SEMI-SYNTHETIC CEPHALOSPORINS VIA EXPANDASE ACTIVITY ON PENICILLIN G

Field of the invention and brief description of the prior art

The present invention concerns new mutant expandases, to be used m a biosynthetic process for preparation and recovery of 7-amιnodesacetoxycephalosporanιc acid (7-ADCA) , one of the key intermediates used m the preparation of semi-synthetic cephalosporins (SSC's) β-Lactam antibiotics constitute the most important group of antibiotic compounds, with a long history of clinical use. Among this group, the prominent ones are the penicillins and cephalosporins. These compounds are naturally produced by the filamentous fungi Penicillium chrvsocrenum and Acremonium chrvsoσenum, respectively.

As a result of classical strain improvement techniques, the production levels of the antibiotics m Penicillium chrvsoαenum and Acremonium chrvsoαenum have increased dramatically over the past decades With the increasing knowledge of the biosynthetic pathways leading to penicillins and cephalosporins, and the advent of recombinant DNA technology, new tools for the improvement of production strains and for the i vivo deπvatization of the compounds have become available.

Most enzymes involved in β-lactam biosynthesis have been identified and their corresponding genes been cloned, as can be found in Ingolia and Queener, Med Res. Rev. 9 _. (1989) , 245-264 (biosynthesis route and enzymes) , and Aharonowitz, Cohen, and

Martin, Ann. Rev. Microbiol. 4_ (1992) , 461-495 (gene cloning) .

The first two steps in the biosynthesis of penicillin m

P chrvsoαenum are the condensation of the three amino acids

L-5-amιno-5-carboxypentanoic acid (L-α-ammoadipic acid) (A ⁾ ,

L-cysteine (C) and L- alme (V) mto the tπpeptide LLD-ACV, followed by cyclization of this tπpeptide to form lsopenicill N. This compound contains the typical β-lactam structure.

The third step involves the exchange of the hydrophillic side chain of L-5-amιno-5-carboxypentanoιc acid by a hydrophobic side chain by the action of the enzyme acyltransferase (AT) . The enzymatic exchange reaction mediated by AT takes place inside a cellular organelle, the microbody, as has been described EP-A-044Θ180. Cephalosporins are much more expensive than penicillins. One reason is that some cephalosporins (e.g. cephalexin) are made from penicillins by a number of chemical conversions. Another reason is that, so far, only cephalosporins with a n- am oadipoyl-carboxypentanoyl side chain could be fermented. Cephalosporin C, by far the most important starting material in this respect, is very soluble water at any pH, thus implying lengthy and costly isolation processes usmg cumbersome and expensive column technology. Cephalosporin C obtained in this way has to be converted mto therapeutically used cephalosporins by a number of chemical and enzymatic conversions.

The methods currently favoured in industry to prepare the intermediate 7-ADCA involve complex chemical steps leading tc the expansion and derivatization of penicillin G. One of the necessary chemical steps to produce 7-ADCA involves the expansion of the 5-membered penicillin rmg structure to a 6- membered cephalosporin ring structure (see for instance US 4,003,894) . This complex chemical processmg is both expensive and noxious to the environment.

Consequently, there is a great desire to replace such chemical processes with enzymatical reactions such as enzymatic catalysis, preferably durmg fermentation. A key to the replacement of the chemical expansion process by a biological process is the central enzyme in the cephalosporin biosynthetic pathway, desacetoxycephalospoπn C synthase, or expandase The expandase enzyme from the bacterium Streptomyces clavuliαerus was found to carry out, some cases, penicillin rmg expansions. When introduced mto P chrvsoαenum, it can

convert the penicillin rmg structure mto the cephalosporin rmg structure, as described in Cantwell e_t al _. . , Proc. R. Soc. Lond. B. 248 (1992) , 283-289. The expandase enzyme has been well characterized (EP-A-0366354) both biochemically and functionally, as has its correspondmg gene. Both physical maps of the cefE gene (EP-A-0341892) , DNA sequence and transformation studies in P . chrvsoαenum with cefE have been described.

