, a ketol-acid reductoisomerase, and an acetolactate synthase.

In another aspect, the invention provides a method that includes incubating a recombinant cell oe genetically modified cell as described herein in medium that comprises a carbon source under conditions effective for the cell to produce isobutyrate, wherein the carbon source comprises one or more of: glucose, Compound 6 of FIG. 1, Compound 7 of FIG. 1, Compound 8 of FIG. 1, Compound 9 of FIG. 1, and Compound 10 of FIG. 1. In some cases, the method further includes one or more steps converting isobutyrate to another compound.

In another aspect, the invention provides a method that includes introducing into a host cell a heterologous polynucleotide encoding a polypeptide that catalyzes conversion of isobutyraldehyde to isobutyrate operably linked to a promoter so that the modified host cell catalyzes conversion of isobutyraldehyde to isobutyrate.

The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The description that follows more particularly exemplifies illustrative embodiments. In several places throughout the application, guidance is provided through lists of examples, which examples can be used in various combinations. In each instance, the recited list serves only as a representative group and should not be interpreted as an exclusive list.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1. (a) Chemical synthesis of isobutyric acid from petrochemical feedstock and its representative applications, (b) Design of a metabolic pathway for biosynthesis of isobutyric acid from renewable carbon source glucose. Enzyme "X" efficiently converts

isobutyraldehyde (10) into isobutyric acid (1). FIG. 2. Biosynthesis of isobutyric acid with the synthetic metabolic pathway, (a) Construction of two synthetic operons for gene overexpression to drive the carbon flux towards isobutyric acid, (b) Production level with different aldehyde dehydrogenases: (i) no aldehyde dehydrogenase; (ii) aldB; (iii) aldH; (iv) gabD; (v) kdh _ba ; (vi) kdh _pp ; (vii) padA; m)ydcW.

FIG. 3. The effect of deleting competing pathway on biosynthesis, (a) Endogenous alcohol dehydrogenases such as YqhD compete with PadA for isobutyraldehyde and produce byproduct isobutanol. (b) The yqhD knockout greatly increases isobutyric acid production and decreases isobutanol level.

FIG. 4. Plasmid map of pIB Al .

FIG. 5. Plasmid map of pIBA3.

FIG. 6. Isobutyrate synthetic pathway in E. coli. Abbreviations: AlsS, acetolactate synthase; IlvC, 2,3-dihydroxy-isovalerate:NADP+ oxidoreductase; IlvD, 2,3-dihydroxy- isovalerate dehydratase; KTVD, a-ketoisovalerate decarboxylase; PadA,

phenylacetaldehyde dehydrogenase.

FIG. 7. Effect of alcohol dehydrogenase knockouts on isobutyrate fermentation in shake flask. (A) Isobutyrate production in different knockout strains. (B) Isobutanol formation in corresponding knockout strains, (i) IBAl-lC, AyqhD; (ii) IBAl l-lC, AyqhD AadhE; (iii) IBA12-1C, AyqhDAadhP; (iv) IBA13-1C, AyqhDAeutG; (v) BIA14-1C, AyqhDAyiaY; (vi) IBA15- 1 C, AyqhD AyjgB.

FIG. 8. Effect of PadA expression level on isobutyrate production in shake flask. (A) Isobutyrate level in different knockout strains with two copies of PadA. (B) Corresponding isobutanol formation, (i) IBA1-2C, AyqhD; (ii) IBA13-2C, AyqhDAeutG; (iii) IBA14-2C, AyqhDAyiaY; (iv) IBA15-2C, AyqhD AyjgB.

FIG. 9. Scale-up fermentation of isobutyate by fed-batch culture in a bioreactor. (A)

50% N¾OH; IBA15-2C strain. (B) lON NaOH; IBA15-2C strain. (C) 20% Ca(OH) ₂ suspension; IBA15-2C strain. (D) 20% Ca(OH)2 suspension, IBA1-2C strain. Symbols:

closed square, biomass; closed up triangle, acetate; open circle, isobutyrate.

FIG. 10. Isobutyrate synthetic pathway in E. coli. Abbreviations: AlsS, acetolactate synthase; IlvC, 2,3-dihydroxy-isovalerate:NADP+ oxidoreductase; IlvD, 2,3-dihydroxy- isovalerate dehydratase; BKDH, branched-chain keto acid dehydrogenase; TesA, thioesterase A; TesB, thioesterase B. FIG. 11. Plasmid map of pIBA16.

FIG. 12. Plasmid map of pIBA17.

FIG. 13. Plasmid map of pIBA18. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Isobutyric acid is a platform chemical with many and varied applications. Current processes for manufacturing isobutyric acid involve the use of nonrenewable, unsustainable petroleum feedstocks and/or toxic materials. No natural organism can produce a commercially significant amount of isobutyric acid. We have, however, constructed recombinant cells that possess synthetic metabolic pathways for high-level biosynthesis of isobutyric acid from renewable feedstock such as, for example, glucose. Thus, we provide a novel route for synthesizing isobutyric acid that is not dependent on petroleum. We further provide novel recombinant microbes for synthesizing isobutyric acid.

As used in the description that follows, the term "and/or" means one or all of the listed elements or a combination of any two or more of the listed elements; the term "comprises" and variations thereof do not have a limiting meaning where these terms appear in the description and claims; unless otherwise specified, "a," "an," "the," and "at least one" are used interchangeably and mean one or more than one; and the recitations of numerical ranges by endpoints include all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).

Fossil-based resources are commonly exploited for energy and as chemical feedstocks. Due to the depletion of oil reserves, there is growing interest in exploring alternatives to petroleum-based products. Biosynthesis is a promising approach that enables sustainable production of certain fuels or certain chemicals from renewable carbon sources. (Atsumi et al., 2008 Nature 451:86-89; Steen et al, 2010 Nature 463:559-562; Causey et al, 2003 Proc. Natl. Acad. Sci. USA 100:825-832; Lin et al., 2005 Metab. Eng. 7:116-127; Zeng and Biebl, 2002 Adv. Biochem. Eng. Biotechnol. 2002:239-259; Alper et al., 2005 Nat. Biotechnol. 23:612-616). One challenge is that many useful chemicals are not naturally produced by biological systems. Therefore, it is often necessary to design or evolve novel metabolic pathways for the production of non-natural metabolites. (Zha et al., 2004 J. Am. Chem. Soc. 126:4534-4535; Zhang et al., 2008 Proc. Natl. Acad. Sci. USA 105:20653-20658; Yan et al, 2005 Appl. Environ. Microbiol. 71:3617-3623; Zhang et al., 2010 Proc. Natl. Acad. Sci. USA 107:6234-6239). Here we report the development of a biosynthetic route for producing isobutyric acid.

Isobutyric acid (FIG. 1(a), Compound 1) is a useful platform chemical. It can be converted to methacrylic acid (FIG. 1(a), Compound 2) by catalytic oxidative

dehydrogenation. (Millet, 1998 Catal. Rev.-Sci. Eng. 40:1-38). Ester of methacrylic acid, methyl methacrylate, is produced in the quantity of 2.2 million tons per year for the synthesis of poly(methyl methacrylate). (Nagai, 2001 Appl. Catal. A-Gen. 221:367-377). Isobutyric acid also can be used to manufacture sucrose acetate isobutyrate (FIG. 1(a), Compound 3), an emulsifier that is used in printing inks, automotive paints, and beverage additives with a market size of 100,000 tons annually. (Godshall, "Sustainability of the Sugar and Sugar-

Ethanol Industries," ACS Symposium Series 1058; American Chemical Society: Washington, DC, 2010; pp 253-268). Another application of isobutyric acid is for the synthesis of 2,2,4- trimethyl-l,3-pentanediol monoisobutyrate (FIG. 1(a), Compound 4; TEXANOL, Eastman Chemical Co., Kingsport, TN) or diisobutyrate (TXIB). TXIB is a non-phthalate plasticizer and TEXANOL is a commonly used coalescent. ("Screening Information Data Set (SIDS) for High Production Volume Chemicals," Organization for Economic Cooperation and

Development 2005). Other exemplary applications of isobutyric acid include preparation of isopropyl ketones such as isobutyrone (FIG. 1(a), Compound 5) by decarboxylative coupling (see, e.g., , U.S. Patent 4,754,074).

One current manufacturing process of isobutyric acid involves an acid-catalyzed Koch carbonylation of propylene (FIG. 1(a); see, e.g., U.S. Patent 4,452,999). This chemical process produces at least two concerns. First, propylene, the starting material, is produced by cracking larger hydrocarbon molecules that are most commonly derived from non-renewable resources such as petroleum and natural gas, whose long-term sustainable supply is not guaranteed. Second, the use of carbon monoxide and hydrogen fluoride may cause

environmental damage. Such problems could be alleviated by replacing chemical synthesis with microbial biosynthesis.

While there are some bacteria that can overproduce butyric acid (Liu et al, 2006 Enzyme Microb. Technol. 38:521-528), no natural organism is known to produce a commercially useful amount of isobutyric acid. We have developed a synthetic metabolic pathway that is based on the natural metabolic route for generating isobutyraldehyde from, for example, glucose. The natural metabolic pathway is augmented with at least one engineered metabolic step that diverts this natural metabolic pathway toward the production of isobutyrate (e.g., FIG. 1(b) and FIG. 10).

