A method for influencing beta-lactam antibiotic production and for isolation of large quantities of ACV synthetase

Abstract: Novel methods and compositions are provided for the enhanced production of fermentable or known and new β-lactam antibiotics. The process is exemplified by the production of penicillin. In addition, the P.chrysogenum and A.chrysogenum δ-(L-α-aminoadipyl)-L-cysteinyl-D-valine synthetase genes have been isolated and sequenced. Also methods are provided for the production of δ-(L-α-aminoadipyl)-L-cysteinyl-D-valine synthetase using recombined DNA techniques.

Description: INTRODUCTION Technical Field This invention relates to methods and compositions to enhance the in vivo and in vitro production of fermentable or known and new secondary metabolites, particularly β-lactams and their biosynthetic intermediates using recombinant DNA techniques. Background β-Lactam antibiotics are the largest family of secondary metabolites produced in nature by microorganisms. The most important classes of the β-lactam antibiotics both clinically and economically are the penicillins (penam) and cephalosporins (cephem). Their biosynthesis occurs via a complex pathway of enzymatic steps; the unravelling of this pathway has been the subject of many studies during the last few decades. The first two steps are the key steps both in the biosynthetic pathways of the penam and cephem classes of β-lactam antibiotics. After these two steps the biosynthetic pathways to the penicillins and cephalosporins diverge. The first step in the biosynthesis of the penicillin, cephalosporin and cephamycin antibiotics is the condensation of the L-isomers of three amino acids, L-α-amino adipic acid (A), L-cysteine (C) and L-valine (V) into a tripeptide, δ-(L-α-aminoadipyl)-L-cysteinyl-D-valine or ACV. This step is catalyzed by δ-(L-α-aminoadipyl)-L-cysteinyl-D-valine synthetase or ACVS. In the second step, the tripeptide ACV is oxidatively cyclised by the action of the Isopenicillin N Synthase (hereinafter referred to as IPNS) or cyclase. The product of this reaction is Isopenicillin N; this compound contains the typical β-lactam and thiazolidine ring structures and possesses antibacterial activity. From Isopenicillin N the penicillins G or V are formed by exchange of the hydrophilic A side chain by a hydrophobic side chain. The side chains commonly used in industrial processes are either phenylacetic acid (PA), yielding penicillin G, or phenoxyacetic acid (POA), yielding penicillin V; this exchange reaction is catalyzed by the enzyme acyltransferase (hereinafter referred to as AT). From Isopenicillin N the route to cephalosporin and the cephamycins proceeds via racemisation of the A side chain, forming penicillin N. This reaction is catalyzed by an enzyme named epimerase or racemase. The five-membered ring in penicillin N is expanded into a six-membered ring by the action of the enzyme deacetoxycephalosporin C synthase or expandase. The fungal enzyme has been shown also to catalyze the next reaction in the pathway, the hydroxylation of the methyl group at the 3'-position of the six-membered ring, forming deacetylcephalosporin C. In Streptomycetes this latter enzyme activity is encoded by a separate gene. From deacetylcephalosporin C, Cephalosporin C is formed by acetylation of the 3'-position. The cephamycins are formed from the same compound by several enzymatic steps. In vivo synthesis of β-lactams and their precursors can be increased by increasing the activity of enzymes involved in β-lactam biosynthetic pathways. This can be achieved by either increasing the amount of enzyme present or by improving the specific activity of the enzyme. However, obtaining enzymes having the desired activity has typically been limited by the availability of spontaneous mutations in enzymes which are active at points in the pathway. Currently, the cost effectiveness of in vitro synthesis of β-lactam precursors in cell-free extracts is poor in comparison to traditional fermentation processes. These in vitro processes are hampered by the limited amounts of enzyme present within the cell, and hence within the cell-free extract, and by the presence of inhibitory factors such as proteases and enzyme inhibitors. In addition, the use of fermentable or known β-lactam antibiotics (i.e. can be produced by fermentation of (β-lactam producing) non-recDNA microorganisms) is complicated by a) the development of resistance or tolerance to fermentable or known β-lactam antibiotics by bacterial species, and b) the limitations in therapeutic use, such as an allergy to penicillin. There is, therefore, substantial interest in the development of systems which allow for the efficient and cost-effective production of new β-lactam antibiotics (i.e. cannot be produced by fermentation of (β-lactam producing) non- recDNA microorganisms) and their precursors, both in vivo and in vitro . Relevant Literature Van Liempt et al ., ( J. Biol. Chem. (1989), 264:3680-3684) have shown in Aspergillus nidulans that the ACV condensation is carried out by a large multifunctional enzyme, ACV synthetase. Similar results have been obtained for the Acremonium chrysogenum ACV synthetase by Banko et al ., ( J. Amer. Chem. Soc. (1987), 109:2858). The gene encoding ACV synthetase ( pcb AB) has been located on the genome of A.nidulans (MacCabe et al ., EMBO J. (1990), 8:279-287) and Penicillium chrysogenum (EP-A-320272; EP-A-357119; D. Smith et al ., Bio/Technology (1990), 8:39-41). pcb AB is located just upstream of the gene encoding