Other sources for a rmg expansion enzyme are the bacteria Nocardia lactamdurans (formerly Streptomyces lactamdurans) and Lvsobacter lactamαenus . Both the biochemical properties of the enzyme and the DNA sequence of the gene have been described

(Cortes et al. , J. Gen. Microbiol. 133 (1987) , 3165-3174; and

Coque e_t al. , Mol. Gen. Genet. 236 (1993) , 453-458, respectively) . Since the expandase catalyses the expansion of the 5- membered thiazolidine rmg of penicillin N to the 6-membered dihydrothiaz e ring of DAOC this enzyme would be of course a logical candidate to replace the rmg expansion steps of the chemical process. Unfortunately, the enzyme works on the penicillin N mtermediate of the cephalosporin biosynthetic pathway, but not efficiently on the readily available inexpensive penicillins as produced by P. chrvsoαenum, like penicillin V or penicillin G. Penicillin N is commercially not available and even when expanded, the D-α-ammoadipoyl side chain cannot be easily removed by penicillin acylases .

It has recently been found that the expandase enzyme is capable of expanding penicillins with particular side chains to the correspondmg 7-ADCA derivative. This feature of the expandase has been exploited in the technology as disclosed in EP-A-0532341, WO95/04148 and WO95/04149. In these disclosures the conventional chemical conversion of penicillin G to 7-ADCA has been replaced by the m v vo conversion of certain 6- aminopenicillanic acid (6-APA) derivatives recombinant Penicillium chrvsoαenum strams containing an expandase gene. More particularly, EP-A-0532341 teaches the m vivo use of the expandase enzyme m P. chrvsoαenum, in combination with a 5-carboxypentanoyl side chain as a feedstock, which is a

substrate for the acyltransferase enzyme n P. chrvsoσenum. This leads to the formation of 5-carboxypentanoyl-6-APA, which is converted by an expandase enzyme introduced into the P. chrvsoαenum strain to yield 5-carboxypentanoyl-7-ADCA. Finally, the removal of the 5-carboxypentanoyl side chain is suggested, yielding 7-ADCA as a final product.

In WO95/04148 and WO95/04149 it has been disclosed that 3 ' - carboxymethylthiopropionic acid and 3 , 3 ' -thiodipropionic acid, respectively were found to be substrates for the expandase, yielding 2- (carboxyethylthio) acetyl- and 3- (carboxymethyl- thio) propionyl-7-ADCA.

However, the process of the present invention provides more advantages, because of the high penG biosynthetic capacity of penicillin producing strains and the more favourable process of extraction of phenylacetyl-7-ADCA acid. Furthermore the phenyl- acetyl side chain of phenylacetyl-7-ADCA is very amenable to enzymatic cleavage , by penicillin G amidases produced by several types of microorganisms yielding 7-ADCA, for instance penG acylase as disclosed m EP-A-0453047. Various publications have reported the expandase not to accept penicillin G or penicillin V as a substrate for expansion (Baldwin & Abraham (1988) , Natural Product Reports, 5(2) , p.129- 145; Maeda et al . (1995) , Enzyme and Microbial Technology, 17, 231-234; Crawford et al . (1995) , Bio/technology, 13, p.58-61) . In contrast to those observations one report mentions an activity of expandase on penicillin G vitro (Baldwin et al . (1994) , Proceedings of the 7 ^lh International Symposium on the Genetics of Industrial Microorganisms, Abstract P.262) .

Recently, the structure of the lsopenicillm N synthase (IPNS) enzyme of A. nidulans has been determined (Roach (1995) , Nature, 375, p700-704) . IPNS and expandase belong to the same family of oxidase enzymes . They share biochemical characteristics and, on the basis of sequence homologies, it has been proposed that structural similarities exist between the two enzymes (Roach et al . , supra; Cooper (1993) , Bioorganic Med. Chem. 1, pl-17) .

The mechanism of IPNS activity has been described m several reports (see for example: Blackburn et al . (1995) , Biochemistry 34, p7548-7562) . It is proposed, from an analysis of the chemistry catalysed by IPNS, that the cysteinyl thiol group of ACV must bind to the ferrous ion at the active site in the enzyme-substrate complex. Given this implicit attachment point between the substrate and the enzyme a large number of conformationally distinct binding modes can be distinguished given the crystallographically determined constraints of the active site. It is therefore not obvious how the ammoadipoyl side-chain of ACV binds to A _^ _ mdulans IPNS (alPNS) and, by inference, the mode of binding of penicillin N to expandase s even less apparent .