In one engineered pathway, shown in FIG. 1(b), glucose is metabolized to pyruvate (Compound 6) through glycolysis. Pyruvate is then converted into 2-ketovaline (Compound 9) by valine biosynthetic enzymes AlsS, IlvC, and IlvD. (Atsumi et al., 2008 Nature 451:86- 89). 2-Ketovaline can be decarboxylated into isobutyraldehyde by Ehrlich pathway enzyme 2- ketoacid decarboxylase (KIVD) from Lactococcus lactis. (de la Plaza et al, 2004 FEMS Microbiol. Lett. 238:367-74). For this synthetic pathway, we needed to identify an enzyme, indicated in FIG. 1(b) as "X", that could effectively catalyze the conversion of

isobutyraldehyde into isobutyrate.

We identified enzymes capable of catalyzing the oxidation of isobutyraldehyde to isobutyrate even though isobutyraldehyde was a known natural or experimental substrate for none of the enzymes. We chose seven aldehyde dehydrogenases as possible candidate enzymes: E. coli acetaldehyde dehydrogenase AldB, E. coli 3-hydroxypropionaldehyde dehydrogenase AldH, E. coli succinate semialdehyde dehydrogenase GabD, E. coli phenylacetaldehyde dehydrogenase PadA, E. coli γ-aminobutyraldehyde dehydrogenase YdcW, Burkholderia ambifaria a-ketoglutaric semialdehyde dehydrogenase (ΚΌ¾ _Ε ), and Pseudomonas putida KT2440 a-ketoglutaric semialdehyde dehydrogenase (KDH _PP ). These enzymes share little homology and cover a wide range of aldehyde substrates, although the wide range of aldehyde substrates does not include isobutyraldehyde. An oligonucleotide encoding one of the aldehyde dehydrogenases was cloned after KTVD to build an expression cassette kivd-x on a high copy plasmid (FIG. 2(a), with X representing the aldyhyde dehydrogenase-encoding oligonucleotide). Another operon on a medium copy plasmid in the transcriptional order UvD-alsS (FIG. 2(a)) was also constructed to drive the carbon flux towards 2-ketovaline (ilvC was not cloned since the chromosomal copy could be

overexpressed upon induction by its substrate acetolactate; Wek and Hatfield, 1988 Mol. Biol. 203:643-663). This was repeated for each of the aldehyde dehydrogenases, resulting in a library of expression cassettes, each expressing one of the aldehyde dehydrogenases.

The cloned plasmids were transformed into wild type E. coli strain BW25113. Shake flask fermentation was performed at 30°C for 48 hours. Cultures were grown in M9 minimal medium containing 40 g/L glucose as carbon source, and 0.1 mM IPTG was added to induce protein expression. Fermentation products were quantified by HPLC analysis with refractive index detection. As can be seen from FIG. 2(b), the aldehyde dehydrogenases provided varying levels of isobutyrate production. Without any plasmid-encoded aldehyde

dehydrogenase, 1.3 g/L isobutyrate was detected (i, FIG. 2(b)), which should come from the function of endogenous aldehyde dehydrogenases. GabD, Kdh _ba , dh _pp and YdcW slightly increased the production level of isobutyrate (FIG. 2(b) iv, v, vi, and viii, respectively). In comparison, transformants possessing AldB and AldH produced 2.3 g/L and 3.8 g/L isobutyrate (FIG. 2(b) ii, iii, respectively). Transformants possessing PadA produced 4.8 g/L isobutyrate (FIG. 2(b), vii).

Because it produced the greatest amount of isobutyrate, we selected PadA for further study. To characterize the enzyme, PadA was tagged with an N-terminal 6xHis-tag, overexpressed, and purified through a Ni-NTA column. The Idnetic parameters for conversion of isobutyraldehyde by PadA were determined by measuring the reduction of NAD+ to NADH at 340 nm. Results are shown in Table 1. PadA activity toward isobutyraldehyde is much lower than that toward its natural physiological substrate phenylacetaldehyde. Though the r _cat value is only 4-fold lower (1494 rnin ^"1 versus 5810 mm ¹ ), the K _m value is 230-fold higher (2.67 mM versus 0.0116 mM). Thus the specificity constant k _cgt /K _m of PadA towards phenylacetaldehyde is almost 1000-fold higher than towards the non-natural substrate isobutyraldehyde. Table 1. Kinetic parameters of PadA.

Substrate Wmirr K K _m fmM- ¹ .min- J

Isobutyraldehyde 2.67±0.17 1494±30 560

Phenylacetaldehyde ^* 0.0116 5810 501 ,000

25

Since PadA has a relatively high K _m for isobutyraldehyde, endogenous alcohol dehydrogenases such as YqhD (K _m 1.8 mM; Atsumi et al, 2010 Appl. Microbiol. Biotechnol. 85:651-657) can compete for the aldehyde substrate and produce isobutanol rather than isobutryric acid (FIG. 3(a)). This may explain, in part, the accumulation of 4.8 g/L isobutanol in the fermentation product, equal to the concentration of isobutyrate (FIG. 3(b)). We deleted the yqhD gene from the chromosome of BW25113. Compared to the wild type strain, the AyqhD mutant decreased the isobutanol production to 0.8 g/L and increased the isobutyrate production to 11.7 g/L (Figure 3(b)). Thus, in shake flask fermentation this modified strain can produce isobutyrate with a yield of 0.29 g/g glucose (FIG. 3(b)) which is 59% of the theoretical maximum.

Thus, we have developed a synthetic metabolic pathway for biosynthesis of isobutyrate from glucose. We discovered that each of the seven aldehyde dehydrogenases we investigated converted isobutyraldehyde to isobutyrate. Of these seven aldehyde

dehydrogenases, PadA was the most effective enzyme in oxidizing isobutyraldehyde to isobutyrate in vivo. Deleting from chromosome the yqhD gene, which encodes an enzyme that competes with PadA for isobutyraldehyde, further increased isobutyrate production to 11.7 g/L from 40 g/L glucose.

In an alternate engineered, pathway, shown in FIG. 10, 2-ketovaline (Compound 9) is converted to isobutyrate (Compound 1). The branched-chain keto acid dehydrogenase BKDH can convert 2-ketovaline to a branched-chain CoA, which can, in turn, be converted to isobutyrate by a thioesterase.

We cloned bkdh from Pseudomonas putida genomic DNA, and tesA and tesB from wild type E. coli genomic DNA. Plasmid pIBA16 contains bkdh, pIBA17 contains bkdh and TesA, and pIBA18 contains bkdh and TesB. The construct that includes BKDH without TesB or TesA (pIBA16) accumulated isobutyrate to a concentration of 5.61 ± 0.67 g/L (Table 4), somewhat higher than the isobutyrate production exhibited by the PadA construct prior to knocking out of yqhD (FIG. 3(a)). The addition of a thioesterase, however, further increased the isobutyrate yield. For example, BKDH plus TesB (pIBA18) produced 8.6 g/L isobutyrate from 40 g/L glucose (0.22 g/g glucose, or about 44% of the theoretical maximum yield).

Consequently, we have demonstrated various modified metabolic routes to achieve isobutyrate biosynthesis. Moreover, we have demonstrated that one can achieve efficient isobutyrate biosynthesis by diverting biosynthesis from various points along an endogenous biosynthetic pathway that does not natively produce isobutyrate. We have, therefore, established a general platform for biosynthesis of isobutyrate. Effect of knockouts on isobutyrate production

We next built on our intial findings by investigating whether further engineering could further increase carbon yield. We constructed six E. coli knockout strains and found that one double knockout (AyqhD, AyjgB) produced 17% more isobutyrate than the single knockout strain (AyqhD). We then introduced an additional copy of aldehyde dehydrogenase under a constitutive promoter on a plasmid. PadA overexpression further reduced isobutanol formation and further increased isobutyrate production. Thus we were successful in constructing an engineered strain that has an isobutyate yield of 0.39 g/g glucose, 80% of the theoretical maximum.

We also scaled up the fermentation process from shake flasks to a bioreactor. We found that Ca(OH) ₂ was much better than N¾OH or NaOH as the base for pH adjustment during fermentation. The use of Ca(OH) ₂ to maintain the pH of the fermentation culture increases cell density, decreases acetate accumulation, and increased the final accumulation of isobutyrate to 90 g/L.

In the engineered biosynthetic pathway illustrated in in FIG. 1(b) and FIG. 6, isobutyraldehyde is the immediate precursor to isobutyrate. In many organisms,

isobutyraldehyde is naturally reduced to isobutanol by an endogenous alcohol dehydrogenase such as, for example, AdhE, AdhP, EutG, YiaY, YjgB and YqhD in E. coli. YqhD is known to be involved in isobutanol formation since yqhD knockouts can exhibit a 50% increase in isobutyrate production. (Zhang et al., 2011 ChemSusChem 4:1068-1070). However, even after yqhD knockout, isobutanol was still present as a fermentation byproduct with a concentration of 0.8 g/L (FIG. 7B, i). Therefore, we investigated whether knockouts of other alcohol dehydrogenase genes - in combination with a deletion of yqhD - can decrease conversion of isobutyraldehyde to isobutanol and thus increase the amount of

isobutyraldehyde available to the cell for conversion to isobutyrate. The additional deletion of adhE or adhP slightly increased isobutanol accumulation to 0.90 g/L (FIG. 7B, ii and iii, respectively), while isobutyrate production was not affected (FIG. 7A, ii and iii, respectively). Knocking out eutG decreased isobutanol level to 0.76 g/L and increased isobutyrate concentration to 12.2 g/L (FIG. 7B, iv and FIG. 7 A, iv, respectively). Interestingly, while the additional deletion of either yiaY or yjgB did not reduce isobutanol formation (FIG. 7B, v and vi, respectively), they increased isobutyrate production level to 12.4 g/L and 12.9 g/L (FIG. 7A, v and vi, respectively). Thus, compared to IBAl-lC strain (AyqhD, i), IBA15-1C strain (AyqhD, AyjgB, vi) exhibited an increase in isobutyrate production. The results in AadhE strain were different from a recently published report (Trinh et al., 2011 Appl. Environ.