IPNS. In vitro synthesis of the tripeptide ACV in cell-free extracts (Adlington et al ., Biochem. J. (1983), 213:573; G. Banko et al ., J. Am. Chem. Soc. (1987), 109:2858; Jensen and Westlake, Developments in Industrial Microbiology (1989), 30:113-119; EP-A-280051) has been used to study parameters of the ACV synthetase reaction and also to study the feasibility of commercial application of in vitro synthesis of β-lactam antibiotics; such as compared to traditional fermentation processes these processes are not commercially attractive (Jensen, supra ). Several inhibitory compounds for the ACV synthetase reaction have been disclosed (Adlington and Banko, supra ). It has also been established that the ACV synthetase has a rather narrow substrate specificity. Only a few amino acids can be substituted for the native α-amino adipic acid, cysteine and valine. Amplification of antibiotic biosynthetic genes by increased copy number resulting in an increase in the production of an antibiotic has been described by Skatrud et al ., Bio/technology (1989), 7:477-485 and EP-A-357119, supra ). Increased cephalosporin production, using the cef EF gene (Skatrud et al ., supra ) or penicillin production, using the pcb C- pen DE gene cluster (EP-A-357119) has been reported. Expression of β-lactam biosynthetic genes, other than the ACV synthetase gene, has been described in Streptomyces lividans (Chen et al ., Bio/technology (1988), 6:1222-1224). Publications relating to enzymes included in β-lactam biosynthesis and the cloning of genes encoding these enzymes are as follows. The IPNS has been purified and the gene encoding this enzyme, pcb C, has been cloned from various organisms (reviewed in Miller and Ingolia, Molecular Microbiology 1989, 3:689-695; Martin and Liras, Advances in Biochemical Engineering/Biotechnology (1989), 39:153-187). The enzyme acyltransferase has been purified (Alvarez et al ., Antimicrob. Agents Chemother. 1987, 31:1675-1682; EP-A-336446) and the gene encoding this enzyme ( pen DE) has been cloned (EP-A-336446; EP-A-354624; Veenstra et al ., In: C.L. Hershberger, S.W. Queener and G. Hegeman, eds: Genetics and Molecular Biology of Industrial Microorganisms , (1989), pp 262-269; Barredo et al ., Gene 1989, 83:291-300). The pcb C and pen DE genes are clustered in the genome of Penicillium chrysogenum (B. Diez et al ., Mol. Gen. Genet. (1989), 218:572-576; Veenstra, supra ). The epimerase has been purified from Streptomyces clavuligerus (Usui and C-A Yu, Biochem. Biophys. Acta (1989), 999:78-85); the presence of this gene on a large DNA fragment was suggested in EP-A-233715. The enzyme expandase has been isolated both from Acremonium chrysogenum (Kupta et al ., FEMS Microbiol. Letters 1983, 16:1-6; Dotzlaf and Yeh, J. Bacteriol. (1986), 16:1611-1618) and from Streptomyces clavuligerus (Rollins et al ., Can. J. Microbiol. (1988), 34:1196-1202) and Streptomyces lactamdurans (Cortes et al ., J. Gen. Microbiol. (1987), 133:3165-3174). The expandase genes have been cloned from both A.chrysogenum ( cef EF, Samson et al ., Bio/technology (1987), 5:1207-1214) and from Streptomyces clavuligerus ( cef E, Kovacevic et al ., J. Bacteriol. (1989), 171:754-760). SUMMARY OF THE INVENTION Methods, and compositions for use therein, are provided for enhanced expression of β-lactam antibiotic biosynthetic genes, leading to increased production of β-lactam antibiotics and precursors thereto. The methods include the steps of stably transforming a host cell with an expression cassette containing at least one DNA sequence encoding an ACV synthetase (ACVS) or a biologically active mutant of ACV synthetase (ACVS') and isolating transformants which produce an enhanced amount of ACV synthetase. The expression cassette includes transcriptional and translational initiation regulatory regions, hereinafter defined as promoter, and transcriptional and translational termination regulatory regions, hereinafter defined as terminator, appropriate for the host cell. The promoter may also be heterologous to the open reading frame. The host cell may be a eukaryote or a prokaryote. Also included is the pcb AB gene from P.chrysogenum and A.chrysogenum , constructs and vectors comprising the pcb AB gene or a mutant thereof, and transformed cells comprising said pcb AB gene. The subject invention finds use particularly in improved production both in vivo and in vitro of β-lactam antibiotics and their precursors, particularly antibiotics of the penam and cephem classes. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic representation of the distribution of β-lactam producing microorganisms. Figure 2 shows a schematic representation of the biosynthetic pathways of the penicillins, cephalosporins and cephamycins. Figure 3 shows a restriction site and functional map of the chromosomal region containing the ACV synthetase gene in P.chrysogenum . Probes that have been used for Northern hybridizations are indicated. The region containing the pcb AB gene is shaded. S= Sal I. Figure 4 shows a detailed physical map of the region encoding the ACV synthetase gene in P.chrysogenum , including the sequencing strategy. Figure 5 shows a schematic representation of the cluster of penicillin biosynthetic genes present in the genome of P.chrysogenum . Figure 6 shows a restriction site and functional map of construct pPCV01. Figure 7 shows a restriction site and functional map of construct pPCV02. Figure 8 shows a restriction site and functional map of construct pPCV03. Figure 9 shows a restriction site