Figures

Figure 1 : Alignment of expandase f rom Streptomyces clavuliαerus versus lsopenicillm N synthase f rom Aspergillus mdulans .

Summary of the invention

The present invention provides new mutant expandase enzymes, especially suitable for the preparation and recovery of 7-ammodesacetoxycephalosporanιc acid (7-ADCA) by: a) transforming a Penicillium chrysoαenum strain with a modified expandase gene, under the transcriptional and translational regulation of fungal expression signals; b) fermenting said strain m a culture medium and adding to said culture medium phenylacetic acid or a salt or ester thereof suitable to yield penicillin G, which is expanded to form phenylacetyl-7-ADCA; c) recovering the phenylacetyl-7-ADCA from the fermentation broth; d) deacylat g phenylacetyl-7-ADCA; and e) recovering the crystalline 7-ADCA. Preferably, step (e) is a filtration step. Preferably, phenylacetyl-7-ADCA is recovered from the fermentation broth by extracting the broth filtrate with an organic solvent immiscible with water at a pH of lower than about 4.5 and back-extracting the same with water at a pH between 4 and 10.

Moreover, the DNA encodmg modified expandase and a recombinant DNA vector comprising the same, functionally linked to the transcriptional and translational control elements of a fungal gene, for instance Aspergillus mdulans αpdA gene, and the Aspergillus niger αlcA gene and host cells transformed with the same, are provided.

Detailed description of the invention

The present invention concerns the use of functional gene constructs encodmg modified expandase enzyme in P . chrvsoαenum for the m v vo expansion of the penicillin G ring structure to form the phenylacetyl acid derivative of a key intermediate m the cephalosporin biosynthesis, 7-amιnodesacetoxycephalosporanιc

acid, or 7-ADCA. This derivative has a chemical composition so as to allow efficient solvent extraction, thus providing an economically attractive recovery process.

Modification of the expandase gene is directed at producing mutants which best expand penicillin G m in vitro and/or in vivo context where other penicillins such as penicillin N and lsopenicillm N can act as competing substrates. This is an important feature of the invention given the observation of significant amounts of penicillin N being produced by P_ chrvsoσenum and the knowledge that penicillin N -is a significantly better substrate than penicillin G for the wildtype expandase (Alvi et al . , J. Antibiotics, 338 (1995)) . By transforming P chrvsoαenum with such targeted mutants of expandase, novel P chrvsoαenum strains can be obtained which have an improved capacity for the production of phenylacetyl-7- ADCA. Mutant expandase enzymes suitable to expand penicillin G have been screened as follows: i) Identifying ammo acid residues of expandase involved in penicillin side-chain recognition and binding; ii) Construction of modified genes encodmg mutant expandase proteins in a form easily puπfiable as fusions to maltose-bmdmg protein (MBP) ,* in) Characterising the binding and catalytic properties of said mutant expanαases towards penicillins specifically those with phenylacetyl and a -ammoadipoyl side-chains.

This allows those skilled in the art to construct strains with improved phenylacetyl-7- DCA production capacity for a 7- ADCA process as described by the non-prepublished international application PCT/EP 96/02434, filing date June 3, 1996 Transformation of P chrvsoαenum can, in principle, be achieved by different means of DNA delivery, like PEG-Ca mediated protoplast uptake, electroporation or particle gun techniques, and selection of transformants. See for example Van den Hondel en Punt, Gene Transfer and Vector Development for Filamentous Fungi, m: Applied Molecular Genetics of Fungi (Peberdy, Laten, Ogden, Bennett, eds.) , Cambridge University Press (1991) . The application of dominant and non-dominant selection markers has

been described (Van den Hondel, supra) . Selection markers of both homologous (P. chrvsoαenum derived) and heterologous (non- P. chrysoαenum derived) origin have been described (Gouka et al . , J. Biotechnol. 2_0 (1991) , 189-200) . In summary, the present invention teaches how a modified expandase gene introduced into P. chrvsoαenum can be used to improve the yield of phenylacetyl-7-ADCA resulting from the in vivo ring expansion of penicillin G by a mutant expandase.