Microbiol. 77:4894-4904). However, that study used anaerobic condition to investigate the function of adhE for isobutanol production, while our fermentation condition was

semianaerobic. AdhE enzyme is known to be inactivated by oxygen (Holland-Staley et al., 2000 J. Bacteriol. 182:6049).

Effect of PadA expression level on isobutyrate production

Next we examined an alternative approach to directing conversion of isobutyraldehyde to isobutyrate that involved increasing the protein expression level of PadA to be more competitive against alcohol dehydrogenases that convert isobutyraldehyde to isobutanol. An additional copy of padA was introduced under a constitutive promoter on a high copy plasmid. We added the second copy of padA to the single knockout strain IBAl (AyqhD), and to double knockout strains IBA13 (AyqhD AeutG), IBA14 (AyqhDAyiaY), and IBA15

(AyqhD AyjgB), each of which produced more isobutyrate than the single knockout IBAl strain when carrying one copy of padA (FIG. 7A).

The double-PadA strain IBA1-2C produced 13.7 g/L isobutyrate (FIG. 8A, i) as compared to 11 g/L from the single-PadA parental strain, IBAl-lC (FIG. 7 A, i). On the other hand, isobutanol concentration was reduced to 0.35 g/L (FIG. 8B, i) from 0.82 g/L (FIG. 7B, i). These results demonstrate that increasing expression of PadA decreases isobutanol accumulation and increases isobutyrate production. The isobutanol decrease (0.47 g/L) was less than the isobutyrate increase (2.7 g/L). One possible reason for the disparity in the difference may be that producing isobutyrate is less stressful on the cells than producing isobutanol so that the cells containing two copies of padA produced smaller amounts of byproducts and therefore direct more biosynthetic energy toward production of isobutyrate. For example, accumulation of acetate, another byproduct, also was reduced: from 0.6 g/L in IBAl-lC to 0.1 g/L in IBA1-2C.

The effect of PadA overexpression was confirmed in IBA13-2C, IBA14-2C, and IBA15-2C strains as well. With two copies of padA, these strains generated around 0.4 g/L isobutanol (FIG. 8B, ii-iv), significantly lower than the strains carrying one copy of PadA (FIG. 7B, iv-vi). More importantly, IBA13-2C, IBA14-2C, and IBA15-2C also increased accumulation of isobutyrate (14.3 g/L, 14.6 g/L, and 15.6 g/L (FIG. 8A, ii-iv), respectively) compared to their respective single-PadA parental strains (FIG. 7A, iv-vi).

Moreover, isobutyrate accumulation in the PadA overexpressing double knockouts IBA-13-2C, EBA14-2C, and IBA15-2C were higher than the isobutyrate accumulation in the PadA overexpressing single (Ayqh) knockout in IBA1-2C, confirming that double knockouts increase isobutyrate production.

Thus, one can engineer microbes to favor production of isobutyrate rather than isobutanol by overexpressing PadA, which favors conversion of isobutyraldehyde to isobutyrate, and/or knocking out one or more aldehyde dehydrogenases that can compete with PadA for isobutyraldehyde but favor conversion of isobutyraldehyde to isobutanol. The PadA overexpressing double knockout (Ayqh, AyjgB) strain IBA15-2C yielded 0.39 g/g glucose, 80% of the theoretical maximum.

Optimize the fermentation conditions in a fed-batch bioreactor

We next investigated whether the effects described above are maintained when fermentation is scaled up from a shake flask to a bioreactor. We performed bioreactor fermentation experiments with the strain IBA15-2C. To avoid overaccumulation of acetate in the bioreactor, the glucose feeding rate was adjusted to keep glucose at a level below 10 g/L.

Since two molecules of NADH are generated for each molecule of isobutyrate produced, the dissolved oxygen (DO) level was maintained at 10% to burn excess NADH. Higher DO levels were avoided in order to prevent excessive oxidation of substrate into C0 ₂ through the TCA cycle.

During biosynthesis of isobutyrate, pH can drop sharply if base is not added to the fermentation culture medium. We investigated the effect of three different bases, NH OH, NaOH, and Ca(OH) ₂ to maintain a pH of 7.0. As seen in FIG. 9A-C (closed square), for all conditions, the biomass increased exponentially at the first 20 hours, and then decreased gradually. The maximum biomass obtained using either NH ₄ OH or NaOH was about 7.5 g/L, whereas the maximum biomass obtained Ca(OH) ₂ was about 10 g/L. This result suggests that excessive ammonium ion or sodium ion might have a negative impact on cell growth.

Ammonium hydroxide has previously been used to control pH and provide a supply a source of nitrogen, but this base apparently is less than optimal for maximizing isobutyrate production. Isobutyrate accumulation reached 51.1 g/L after 140 hours using NH ₄ OH (FIG. 9A, open circle), 65.4 g/L with NaOH (FIG. 9B, open circle), and 90.3 g/L with Ca(OH) ₂ (FIG. 9C, open circle). Generally, the final accumulation of isobutyrate was inversely related to the final accumulation of acetate in each culture: the NHUOH-adjusted culture accumulated 12.6 g/L acetate, while acetate decreased to 7.1 g/L in the NaOH-adjusted culture and only 3.4 g/L in the Ca(OH) ₂ -adjusted culture (FIG. 9A-C, closed triangle). This is consistent with previous reports that acetate was a major inhibitor of E. coli fermentation (Eiteman and Altaian, 2006 Trends Biotechnol. 24:530-536; Koh et al., 1992 Biotechnol. Lett. 14:1115- 1118). In summary, using Ca(OH) ₂ to maintain a culture pH of 7.0 increased cell density, increased isobutyrate accumulation and decreased acetate byproduct compared to the use of Ν¾ΟΗ or NaOH.

As a control, the fermentation of the PadA overexpressing single gene (yqhD) knockout strain IBA1-2C in a bioreactor was also investigated. This strain produced 57.6 g/L isobutyrate and 1.0 g/L acetate after 122 hours (FIG. 9D), confirming that calcium hydroxide helped decrease acetate formation and increase isobutyrate production. However, IBA15-2C strain produced 57% more isobutyrate than IBA1-2C strain under the same conditions, which suggests that the increased isobutyrate production observed shake flask cultures of the AyqhD/AygfB double knockout can be scaled up to bioreactor volumes.

This work demonstrates that isobutyrate can be produced from engineered microbes with a high accumulation and high yield. Since the production of isobutyrate described in this work is amenable to microbial fermentation, the modified microbial strains and the methods described herein can provide a new platform for commercial production of isobutyrate.

We have, therefore, developed a platform for producing isobutyrate in a renewable fashion. We have addressed problems associated with chemical synthesis such as the use of unsustainable petroleum feedstocks and toxic materials. Our biosynthetic approach provides an attractive option for the benefit of both economy and environment (Dale, 2003 J. Chem. Technol. Biotechnol. 78:1093-1103).

Thus, in one aspect, the invention provides a recombinant microbial cell modified to exhibit increased biosynthesis of isobutyic acid compared to a wild type control. As used herein, "increased production" can be characterized as a relative increase in biosynthesis of isobutyrate compared to a wild type contol, as biosynthesis sufficient for a culture of the microbial cell to accumulate isobutyrate to a predetermined concentration, as an increase in the ratio of isobutyratedsobutanol produced by the cell, or as an increase in the percentage of maximum theoretical yield using a specified reference feedstock such as, e.g., glucose.

Specifying a reference feedstock such as glucose does not require that the microbial culture be grown using the specified reference feedstock as a carbon source or energy source. Indeed, as described in more detail below, feedstock can include, for example, any of

Compounds 6-9 shown in FIG. 1(b). Those of ordinary skill in the art, however, are able to arithmetically convert a theoretical maximum yield using any alternative feedstock to a corresponding theoretical maximum yield based on a metabolically equivalent amount of any reference feedstock.

Thus, in some cases, a modified microbial cell can exhibit an increase in biosynthsis of isobutyrate that reflects at least 110%, at least 125%, at least 150%, at least 175%, at least 200% (two-fold), at least 250%, at least 300% (three-fold), at least 400% (four-fold), at least 500% (five-fold), at least 600% (six-fold), at least 700% (seven-fold), at least 800% (eightfold), at least 900% (nine-fold), at least 1000% (10-fold), at least 2000% (20-fold), at least 3000% (30-fold), at least 4000% (40-fold), at least 5000% (50-fold), at least 6000% (60-fold), at least 7000% (70-fold), at least 8000% (80-fold), at least 9000% (90-fold), at least 10,000% (100-fold), or at least 100,000%) (1000-fold) of the isobutyrate produced by an appropriate wild type control, up to and including the fold increase necessary for a given host cell to produce isobutyrate at the theoretical maximum of 0.49 g isobutyrate/g glucose.

In other cases, a modified microbial cell can exhibit an increase in the biosynthesis of isobutyrate reflected by accumulation of isobutyrate to a predetermined concentration when the microbial cell is grownfor a specified time in culture. The predetermined concentration may be any predetermined concentration of isobutyrate suitable for a given application. Thus, a predetermined concentration may be, for example, a concentration of at least 0.1 g/L such as, for example, at least 0.5 g/L, at least 1.0 g/L, at least 2.0 g/L, at least 3.0 g/L, at least 4.0 g/L, at least 5.0 g/L, at least 6.0 g/L, at least 7.0 g/L, at least 8.0 g/L, at least 9.0 g/L, at least 10 g/L, at least 20 g/L, at least 50 g/L, at least 55 g/L, at least 60 g/L, at least 65 g/L, at least 70 g/L, at least 75 g/L, at least 80 g/L, at least 85 g/L, at least 90 g/L, at least 95 g/L, at least 100 g/L, at least 110 g/L, at least 120 g/L, at least 130 g/L, at least 140 g/L, at least 150 g/L, at least 160 g/L, at least 170 g/L, at least 180 g/L, at least 190 g/L, or at least 200 g/L.

In batch culture, the specified time may have a minimum of at least 12 hours such as, for example, at least 24 hours, at least 36 hours, at least 48 hours, at least 60 hours, at least 72 hours, at least 84 hours, at least 96 hours, at least 108 hours, at least 120 hours, at least 132 hours, or at least 144 hours. In batch culture, the specified time may have a miximum of no more than 240 hours such as no more than 216 hours, no more than 192 hours, no more than 168 hours, no more than 144 hours, no more than 120 hours, no more than 108 hours, no more than 96 hours, no more than 84 hours, no more than 72 hours, no more than 60 hours, or no more than 48 hours. In batch culture, the specified time also may be expressed as a range with endpoints defined by any minimum time and any appropriate maximum time. In continuous culture, the specified time may be expressed as an absolute amount of time in the same way as for a batch culture. Alternatively, the specified time in continuous culture may be expressed in terms of a stage of the culture such as, for example, homeostasis.

In certain embodiments, therefore, a modified cell can exhibit an incrtease in the biosynthesis of isobutyrate that can be characterized as producing at least 4.7 g/L isobutyrate after 48 hours. In other exemplary embodiments, the increase in isobutyrate production may be expressed in terms of accumualtiong at least 90 g/L isobutyrate after 120 hours of culture.

In other cases, a modified microbial cell can exhibit an increase in biosythesis of isobutyrate that is characterized in terms of the ratio of isobutyratedsobutanol produced by the cell. An increase in the biosynthesis of isobutyrate can be expressed as an

isobutyratedsobutanol ratio of at least 1:1 such as, for example, at least 2:1, at least 3:1, at least 4:1, at least 5:1, at least 6:1, at least 7:1, at least 8:1, at least 9:1, at least 10:1, at least 11:1, at least 12:1, at least 13:1, at least 14:1, at least 15:1, at least 20:1, at least 25:1, at least 30:1, at least 50:1, at least 60:1, at least 70:1, at least 80:1, at least 90:1, or at least 100:1.

In still other cases, a modified microbial cell can exhibit an increase in biosythesis of isobutyrate that reflects a predetermined isobutyrate yield of at least 40% of the theoretical yield from a specified reference feedstock such as, for example, glucose. The predetermined isobutyrate yield can be, for example, at least 40% of the theoretical maximum yield, at least 50% of the theoretical maximum yield, at least 60% of the theoretical maximum yield, at least 70% of the theoretical maximum yield, at least 80% of the theoretical maximum yield, at least 90% of the theoretical maximum yield, at least 95% of the theoretical maximum yield, at least 96% of the theoretical maximum yield, at least 97% of the theoretical maximum yield, at least 98% of the theoretical maximum yield, or at least 99% of the theoretical maximum yield.

Certain embodiments can produce isobutyrate at about 44%, about 59%, or about 80% of the theoretical maximum yield from glucose. The recombinant cell can be, or be derived from, any suitable microbe including, for example, a prokaryotic microbe or a eukaryotic microbe. In some embodiments, the cell can include at least one heterologous DNA molecule. As used herein, the term "or derived from" in connection with a microbe simply allows for the "host cell" to possess one or more genetic modifications before being modified to include a heterologous DNA molecule that encodes a polypeptide that is involved in an engineered biosynthetic pathways that results in

biosynthesis of isobutyrate. Thus, the term "recombinant cell" encompasses a "host cell" that may contain nucleic acid material from more than one species before having the heterologous DNA molecule that encodes a polypeptide that is involved in, for example, either the conversion of isobutyraldehyde to isobutyrate or the conversion of 2-ketovaline to isobutyrate introduced into the cell.

In some embodiments, the recombinant cell may be, or be derived from, a eukaryotic microbe such as, for example, a fungal cell. In some of these embodiments, the fungal cell may be, or be derived from, a member of the Saccharomycetaceae family such as, for example, Saccharomyces cerevisiae, a member of the genus Candida such as, for example, Candida albicans, a member of the genus Kluyvermyces, or a member of the genus Pichia such as, for example, Pichia pastoris. In other embodiments, the fungal cell may be a member of the family Dipodascaceae such as, for example, Yarrowia lipolytica.

In other embodiments, the recombinant cell may be, or be derived from, a prokaryotic microbe such as, for example, a bacterium. In some of these embodiments, the bacterium may be a member of the phylum Protobacteria. Exemplary members of the phylum Protobacteria include, for example, members of the Enterobacteriaceae family (e.g., Escherichia coli) and, for example, members of the Pseudomonaceae family (e.g., Pseudomonas putida). In other cases, the bacterium may be a member of the phylum Firmicutes. Exemplary members of the phylum Firmicutes include, for example, members of the Bacillaceae family (e.g., Bacillus subtilis) and, for example, members of the Streptococcaceae family (e.g., Lactococcus lactis).

In the description that follows, descriptions of various embodiments refer to a heterologous DNA molecule that encodes a genetic modification. Combinations of the various embodiments are also possible. In such embodiments, more than one genetic modification can be included on a single heterologous DNA molecule such as, for example, a plasmid vector. Alternatively, different genetic modifications may be included on different vactors, each opf which is introduced into the host cell. In some embodiments, the cell can include at least one heterologous DNA molecule that encodes a polypeptide that catalyzes conversion of isobutyraldehyde to isobutyrate. In some embodiments, the polypeptide that catalyzes conversion of isobutyraldehyde to isobutyrate comprises an aldehyde dehydrogenase. As used herein, the term "aldehyde dehydrogenase" refers to a polypeptide that, regardless of its common name or native function, catalyzes the conversion of isobutyraldehyde to isobutyrate. Exemplary aldehyde dehydrogenases include, for example, E. coli phenylacetaldehyde dehydrogenase (PadA), E. coli acetaldehyde dehydrogenase (AldB), E. coli 3-hydroxypropionaldehyde dehydrogenase (AldH), E. coli succinate semialdehyde dehydrogenase (GabD), E. coli γ-aminobutyraldehyde dehydrogenase (YdcW), B. ambifaria a-ketoglutaric semialdehyde dehydrogenase (KDH _ba ), or P. putida a-ketoglutaric semialdehyde dehydrogenase ( DH _PP ). In certain embodiments, the recombinant cell can include a heterologous DNA molecule - or a plurality of heterologous DNA molecules - that encodes a combination of two or more aldehyde dehydrogenases.

In other embodiments, the polypeptide encoded by the heterologous DNA molecule (i.e., the heterologously-encoded polypeptide) that catalyzes conversion of isobutyraldehyde to isobutyrate comprises, or is structurally similar to, a reference polypeptide that comprises the amino acid sequence of one or more of SEQ ID NO:l through SEQ ID NO: 106.

As used herein, a heterologously-encoded polypeptide is "structurally similar" to a reference polypeptide if the amino acid sequence of the heterologously-encoded polypeptide possesses a specified amount of similarity and/or identity compared to the reference polypeptide. Structural similarity of two polypeptides can be determined by aligning the residues of the two polypeptides (for example, a heterologously-encoded polypeptide and the polypeptide of, for example, any one of SEQ ID NO:l through SEQ ID NO: 106) to optimize the number of identical amino acids along the lengths of their sequences; gaps in either or both sequences are permitted in making the alignment in order to optimize the number of identical amino acids, although the amino acids in each sequence must nonetheless remain in their proper order.

A pair- wise comparison analysis of amino acid sequences can be carried out using the BESTFIT algorithm in the GCG package (version 10.2, Madison, WI). Alternatively, polypeptides may be compared using the Blastp program of the BLAST 2 search algorithm, as described by Tatiana et al, (1999 FEMS Microbiol Lett, 174:247-250), and available on the National Center for Biotechnology Information (NCBI) website. The default values for all BLAST 2 search parameters may be used, including matrix = BLOSUM62; open gap penalty = 11, extension gap penalty = 1, gap x_dropoff = 50, expect = 10, wordsize = 3, and filter on.

In the comparison of two amino acid sequences, structural similarity may be referred to by percent "identity" or may be referred to by percent "similarity." "Identity" refers to the presence of identical amino acids. "Similarity" refers to the presence of not only identical amino acids but also the presence of conservative substitutions. A conservative substitution for an amino acid in a polypeptide of the invention may be selected from other members of the class to which the amino acid belongs. For example, it is well-known in the art of protein biochemistry that an amino acid belonging to a grouping of amino acids having a particular size or characteristic (such as charge, hydrophobicity, and hydrophilicity) can be substituted for another amino acid without altering the activity of a protein, particularly in regions of the protein that are not directly associated with biological activity. For example, nonpolar (hydrophobic) amino acids include alanine, leucine, isoleucine, valine, proline, phenylalanine, tryptophan, and tyrosine. Polar neutral amino acids include glycine, serine, threonine, cysteine, tyrosine, asparagine and glutamine. The positively charged (basic) amino acids include arginine, lysine and histidine. The negatively charged (acidic) amino acids include aspartic acid and glutamic acid. Conservative substitutions include, for example, Lys for Arg and vice versa to maintain a positive charge; Glu for Asp and vice versa to maintain a negative charge; Ser for Thr so that a free -OH is maintained; and Gin for Asn to maintain a free -NH2. Likewise, biologically active analogs of a polypeptide containing deletions or additions of one or more contiguous or noncontiguous amino acids that do not eliminate a functional activity of the polypeptide are also contemplated.