Methods for the recovery of phenylacetyl-7-ADCA from the fermentation broth and for the conversion to 7-ADCA and for the recovery of 7-ADCA have been disclosed in the non-pre-published application No. PCT/EP 96/024340 mentioned above.

The followmg examples are offered by way of illustration and not by way of limitation. The overall approach entails i) identification of residues of expandase involved in penicillin side-chain recognition and binding specificity, ii) construction of mutant expandase proteins in a form easily purified as fusion proteins to maltose-binding-protein (MBP) , iii) assessment of activity towards different penicillins as penicillin N and penicillin G following expression in E. coli and purification.

Example 1

Identification of residues involved in substrate side-chain binding

Central to the invention is the proposal that, in the case of alPNS (IPNS from Aspergillus nidulans) , upon ACV binding, the L-α-aminoadipoyl side chain of ACV displaces the C-terminal tail of the enzyme (glutamine 330, threonine 331 and a number of preceeding residues) by virtue of the similarity between the L- α-aminoadipoyl side chain of ACV and the C-terminal dipeptide in steric and electronic terms. Comparison of the C-terminal tail and ACV reveals the similarity between the L-α-ammoadipoyl side-chain of ACV and the glutaminyl-threonine end of the tail; specifically the carboxylates in both cases are functionally homologous. In the resting state of the alPNS enzyme, the

carboxylate of the C-termmal threonine residue is in a position to form hydrogen-bonds to argmme 87, polar contacts to serine 183 and hydrophobic contacts with valine 185 and phenylalanine 285 Upon ACV binding to alPNS it is proposed that serine 183 and argmme 87 can interact with the L-α-aminoadipoyl side- chain carboxylate by polar effects and that hydrophobic contacts can be made between the methylene groups and amide of the side cham to valine 185 and phenylalanine 285 The sequence similarities of expandase and alPNS along with common spectroscopic properties and similar chemistries suggest that the D-α--ammoadipoyl side chain of the substrate penicillin N binds in a similar fashion to expandase as does the L-α- aminoaαipoyl side chain of ACV to alPNS

At the heart of the invention is the proposal that the oi¬ aminoammoadipoyl side chain of penicillin N will be bound by am o acid residues of expandase that are structurally analogous in the context of the 3D protem structure (and can be identified by sequence homology analysis homology analysis) to the ammo acid residues of alPNS involved m binding the α- ammoadipoyl side chain of ACV. In addition to these residues other residues that can be similarly identified by examination of the alPNS crystal structure using the aforementioned substrate-binding model.

The ammo acid sequence of Streptomyces clavuliαerus expandase is shown figure 1. The numbers indicate the position of an ammo acid m the sequence and will be used as an indication for the ammo acid position the description of am o acid changes. Regarding the corresponding ammo acid changes expandase which are homologous with Streptomyces clavuliαerus expandase, the skilled person will understand that the Streptomyces clavuliαerus ammo acid position used herein refer to the correspondmg conserved ammo acids in the ammo acid sequence of these related enzymes and not necessarily to their ammo acid positions in those enzymes. It is also to be understood that these corresponding conserved ammo acids are not necessarily identical to those of Streptomyces clavuliαerus expandase. Correspondmg position with respect to Streptomyces

clavuligerus can be obtamed by standard alignment procedures as are known by persons skilled m the art.