A heterologously-encoded polypeptide can include a polypeptide with at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence similarity to the reference amino acid sequence.

A heterologously-encoded polypeptide can include a polypeptide with at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity to the reference amino acid sequence.

Exemplary reference amino acid sequences include the amino acid sequence of any one of SEQ ID NO:l through SEQ ID NO: 106.

1 WNB is the crystal structure of E. coli protein YdcW complexed with NADH and betaine aldehyde (Gruez et al. 2004 J. Mol. Biol. 343:29-41). Based on the crystal structure, residues Y150, D279, F436, and L438 are within a radius of 5 A of the a-carbon of betaine aldehyde substrate. While the homology between PadA and YdcW is low, the binding pocket is well conserved. The corresponding residues in the active site of PadA are F175, V305, T461, and 1463. Similar analyses may be performed to identify amino acids residues that may be modified without interfering with the catalytic activity of the polypeptide and, just as important, to identify amino acid residues that are likely to be involved in substrate binding and/or catalytic activity.

In some embodiments, the recombinant cell can include a heterologous DNA molecule that encodes a polypeptide having at least 80% amino acid sequence similarity to the amino acid sequence of any one of SEQ ID NO:l through SEQ ID NO: 106. Thus, exemplary heterologous DNA molecules include those that encode a polypeptide having, for example, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence similarity to the reference amino acid sequence.

In other embodiments, the heterologous DNA molecule encodes a polypeptide having at least 80% amino acid sequence identitity to the amino acid sequence of any one of SEQ ID NO:l through SEQ ID NO: 106. Thus, exemplary heterologous DNA molecules include those that encode a polypeptide having, for example, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%), at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity to the reference amino acid sequence.

A heterologously-encoded polypeptide can further be designed to provide additional sequences, such as, for example, the addition of coding sequences for added C-terminal or N- terminal amino acids that would facilitate expression or purification by trapping on columns or use of antibodies. Such tags include, for example, histidine-rich tags that allow purification of polypeptides on nickel columns. Such gene modification techniques and suitable additional sequences are well known in the molecular biology arts.

In other embodiments, a recombinant cell can include at least one heterologous DNA molecule that encodes a polypeptide that catalyzes conversion of 2-ketovaline to isobutyrate. In some embodiments, the polypeptide that catalyzes conversion of isobutyraldehyde to isobutyrate comprises a branched-chain keto acid dehydrogenase (BKDH). As used herein, the term "branched-chain keto acid dehydrogenase" refers to a polypeptide that, regardless of its common name or native function, catalyzes the conversion of 2-ketovaline to a branched- chain Co A. Exemplary branched-chain keto acid dehydrogenases can include, for example, BKDH of Pseudomonas putida. In some of these embodiments, the recombinant cell can include at least one heterologous DNA that encodes a thioeserase. As used herein, the term "thioesterase" refers to a polypeptide that, regardless of its common name or native function, catalyzes the conversion of a a branched-chain CoA to isobutyrate. Exemplary thioesterases can include, for example, TesA or TesB of E. coli.

In some embodiments, a recombinant cell can further include one or more

polypeptides that catalyze a biosynthetic conversion illustrated in FIG. 1(b). Thus, for example, the recombinant cell can further include a polypeptide that catalyzes conversion of 2-ketoisovalerate to isobutyraldehyde such as, for example, 2-ketoacid decarboxylase; or, for example, any one or more of the polypeptides that catalyze a step in the conversion of pyruvate to 2-ketoisovalerate such as, for example, one or more of: a dihydroxyacid dehydratase, a ketol-acid reductoisomerase, and an acetolactate synthase.

In another aspect, the invention provides a genetically modified cell in which at least one endogenous enzyme is modified to increase its ability to convert isobutyraldehyde to isobutyrate. In some embodiments, the genetically modified cell can include one or mutations to one or more endogenous enzymes. In other embodiments, the genetically modified cell can include one or more mutations to one or more polypeptides that increase the ability of the cells to tolerate high levels of isobutyrate in culture. The mutations may be produced using molecular biology techniques including, for example, one or more of: transcriptome analysis, genome sequencing, cloning, site-specific mutagenesis, and transformation of the microbe with a vector that includes a polynucleotide that encodes the modified enzyme or enzymes. Alternatively, the mutations may be produced using classical microbial genetic techniques such as, for example, growth in or on a medium designed to select and/or identify microbes possessing desired spontaneous mutations.

In some embodiments, the recombinant cell or genetically modified cell can further include a genetically modified version of a polypeptide that catalyzes the conversion of isobutyraldehyde to a product other than isobutyrate such as, for example, isobutanol, lactate, ethanol, or acetyl-P, and thereby directs more isobutyraldehyde toward the biosynthesis of isobutyrate. Generally, the genetically modified version polypeptide can exhibit reduced catalytic activity compared to the wild type polypeptide. Such a genetic modification can decrease the extent to which isobutyraldehyde is metabolized in a manner that results in biosynthesis of products other than isobutyrate such as, for example, isobutanol, lactate, ethanol, or acetyl-P, and thereby increase the extent to which isobutyraldehyde is converted to isobutyrate.

In some cases, the recombinant cell or genetically modified cell can include a genetically modified version of a polypeptide that catalyzes the conversion of

isobutyraldehyde to isobutanol. An exemplary polypeptide of this type can include, for example, can be a genetically modified version of an alcohol dehydrogenase such as, for example, a polypeptide encoded by a genetically modified adhE or a genetically modified adhP. In other embodiments, the genetically modified polypeptide can be genetically modified version of an ethanolamine utilization protein such as, for example, a polypeptide encoded by a genetically modified eutG. In some embodiments, the genetically modified polypeptide can be a polypeptide encoded by a genetically modified dkgA, genetically modified yia Y, a genetically modified yqhD, or a genetically modified yjgB

In some cases, the recombinant cell or genetically modified cell can include a genetically modified version of a polypeptide that catalyzes the conversion of pyruvate to any one or more of lactate, formate, and acetate, wherein the genetically modified version polypeptide exhibits a decrease in catalytic activity compared to the wild type polypeptide. An exemplary polypeptide of this type can include, for example, a genetically modified version of a lactate dehydrogenase such as, for example, a polypeptide encoded by a genetically modified IdhA; a genetically modified version of a pyruvate formate lyase I such as, for example, a polypeptide endcoded by a genetically modified pflB; or a genetically modified version of a pyruvate oxidase such as, for example, a polypeptide encoded by a genetically modified poxB. In some case, the recombinant cell or genetically modified cell can include a genetically modified version of a polypeptide that catalyzes the conversion of acetyl-CoA to ethanol or acetyl-P, wherein the genetically modified version of the polypeptide exhibits a decrease in catalytic activity compared to the wild type polypeptide. An exemplary

polypeptide of this type can include, for example, a genetically modified version of an alcohol dehydrogenase such as, for example, a polypeptide encoded by a genetically modified adhE; or a genetically modified version of a phosphate acetyltransferase such as, for example, a polypeptide encoded by a genetically modified pta.

A decrease in catalytic activity can be quantitatively measured and described as a percentage of the catalytic activity of an appropriate wild type control. The catalytic activity exhibited by a genetically modified polypeptide can be, for example, no more than 95%, no more than 90%, no more than 85%, no more than 80%, no more than 75%, no more than 70%, no more than 65%, no more than 60%, no more than 55%, no more than 50%), no more than 45%, no more than 40%, no more than 35%, no more than 30%), no more than 25%>, no more than 20%, no more than 15%, no more than 10%), no more than 5%, no more than 4%, no more than 3%, no more than 2%, no more than 1% of the activity, or 0% of the activity of a suitable wild type control.

Alternatively, a decrease in catalytic activity can be expressed as an appropriate change in a catalytic constant. For example, a decrease in catalytic activity may be expressed as at a decrease in & _C at such as, for example, at least a two-fold decrease, at least a three-fold decrease, at least a four-fold decrease, at least a five-fold decrease, at least a six-fold decrease, at least a seven-fold decrease, at least an eight-fold decrease, at least a nine-fold decrease, at least a 10-fold decrease, at least a 15 -fold decrease, or at least a 20-fold decrease in the k _cat value of the enzymatic conversion.

A decrease in catalytic activity also may be expressed in terms of an increase in K _m such as, for example, an increase in K _m of at least two-fold, at least three-fold, at least fourfold, at least five-fold, at least six-fold, at least seven-fold, at least an eight-fold, at least ninefold, at least 10-fold, at least 15-fold, at least 20-fold, at least 25-fold, at least 30-fold, at least 35-fold, at least 40-fold, at least 45-fold, at least 50-fold, at least 75-fold, at least 100-fold, at least 150-fold, at least 200-fold, at least 230-fold, at least 250-fold, at least 300-fold, at least 350-fold, or at least 400-fold. An increase in catalytic activity can be quantitatively measured and described as a percentage of the catalytic activity of an appropriate wild type control. The catalytic activity exhibited by a genetically modified polypeptide can be, for example, at least 110%, at least 125%, at least 150%, at least 175%, at least 200% (two-fold), at least 250%, at least 300% (three-fold), at least 400% (four-fold), at least 500% (five-fold), at least 600% (six-fold), at least 700% (seven-fold), at least 800% (eight-fold), at least 900% (nine-fold), at least 1000% (10-fold), at least 2000% (20-fold), at least 3000% (30-fold), at least 4000% (40-fold), at least 5000% (50-fold), at least 6000% (60-fold), at least 7000% (70-fold), at least 8000% (80-fold), at least 9000% (90-fold), at least 10,000% (100-fold), or at least 100,000% (1000-fold) of the activity of an appropriate wild type control.

Alternatively, an increase in catalytic activity may be expressed as at an increase in k _cat such as, for example, at least a two-fold increase, at least a three-fold increase, at least a fourfold increase, at least a five-fold increase, at least a six-fold increase, at least a seven-fold increase, at least an eight-fold increase, at least a nine-fold increase, at least a 10-fold increase, at least a 15-fold increase, or at least a 20-fold increase in the k _czt value of the enzymatic conversion.

An increase in catalytic activity also may be expressed in terms of a decrease in K _m such as, for example, at least a two-fold decrease, at least a three-fold decrease, at least a fourfold decrease, at least a five-fold decrease, at least a six-fold decrease, at least a seven-fold decrease, at least an eight-fold decrease, at least a nine-fold decrease, at least a 10-fold decrease, at least a 15-fold decrease, or at least a 20-fold decrease in the K _m value of the enzymatic conversion.

In another aspect, the invention provides a method that includes introducing into a host cell a heterologous polynucleotide encoding a polypeptide that catalyzes conversion of isobutyraldehyde to isobutyrate operably linked to a promoter so that the modified host cell catalyzes conversion of isobutyraldehyde to isobutyrate. In various embodiments, the method includes further introducing into the cell one or more heterologous polynucleotides that encode genetic modifications and/or polypeptides described above. Such heterologous polynucleotides can include, for example, a genetic modification that decreases biosynthetic competition for isobutyraldehyde and thereby promotes accumulation of isobutyraldehyde for subsequent conversion to isobutyrate. Such heterologous polynucleotides also may encode a polypeptide that catalyzes a step in the conversion of a carbon source substrate to

isobutyraldehyde.

In another aspect, the invention provides a method that includes incubating a recombinant cell as described herein in medium that comprises a carbon source under conditions effective for the recombinant cell to produce isobutyrate. Referring to FIG. 1(b), in some embodiments, the recombinant cell can convert glucose to isobutyrate by converting glucose to pyruvate, then converting pyruvate to isobutyraldenyde by, for example, the biosynthetic pathway illustrated in FIG. 1(b), then converting the isobutyraldehyde to isobutyrate through the activity of the heterologously-encoded polypeptide. In other embodiments, however, the isobutyraldehyde need not necessarily result from the metabolism of any particular feedstock through any particular biosynthetic pathway. For example, isobutyraldehyde may be provided directly to the recombinant cell in the culture medium. In other examples, the culture medium can include one or more intermediates of the biosynthetic pathway shown in FIG. 1(b) or any other biosynthetic pathway that produces

isobutyraldehyde or feeds into the biosynthetic pathway shown in FIG. 1(b) to produce isobutyraldehyde. Thus, in various embodiments, the carbon source can include one or more of: glucose, Compound 6 of FIG. 1(b), Compound 7 of FIG. 1(b), Compound 8 of FIG. 1(b), Compound 9 of FIG. 1(b), and Compound 10 of FIG. 1(b).

Because isobutyrate is a commodity chemical, isobutyrate synthesis can be extended to the synthesis of other compounds. For example, isobutyrate may be a starting material for the synthesis of 3 -hydroxy butyrate (Formula 5 of Reaction Scheme I, below) which can be dehydrated to produce methacrylic acid. Reaction Scheme I illustrates the synthesis of 3- hydroxy butyrate. Reaction Scheme I consists of four steps downstream of isobutyrate synthesis. Candidate genes coding enzymes for catalyzing each of these steps have been identified. Isobutyrate may be converted to isobutyryl-CoA (Formula 2 of Reaction Scheme I) by the enzyme butyryl-CoA:acetoacetate CoA-transferase (I, Reaction Scheme I) from, for example, Clostridium SB4 or Fusobacterium nucleatum. Isobutyryl-CoA may then be dehydrogenated to methylacrylyl-CoA (Formula 3 of Reaction Scheme I) by the enzyme 2- methylacyl-CoA dehydrogenase (II, Reaction Scheme I) - e.g., acdH from Streptomyces avermitilis or Acadsb from Rattus norvegicus. Methylacrylyl-CoA may then be hydrated to 3- hydroxy isobutyryl-CoA (Formula 4, Reaction Scheme I) by enoyl-CoA hydratase (III,

Reaction Scheme I) - e.g., ECHS1 from Bos taurus or ech from Pseudomonas fluoresceins. 3- hydroxy isobutyryl-CoA is converted to 3 -hydroxy butyrate by 3-hydroxyisobutyryl-CoA hydrolase (IV, Reaction Scheme I) - e.g., Hibch from Rattus norvegicus.

Reaction Scheme I

5 4

Exemplary enzymes involved in Reaction Scheme I:

I: Enzyme: butyryl-CoA:acetoacetate CoA-transferase

Species: Clostridium SB4

Species: Fusobacterium nucleatum (Entrez Gene IDs 993155, 991616, or 992527, 992528)

II: Enzyme: 2-methylacyl-CoA dehydrogenase

Gene: acdH Accession Number: G-9098 (MetaCyc) Species: Streptomyces avermitilis

Gene: Acadsb Accession Number: G-9097 (MetaCyc) Species: Rattus norvegicus

III: Enzyme: short chain enoyl-CoA hydratase

Gene: ECHS1 Accession Number: G-9101 (MetaCyc) Species: Bos taurus Enzyme: enoyl-CoA hydratase

Gene: ech Accession Number: G-9099 (MetaCyc) Species: Pseudomonas fluorescens rV: Enzyme: 3-hydroxyisobutyryl-CoA hydrolase

Gene: Hibch Accession Number: G-9102 (MetaCyc) Species: Rattus norvegicus The biosynthesis of other compounds from isobutyrate may be accomplished by co- culturing a recombinant cell described herein or a genetically modified cell described herein with a microbe that (a) can use isobutyrate as a sole carbon source and (b) possesses the metabolic ability to prodice the desired product, whether naturally or through genetic manipulation.

For example, engineered E. coli can be employed as the isobutyrate source during fermentation. To synthesize, for example, (5)-3-hydroxyisobutyrate, the engineered E. coli may be co-cultured with a strain of Pseudomonas putida (ATCC 21244) that can produce the S isomer of 3-hydroxyacid from isobutyrate with 48% yield. To synthesize the R isomer, one can co-culture the engineered E. coli with a yeast strain Candida rugosa (ATCC 10571). This species can produce 150 g/L (i?)-3-hydroxyisobutyrate with a molar conversion yield of 81.8% from isobutyrate.

Alternatively, the biosynthesis of other compounds from isobutyrate may be accomplished by further modifying a recombinant cell described herein or a genetically modified cell described herein (collectively, a "biocatalyst") by introducing isobutyrate- assimilation capability into the microbe so that a single biocatalyst is needed for

biotransformation. For example, isobutyrate may be converted into isobutyryl-CoA by acyl- CoA synthetase (Acs). Isobutyryl-CoA may then be turned into methylacrylyl-CoA by acyl- CoA dehydrogenase (AcdH). Hydration of methylacrylyl-CoA by enoyl-CoA hydratase (Ech) can generate 3-hydroxy-isobutyryl-CoA, which may be hydrolyzed into 3-hydroxyisobutyrate by 3-hydroxyisobutyryl-CoA hydrolase (Hibch). 3-hydroxyisobutyrate may be oxidized to methylmalonate-semialdehyde by 3-hydroxyisobutyrate dehydrogenase (MmsB). Finally, the aldehyde may be converted to propanoyl-CoA by methylmalonate-semialdehyde

dehydrogenase (MmsA). Propanoyl-CoA can enter central metabolism for biosynthesis to support growth. Acs, AcdH, Hibch, MmsB, and MmsA have been cloned from various organisms into E. coli and have demonstrated suitable expression levels and enzymatic activities in the new host. One can clone genes encoding such proteins and organize them into synthetic operons for optimal expression. In contrast, Ech has not been cloned into and expressed in E. coli. No study has been performed to identify and characterize this catabolic enzyme in bacteria. From KEGG pathway database, for Pseudomonas putida KT2440, 10 enzymes (PP_ 1412, PP_1845, PP_2136, PP_2217, PPJ283, PP_3284, PP_3358, PP_3491, PP_3726, PP_3732 and PP_4030) have been annotated to be Ech candidates. One can clone these proteins individually into the E. coli strain harboring other pathway enzymes and test the growth of transformants in medium with isobutyrate as the carbon source. This growth-based selection strategy can also be used to evolve any enzyme in the pathway if improved enzymatic activities in E. coli are desired. One can clone Acs, AcdH , Ech, and Hibch into the isobutyrate-producing E. coli strain. The resultant novel E. coli strain will be able to biosynthesize 3-hydroxybutyrate from glucose.

For any method disclosed herein that includes discrete steps, the steps may be conducted in any feasible order. And, as appropriate, any combination of two or more steps may be conducted simultaneously.