Residues of Streptomyces clavuligerus expandase so identified mclude, but are not restricted to argmme 74 (homologous to argmme 87 of alPNS) , cysteine 155 (homologous to serine 183 of alPNS) , proline 157 (homologous to valine 185 of alPNS) , leucine 159 (homologous to isoleucine 187 of alPNS) , phenylalanine 264 (homologous to phenylanalme 285 of alPNS) , isoleucine 298 (homologous to leucine 317 of alPNS) , tyrosine 302 (homologous to leucine 321 of alPNS) , argmme 306 (homologous to isoleucine 325 of alPNS) and argmme 266 (homologous to aspargine 287 of alPNS) . Mutation of these residues individually or in combination will alter the relative binding of penicillin N ana penicillin G to expandase m the ground state and subsequent intermediates and transition states for the expansion of these penicillins to DAOC and phenylacetyldesacetoxycephalospor , respectively Mutations at the aforementioned positions of expandase will increase the expansion of penicillin G, decrease the expansion of penicillin N and/or increase the relative ratio of penicillin G to other penicillin expansion in a competitive scenario. Many benefits accrue to a process involving m. vivo expansion of penicillin G to pnenylacetyl-7-ADCA as described m the non- pre-published application No PCT/EP 96/02434, mentioned above. The wild type expandase accepts penicillin N as its normal substrate with conflicting reports concerning the acceptance of penicillin G as a substrate In order to improve penicillin G as an isolated substrate it is necessary to improve v _max and, in a context where the concentration of penicillin G is non- saturating, to lower the K _ra . This is not only the case when penicillin G is an isolated substrate but also when penicillin G is a substrate in the presence of other penicillins, in the first place penicillin N but also lsopenicillm N. Thus, for example, m the microbody location of P chrvsoσenum cefE transformants precursed with phenylacetic acid, the expandase enzyme can act on penicillin N competition with penicillin G.

The relative and absolute amounts of each penicillin expanded depend on the ratio of the individual rates which can be broken down mto an equation of the form.

rate of expansion of penicillin G = V° = V ⁰ ^ [G] K ^N _M

rate of expansion of penicillin N = V ^N = V^ [N] K ^G _M

where V° _m:a and V ^N _ma- correspond to the maximum enzyme velocities, K ^N _M and K ^G _M are the Michaelis constants, and [G] and [N] are the concentrations of penicillin G and penicillin N respectively Mutations at positions of the expandase listed below which result m an increase of the ratio of V° V are part of the invention The specificity changes required can result from any single or multiple mutant that has values of V _maΛ and/or K _M for either or both substrates altered in any way such as to mcrease the ratio of V ³ V ^N vitro or the relative yield of phenylacetyl-7-ADCA compared to DAOC from a phenylacetate derivative precursed fermentation of a strain of P chrvsoαenum transformed with the mutant cefE gene. In the first instance improvement m the competition between penicillin G and penicillin N will occur but also between penicillin G and lsopenicillm N such as to improve the yield of phenyιacetyi-7- ADCA.

Example 2

Construction of mutants

a) General techniques for gene cloning and manipulation are well described m Sambrook et al , Molecular Cloning, a laboratory

Manual, Cold Spring Harbour, USA (1989)

b) Vector construction*

A chloramphenicol gene cartridge ( Pharmacia, Hindlll cartridge) is blunt-ended by treatment with Klenow fragment and four dNTPs and ligated to Seal linearised pMAL-c2 (New England Biolabs) then used to transform competent E coli cells to

chloramphenicol resistance. Resultant clones are restriction mapped with Ncol, Apal and other enzymes and a plasmid in which the direction of transcription of the chloramphenicol resistance gene is the same as that of the malE gene is designated pAJLlOO. A linker prepared by self-annealing of the palindromic oligonucleotide, AL5 (5'TAC CGA ATT CGG3 ' ) is ligated to Ndel linearised pNM88 (Morgan et al . (1994) , Bioorg. Med. Lett. 4, pl595-1600) and the resultmg ligation is sanitised by digestion with Ndel before bemg used to transform E. coli. Resultant clones are restriction mapped by digestion with EcoRI , Sail and other enzymes an-, a plasmid giving the anticipated restriction pattern is designated pAJL103. The Streptomyces clavuligerus cefE gene is subcloned as an EcoRI/Sall fragment from pAJL103 mto the correspondmg polylinker sites of pAJLlOO giving pAJL104 which is characterised by restriction mapping with Apal, Ball , EcoRI , Seal and other enzymes .

c) Expression in E coli :