The present invention is illustrated by the following example. It is to be understood that the particular examples, materials, amounts, and procedures are to be interpreted broadly in accordance with the scope and spirit of the invention as set forth herein.

EXAMPLES

Example 1

1. Vector construction

All cloning procedures were carried out in the E. coli strain XLlO-gold (Stratagene, Agilent Technologies, Inc.; Santa Clara, CA). Primers (Table 2) were purchased from

Eurofms MWG Operon. PCR reactions were performed with PHUSION High-Fidelity DNA polymerase (New England Biolabs, Inc.; Ipswich, MA) according to the manufacturer's instructions. The sequences of the plasmids produced from all cloning steps were verified using restriction mapping and DNA sequencing.

A gene fragment encoding lac repressor Lacl was inserted respectively into the Sacl site of plasmid pZE12 and pZA22 (Lutz and Bujard, 1997 Nucleic Acids Res. 25:1203-1210) to yield plasmid pZElac and pZAlac. E. coli genomic DNA was amplified with primers ilvd_accfwd and ilvd_nherev. The obtained ilvD gene fragment was digested with Acc65I and Nhel. The Bacillus acetolactate synthase gene alsS was amplified from the genomic DNA of Bacillus subtilis with primers als_accremov/als_accremov_rev (to remove Acc65I site in alsS), as well as the flanking oligos als_nhefwd and als_blprev using overlap PCR. The alsS PCR product was then digested with Nhel and Blpl. Purified ilvD and alsS gene fragments were then ligated into pZAlac to create plasmid pIBAl (see FIG. 4 for plasmid map).

The 2-ketoacid decarboxylase gene kivd was amplified from the genomic DNA of

Lactococcus lactis (ATCC) using the primers kivd_accfwd and kivd_xbarev. The PCR product was digested with Acc65I and Xbal, and ligated into pZElac to yield plasmids pIBA2. Kivd was also amplified with kivd_accfwd and kivd_sphrev, and the PCR product was digested with Acc65I and SphI. YdcWv as amplified from the E. coli genomic DNA with primers ydcw_sphfwd and ydcw_xbarev, which was then digested with SphI and Xbal.

Purified kivd and ydcW gene fragments were then ligated into pZElac to create plasmid pIBA3 (see FIG. 5 for plasmid map). AldB was amplified from the E. coli genomic DNA with primers aldB_sphfwd and aldB_xbarev, digested with SphI and Xbal, and then inserted into pIBA3 to yield plasmid pIBA4. AldHwas amplified from the E. coli genomic DNA with primers aldH sphfwd and aldH_sphremov (to remove SphI site in aldH). And the PCR product was amplified again with primers aldH_sphfwd and aldB_xbarev, digested with SphI and Xbal, and then inserted into pIBA3 to yield plasmid pIBA5. Similarly, gabD was amplified with primers gabD_sphfwd and gabD_ xbarev, and padA was amplified with primers padA_sphfwd and padA_ xbarev from the E. coli genomic DNA. They were cloned into pIBA3 to yield plasmid pIBA6 and pIBA7. While kdhba was amplified from

Burkholderia ambifaria (ATCC BAA-244) with primers kdhba_sphfwd and kdhb _a _xbarev, digested and ligated into into pIBA3 to yield plasmid pIBA8. And kdh _pp was amplified from Psuedomonas putida KT2440 (ATCC 47054D-5) with primers kdh _pP _sphfwd and

kdhpp_xbarev, digested and ligated into into pIBA3 to yield plasmid pIBA9.

PadA gene fragment was amplified using primers padA_bamfwd and padA_bamrev.

After digestion with BamHI, the gene fragments were inserted into expression plasmid pQE9 (Qiagen, Inc. Valencia, CA) to yield pIBAlO. Table 2. Oligonucleotides for cloning.

2. Fermentation procedure and product analysis

Host strain

A wild type E. coli K-12 strain BW25113 (rrnBru A/ cZwj ₁₆ hsdR514 AaraBAD H33 ArhaBADi _D js) was transformed with pIBAl, and any plasmid from pIBA2 to pIBA9 for isobutyrate production.

The yqhD gene deletion strain was from the Keio collection (Baba et al., 2006 Mol. Syst. Biol. 2:10.1038). It was transformed with plasmid pCP20 to remove the kanamycin resistance marker. This AyqhD strain was transformed with pIBAl and pIBA7 for isobutyrate production.

Fermentation process

Overnight cultures incubated in LB medium were diluted 25 -fold into 5 rriL M9 medium supplemented with 0.5% yeast extract and 4% glucose in 125-ml conical flasks. Antibiotics were added appropriately (ampicillin 100 mg/L and kanamycin 25 mg/L). 0.1 mM isopropyl-b-D-thiogalactoside (IPTG) was added to induce protein expression. The culture medium was buffered by addition of 0.5 g CaC0 ₃ . Cultures were placed in a 30°C shaker (250 rpm) and incubated for 48 hours.

Fermentation products were quantified by HPLC analysis with refractive index detection using an Agilent 1100 Capillary HPLC.

3. Enzymatic assay

Protein overexpresion and purification

pIBAlO was transformed into BL-21 E. coli host harboring the pREP4 plasmid

(Qiagen; Valencia, CA). An overnight pre-culture was diluted 100-fold in 300 mL 2X YT rich medium containing 50 mg/L ampicillin, 25 mg/L kanamycin and 0.1 mM IPTG. Expression of recombinant protein was performed at 30°C overnight.

The cell pellet was sonicated in 30 mL lysis buffer (250 mM NaCl, 2 mM DTT, 5 mM imidazole and 50 mM Tris pH 8.0). The cellular debris was removed by centrifugation at

15,000 RPM for 20 minutes. The supernatant was passed through aNi-NTA column. Then the column was washed with 10 mL wash buffer (250 mM NaCl, 20 mM imidazole and 50 mM Tris pH 8.0) four times. Finally, the target protein was eluted with 10 mL elution buffer (250 mM NaCl, 250 mM imidazole and 50 mM Tris pH 8.0). The eluate was buffer- exchanged and concentrated into storage buffer (100 μΜ tris buffer, pH 8.0, and 20% glycerol) using AMICON ULTRA centrifugal filter (Millipore Corp.; Billerica, MA). Protein concentration was determined by measuring the UV absorbance at 280 nm (extinction coefficient, 75070 cm ^"1 M ^"1 ). The concentrated protein solution was aliquoted (100 μΐ) into PCR tubes and flash frozen at -80°C for long term storage.

Measurement of PadA activity

Substrate isobutyraldehyde was purchased from Fisher Scientific International, Inc. (Hampton, NH), and NAD ⁺ was from New England Biolabs, Inc., (Ipswich, MA). The reaction mixture contained 0.5 mM NAD ⁺ and 0.2-4 mM isobutyraldehyde in assay buffer (50 mM NaH ₂ P04, pH 8.0, lmM DTT) with a total volume of 80 μΐ. The reactions were started by adding 2 μΐ KTVD (final enzyme concentration 25 nM), and the generation of NADH was monitored at 340 nm (extinction coefficient, 6.22 mM ^"1 cm ^"1 ). Kinetic parameters (k _cat and K _m ) were determined by fitting initial velocity data to the Michaelis-Menten equation using Origin software.

Example 2

Bacterial strains and plasmids.

All the primers were from Eurofins MWG Operon (Huntsville, AL) and listed in Table 3. The E. coli strains used in this study were listed in Table 3, which were all derived from the wild type E. coli K-12 strain BW25113 with yqhD deletion. All cloning procedures were carried out in the E. coli strain XLlO-gold (Stratagene; Santa Clara, CA). Plasmids ρΓΒΑΙ and pIBA7 used to produce isobutyrate were from previous work (Zhang et al., 2011

ChemSusChem 4:1068-1070). To build the pIBAl plasmid 1 carrying two copies of padA, the padA gene was amplified by PCR with oligos padA_SacIfwd and padA_Saclrev, digested with Sacl and then ligated into pIBA7 to create pIBAl 1. In pIBAl 1, the additional copy of padA is in the same operon with ampicillin resistance gene bla, under the regulation of a constitutive promoter. PI phages of adhE, adhP, eutG, yiaY and yjgB were obtained from the Keio collection (Baba et al, 2006 Mol. Syst. Biol. 2:10.1038). The phages were used to transfect the IBA1 strain to construct double knockout strains. All the knockout strains were then transformed with pCP20 plasmid to remove the kanamycin marker. The correct knockouts were verified by PCR. To produce isobutyrate, each strain was transformed with plasmids pIBAl plus pIB A7, or pIBAl plus pIBAl 1.