Expression of the malE-cefE gene fusion is achieved by induction of a culture of E. coli NM554 [pAJL104] with IPTG. Induced cells are lysed by treatment with lysozyme followed by sonication and released protem is quantitated by Bradford assay. Purification of the MBP-expandase fusion protem is achieved by application of crude cellular lysates to an amylose column, washing and subsequent elution with buffer contammg maltose to give a protem of approximately 77 kDa molecular weight as assessed by SDS-PAGE against molecular weight standards. MBP-DAOCS fusion is assayed for bioactivity using penicillin N as a substrate usmg reverse-phase HPLC and hole-plate bioassay against super sensitive E coli in the presence of penicillmase. Active protein results m zones of lysis on bioassay plates and a new product with the same retention time as synthetic DAOC on reverse phase HPLC with a variety of buffer conditions and monitoring at 220 and 254nm. MBP-DAOCS fusion is assayed for bioactivity using penicillin G as a substrate using reverse- phase HPLC and 500 MHz Η NMR spectroscopy with confirmatory sample spiking both cases.

d) Mutagenesis of expandase gene:

Uracil-containing single-stranded pAJL104 DNA is obtamed from cultures of E. coli BW313 [pAJL104] followmg supermfection with helper phage M13 K07. This uracil-containmg single-stranded pAJL104 DNA is used in mutagenesis experiments m which 5'- phosphorylated synthetic oligonucleotides are annealed and extended m the presence of T7 DNA polymerase, T4 DNA ligase, dNTPs and ATP. Followmg second-strand synthesis the reactions are used to transform competent E. col N 554 or XL1 Blue to chloramphenicol resistance. Resultant clones are restriction mapped or sequenced to confirm the presence of the desired mutation. As examples of mutants constructed in this manner.

Mutation of argme 7a (argmme 87 m alPNS)

The R74F mutation is introduced using the phosphorylated primer: 5'ACC ATG TTT CGC GGC TTC ACC3 ' . Resultant clones are mapped by digestion with SacII , Ncol and other enzymes and the plasmid giving the anticipated fragments is designated pAJL211. The loss of a SacII site m pAJL211 relative to pAJL104 confirms the introduction of the TTT codon encodmg phenylalanine in pAJL211. The R74I, R74M and R74Q mutations are introduced in the following way Xhol and Spel sites flanking the R74 coding region m pAJL104 were introduced using two phosphorylated primers :

5 'GAA GCG CGC CGT CAC TAG TCC CGT CCC CAC CAT G3 ' and

5' CTT CAC CGG GCT CGA GTC GGA GAG C ' .

The resultmg plasmid is designated pGEN50.

The R74M mutation is then introduced by subcloning a synthetic cassette prepared by annealing two oligonucleotides:

5 'CTA GTC CGG TAC CGA CCA TGA TGA GGG GAT TCA CTG GTC3 ' and 5'TCG AGA CCA GTG AAT CCC CTC ATC ATG GTC GGT ACC GGA3 ' The introduction of the cassette was indicated by the presence of a new Kpnl site in the resultant plasmid which was designated pGEN51.

The R74Q mutation is then introduced by subcloning a synthetic cassette prepared by annealing two oligonucleotides ^•

5' CTA GTC CGG TAC CGA CCA TGC CAA GGG GAT TCA CTG GTC3 ' and 5'TCG AGA CCA GTG AAT CCC CTT TGC ATG GTC GGT ACC GGA3 ' . The introduction of the cassette was indicated by the presence of a new Konl site in the resultant plasmid which was designated pGEN52.

The R74Q mutation is then introduced by subcloning a synthetic cassette prepared by annealing two oligonucleotides: 5 ' CTA GTC CGG TAC CGA CCA TGA TCA GGG GAT TCA CTG GTC3 ' and 5'TCG AGA CCA GTG AAT CCC CTG ATC ATG GTC GGT ACC GGA ' . The introduction of the cassette was indicated by the presence of a new Kpnl site in the resultant plasmid which was designated pGEN53.

The sequence of the clones m the region of the residue 74 coding sequence was verified by sequence analysis using a primer:

5'GAG CTG AAG TCG GCC AAG3 '

Mutation of cysteine 155 (serine 183 in alPNS) ^• The C155L mutation is introduced using the phosphorylated primer:

5'GAG GCC TTC CTC GAC CTC GAG CCG CTG CTG CGG3 ' Resultant clones are mapped by digestion with Xhol , Ncol and other enzymes and the plasmid giving the anticipated fragments is designated pAJL201. The Xhol site in pAJL201 confirms the presence of the

CTC codon encodmg leucine.

The C155A mutation is introduced usmg the phospnorylated primer:

5'GTC GAG GCC TTC CTC GAC GCT GAG CCG CTG CTG CGG TTC CG3' Resultant clones are mapped by digestion with ApaLI , Bpul 1021 and Seal and the plasmid giving the anticipated fragments is designated pGEN70.

The C155V mutation is introduced using the phosphorylated primer: 5'GTC GAG GCC TTC CTC GAC GTC GAG CCG CTG CTG CGG TTC CG3 ' .

Resultant clones are mapped by digestion with Aatll , EcoRI and

other enzymes and the plasmid giving the anticipated fragments is designated pJC14.

The sequence of the clones m the residue 155 coding region was verified by sequence analysis using a primer: 5'AGC GGA TCT GGA CCC AGT3 ' .

Mutation of proline 157 (valine 185 in alPNS)

The P157G and P157A mutations are introduced using the phosphorylated primer mixture:

5'GCC TTC CTT GAC TGC GAA NNN CTT CTC CGT TTT CGC TAC TTC CCG3 ' where N represents a mixture of G, A, T and C. Resultant clones are mapped by digestion with XmnI , Ncol and other enzymes and plasmids having the extra XmnI site are analysed further by digestion with StuI and Eco47III . Plasmids having an added StuI site are designated pAJL232; the added XmnI and StuI sites confirm the presence of a GGC codon encodmg glycine. Plasmids having an added Eco47III site are designated pAJL233; the added XmnI and Eco47III sites confirm the presence of a GCG codon encoding alanine.

Mutation of argmme 266 (asparagine 287 m alPNS)

The R266N mutation is introduced using the phosphorylated primer:

5' CC TCC AGT GTG TTC TTT TTA AAT CCC AAC GCG GAC TTC3 ' . Resultant clones are mapped by digestion with Dral, Ncol and other enzymes and the plasmid giving the anticipated fragments is designated pMJSl. The introduction of a fifth Dral site confirms the presence of an AAT codon encodmg asparagine.

The R266Q mutation is introduced using the phosphorylated primer:

5'AGT GTG TTC TTC CTG CAG CCC AAC GCG GAC3 ' . Resultant clones are mapped by digestion with PstI , EcoRI and other enzymes and the plasmid giving the anticipated fragments is designated pMJS2. The introduction of a PstI site confirms the mutation.

The R266M mutation is introduced usmg the phosphorylated primer:

5'GCG GGC AGC AGC CGC ACG AGC TCT GTG TTC TTC CTC ATG CCC AAC GCG GAC TTC3 ' . Resultant clones are mapped by digestion with SacI , EcoRI and other enzymes and the plasmid giving the anticipated fragments is designated pMJS3. The introduction of a SacI site confirms the mutation.

Example 3

Characterisation of mutants

The mutants are characterised by HPLC analysis of expansion reactions of penicillin N and penicillin G both individually and in mixtures of differing proportions and amounts. DAOC and phenylacetyldeacetoxycephalospoπn are identified by retention times using several different buffers and elution conditions with monitoring at 220 and 254nm, and by confirmatory sample spiking. 500 MHz IH NMR analysis is also used to confirm the nature of products by comparison with synthetic standards. As examples the following data are obtained:

The R266N mutant is found to expand penicillin N and penicillin G less well than the wild-type expandase. In mixtures comprising penicillin N and G, the R266N mutant displays a higher ratio of V ^G _: v ^N than the wild-type expandase.

The R74F mutant is found to expand penicillin N poorly and peni¬ cillin G very poorly. The R74I mutant shows slightly lowered activity on penicillin N, this activity is greatly reduced m the presense of penicillin G. The C155L mutant is found to expand penicillin N well and penicillin G poorly relative to the wild-type expandase. In mixtures contammg penicillin N and penicillin G, penicillin N expansion is inhibited by penicillin G. The R74I and C155L mutants both show increased affinity for penicillin G relative to penicillin N, the former more so than the latter.

These examples demonstrate that mutations these positions affect the ratio of V ^G V ^N , the turnover rate of

penicillin N and the affinity for penicillin G in competition with ammoadipoyl penicillins .