TABLE 3. Strains, plasmids and primers used in this study

Name Relevant genotype Reference

Strains

BW25113 rrnBju AlacZ _WJ i ₆ hsdR514 Ai-haBAD _hm g Datsenko and Wanner,

2000 Proc. Natl. Acad.

Sci. U.S.A. 97:6640-6645

IBA1 BW25U3 AyqhD Zhang et al., 2011

ChemSusChem 4:1068-

1070

IBA11 BW25113 AyqhD AadhE This work

EBA12 BW25113 AyqhD AadhP This work

IBA13 BW25113 AyqhD Ae tG This work

IBA14 BW25113 AyqhD AyiaY This work

mA15 BW25113 AyqhD AygjB This work

IBAl-lC BW25113 AyqhD + pIBAl and pIBA7 This work

IBAl l-lC BW25113 AyqhD AadhE + pIBAl and pD3A7 This work

IBA12-1C BW25113 AyqhD AadhP + pIBAl and pIBA7 This work

IBA13-1C BW25113 AyqhD AeutG + pIBAl and pIBA7 This work

EBA14-1C BW25113 AyqhD AyiaY + pIBAl and pIBA7 This work

mA15-lC BW25113 AyqhD AygjB + pIBAl and pIBA7 This work

IBA1-2C BW25113 AyqhD + pIBAl and pIBAl 1 This work

mA15-2C BW25113 AyqhD AygjB + pIBAl and pIBAl 1 This work

plasmids

pIBAl pl5A ori, Kan ^R , PJacO .-.alsS ilvD Zhang et al, 2011

ChemSusChem 4:1068-

1070

pD3A7 CoEl ori, Amp ^R , P _L lacOi::fo ^" v padA Zhang et al., 2011

ChemSusChem 4:1068-

1070

ΓΒΑΙ Ι ColEl ori, Amp ^R , P _L lacOi::foVO padA padA This work

Primers SEQ ID NO:

adhEKOC-F TTGCTTACGCCACCTGGAAGT 133

adhEKOC-F GAACGGTCGCATGAGCAGAAAG 134

adhPKOC-F TGACGATAATTTCTGGCAAGC 135

adhPKOC-R GCAGGCTGACATTAAGTTCGT 136

eutGKOC-F AGATTTGGCCTGCGGTGAAA 137

eutGKOC-R CTGTTAGTTGTTATTTATTGGCGG 138

yiaY OC-F CATTTATTGCGCGACGCATTAT 139

yiaYKOC-R ATAGCGGGCTTTTAACTTGAGG 140

yjgBKOC-F CACTGAAGAGGTATGCGGAAAA 141

yjgBKOC-R CTGGGCATTTTATGCCGGTAG 142

padA SacIfwd ctagtagagctcaAGGAGATATACCatgacagagccgcatgtagcagt 143

padA_SacIrev GACTATGAGCTCTTAATACCGTACACACACCGACTTAGTT 144

Cell cultivation and shake flask fermentation.

Unless otherwise stated, cells were grown in test tubes at 37°C in 2XYT rich medium (16 g/L Bacto-tryptone, 10 g/L yeast extract and 5 g/L NaCl) supplemented with 100 mg/L ampicillin and 50 mg/L kanamycin. 200 μΐ of overnight cultures incubated in 2XYT medium were transferred into 5 ml M9 minimal medium supplemented with 5 g/L yeast extract, 40 g/L glucose, 100 mg/L ampicillin and 50 mg/L kanamycin in 125 ml conical flasks. Isopropyl-β- D-thiogalactoside (IPTG) was added at a concentration of 0.1 mM to induce protein expression. The fermentation broth was buffered by the presence of 0.5 g CaC0 ₃ .

Fermentation cultures were placed at 30°C in a shaker with a speed of 250 rpm. Culture media for fermentor.

The following composition is the seeding medium for E. coli culture, in grams per liter: glucose, 10; (NFL ₂ SO ₄ , 1.8; K ₂ HP0 ₄ , 8.76; KH ₂ P0 ₄ , 2.4; sodium citrate, 1.32; yeast extract, 15; ampicillin, 0.1; kanamycin, 0.05. Fermentation media for bioreactor cultures contained the following composition, in grams per liter: glucose, 30; (N¾) ₂ S0 ₄ , 3; K ₂ HP0 ₄ , 14.6; K¾P0 ₄ , 4; sodium citrate, 2.2; yeast extract, 25; MgS0 ₄ .7H ₂ 0, 1.25; CaCl ₂ .2H ₂ 0, 0.015, calcium pantothenate, 0.001; Thiamine, 0.01; ampicillin, 0.1; kanamycin, 0.05; and 1 mL/L of trace metal solution. Trace metal solution contained, in grams per liter: NaCl, 5; ZnS0 ₄ .7H ₂ 0, 1; MnCl ₂ .4H ₂ 0, 4; CuS0 ₄ .5H ₂ 0, 0.4; H ₃ B0 ₃ , 0.575; Na2Mo04.2H ₂ 0, 0.5; FeCl ₃ .6H ₂ 0, 4.75; 6N H ₂ S0 ₄ , 12.5 mL. The feeding solution contained, in grams per liter: glucose, 600; (NH ₄ ) ₂ S0 ₄ , 5; MgS0 ₄ .7H ₂ 0, 1.25; yeast extract, 5; CaCl ₂ .2H ₂ 0, 0.015; calcium pantothenate, 0.001; Thiamine, 0.01; ampicillin, 0.1; kanamycin, 0.05, 0.2 mM of IPTG; and 1 mL/L of trace elements.

Fermentor culture conditions.

Cultures of E. coli were performed in a 1.3 L Bioflo 115 fermentor (NBS; Edison, NJ) using a working volume of 0.6 L. The fermentor was inoculated with 10% of overnight pre- culture with seeding medium and then the cells were grown at 37°C, 30% dissolved oxygen (DO) level, and pH 7.0. After OD ₆₀₀ reached 8.0, 0.2 mM IPTG was added and the temperature was shifted to 30°C to start isobutyrate production. The pH was controlled at 7.0 by automatic addition of 10 M sodium hydroxide solution, 50% ammonia hydroxide, or 200 g/L calcium hydroxide suspension, respectively. Air flow rate was maintained at 1 wm in the whole process. DO was maintained at about 10% with respect to air saturation by adjusting stirring speed (from 300 to 800 rpm). The glucose level in the fermentor was kept around 10 g/L by adding feeding medium automatically. When DO went over 40% and isobutyrate level did not increase, the fermentation process was stopped. Fermentation samples at different time points were collected to determinate optical density and metabolite concentration. Metabolite analysis and dry cell weight determination.

Fermentation products were analyzed using an Agilent 1260 Infinity HPLC equipped with an Aminex HPX 87H column (Bio-Rad; Hercules, CA) and a refractive-index detector. The mobile phase was 5 mM H ₂ S0 ₄ with a flow rate 0.6 mL/min. The column temperature and detection temperature were 35°C and 50°C, respectively. Cell dry weight was determined by filtering 5 mL culture through a 0.45 μπι glass fiber filter (Michigan Fiberglass Sales; St. Clair Shores, MI). After removal of medium, the filter was washed with 15 mL of MilliQ water, dried in an oven and then weighed. Cell dry weight was determined in triplicate. Example 3

Cloning Procedure

BKDH enzyme complex genes were amplified from Pseudomonas Putida KT2440 genomic DNA with primers bkdh_ecofwd (TGCATCGAATTCAGGAGAAATTAACTAT GAACGAGTACGCCCCCCTGCGTTTGC (SEQ ID NO: 145)) and bkdh_hindrev

(TGCATCAAGCTTTCAGATATGCAAGGCGTGGCCCAG (SEQ ID NO: 146)). The PCR product was then digestion with EcoRI and Hindlll, and inserted into pZE12 to make pIBA16.

The tesA gene was amplified from E. coli strain K12 genomic DNA using the primer pair TesAJffindlllJF (GGGCCCAAGCTTAGGAGAAATTAACTATGATGAACTTCAAC AATGTTTTCCG (SEQ ID NO: 147)) and TesA_Xbal__R (GGGCCCTCTAGATTATGAGT CATGATTTACTAAAGGCT (SEQ ID NO: 148)); tesB was amplified with primer pair TesB Hindlll F (GGGCCCAAGCTTAGGAGAAATTAACTATGATGAGTCAGGCGCT AAAAAATTTACT (SEQ ID NO: 149)) and TesB_Xbal_R (GGGCCCTCTAGATTAATT GTGATTACGCATCACCCCTT (SEQ ID NO: 150)). After PCR, the DNA fragments were purified and digested using the restriction enzymes Hindlll and Xbal . The digested fragments containing tesA were inserted into pIBA16 to make pIBA17; the digected fragments containing tesB were inserted into pIBA16 to make ρΓΒΑΙδ. Fermentation process

Overnight cultures incubated in LB medium were diluted 25-fold into 5 mL M9 medium supplemented with 0.5% yeast extract and 4% glucose in 125-mL conical flasks. Antibiotics were added appropriately (ampicillin 100 mg/L and kanamycin 25 mg/L). 0.1 mM isopropyl-b-D-thiogalactoside (EPTG) was added to induce protein expression. The culture medium was buffered by addition of 0.5 g CaC0 ₃ . Cultures were placed in a 30°C shaker (250 rpm) and incubated for 48 hours.

Fermentation products were quantified by HPLC analysis with refractive index detection using an Agilent 1100 Capillary HPLC. Results are shown in Table 4.

Table 4. Production of isobutyrate with the new pathway.

The complete disclosure of all patents, patent applications, and publications, and electronically available material (including, for instance, nucleotide sequence submissions in, e.g., GenBank and RefSeq, and amino acid sequence submissions in, e.g., SwissProt, PIR, PRF, PDB, and translations from annotated coding regions in GenBank and RefSeq) cited herein are incorporated by reference in their entirety. In the event that any inconsistency exists between the disclosure of the present application and the disclosure(s) of any document incorporated herein by reference, the disclosure of the present application shall govern. The foregoing detailed description and examples have been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described, for variations obvious to one skilled in the art will be included within the invention defined by the claims.

Unless otherwise indicated, all numbers expressing quantities of components, molecular weights, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless otherwise indicated to the contrary, the numerical parameters set forth in the specification and claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. All numerical values, however, inherently contain a range necessarily resulting from the standard deviation found in their respective testing measurements.

All headings are for the convenience of the reader and should not be used to limit the meaning of the text that follows the heading, unless so specified.