and functional map of the E.coli vector pMAtrp. Figure 10 shows a restriction site and functional map of pMA-ACV synthetase. Figure 11 shows a restriction site and functional map of pSLACV-01. Figure 12A shows a restriction site and functional map of pSLACV-03A. Figure 12B shows a restriction site and functional map of pSLACV-03B. Figure 13 shows a scheme that shows the sequence homology between domains found within the ACVS protein and Gramicidin and Tyrocidin Synthetases. Figure 14 shows a scheme that shows the sequence homology between the ACVS protein and the thioesterase domain of rat fatty acid synthetase. Figure 15 shows a scheme listing the domains and sub-domains within the ACVS protein. Figure 16 is a restriction site and functional map of the chromosomal region encoding the ACVS gene in A.chrysogenum . Arrows indicate the positions of the IPNS-( pcb C) and ACVS-( pcb AB) genes. The position of phages which have been isolated from a gene library and which contain part of the region are indicated. Figure 17 is a restriction site and functional map of the chromosomal region encoding the ACVS gene in A.chrysogenum . Probes that have been used in Northern Blot Hybridizations, and the results obtained, are indicated. Figure 18 shows the similarity between the deduced amino acid sequences from the ACVS genes from P.chrysogenum and A.chrysogenum. BRIEF DESCRIPTION OF THE SEQUENCE LISTINGS Sequence listing 1 shows a nucleotide sequence and deduced amino acid sequence of the P.chrysogenum ACV synthetase gene. Sequence listing 2 shows the nucleotide sequence and deduced amino acid sequence of the A.chrysogenum ACV synthetase gene. DESCRIPTION OF THE SPECIFIC EMBODIMENTS In accordance with the subject invention, methods and compositions are provided which allow for increased, cost-effective production of β-lactam antibiotics and their precursors both in vivo and in vitro . The method includes the steps of transforming a host cell using an expression cassette which includes in the 5'-3' direction of transcription, a promoter, an open reading frame encoding an ACVS or ACVS', optionally having a signal sequence for secretion recognized by the host cell; and a terminator. The promoter and terminator are functional in the host cell and provide for efficient expression of ACVS or ACVS' without undesirable effects on the viability and proliferation of the host cell. Transcription and translation of the ACV synthetase DNA sequence will augment levels of ACV synthetase already present in the cell. Transcription and translation of the ACVS' DNA sequence will provide for expression of ACVS' in the host cell, which may have enhanced properties as compared to the native enzyme. Optionally, the expression cassette may include a promoter which is not subject to repression by substances present in the growth medium. The expression systems may be used to prepare fermentable β-lactam antibiotics directly or they may be used to prepare cell-free extracts containing large quantities of ACVS or ACVS' for in vitro preparation of said antibiotics. Alternatively, the expression hosts may be used as a source of large quantities of enzyme which can be purified and used in in vitro systems. Enhanced production of ACV synthetase is achieved by the introduction of extra copies of the gene encoding ACV synthetase ( pcb AB gene) into a host cell by transformation: protoplasts are mixed with DNA constructs that contain at least one copy of the gene to be amplified linked to a selectable marker. By choosing the appropriate conditions, some protoplasts will take up the DNA construct which is thereafter stably maintained because the construct has become integrated into the host cell genome. Transformed cells can be selected from the background of so-called "non-transformed" cells by screening for expression of the selectable marker. Amplification of the pcb AB gene, which may be used to produce cephalosporin, penicillin, and cephamycin thus can be expected to result in an increase in intracellular enzyme activity, which in turn will be accompanied by an increase in production of the desired antibiotics. Since ACV synthetase forms part of the biosynthetic pathway of both the penam and the cephem classes of β-lactam antibiotics, increased production of this enzyme has several applications. Production of ACVS' is achieved by the introduction of at least one copy of a mutant gene encoding ACV synthetase ( pcb AB' gene) into a host cell by transformation, as described hereinabove. The existence of one enzyme indicates the presence of one gene encoding this enzyme, pcb AB. The localization of this gene in P.chrysogenum , just upstream of the gene encoding IPNS, is described in EP-A-357119 (non-prepublished). Similar results about the localization of the gene in P.chrysogenum are also described in EP-A-320272, and in Smith et al ., cited above. These data still allow for the ACVS activity to be encoded in more than one gene, however. Based on the results obtained for A.nidulans (250 kDa enzyme: Van Liempt et al .) and P.chrysogenum (gene present on 8 x 10 3 nucleotides DNA fragment: EP-A-320272) a gene with a size of at most 8 x 10 3 nucleotides would be predicted. Surprisingly, the data presented in the present patent application indicate that the coding region of the P.chrysogenum pcb AB gene is 11,337 nucleotides long and encodes a protein of 413 kDa. In Figure 6B of EP-A-320272 a restriction map of the DNA comprising the asserted intact ACVS gene has been given. From the expression of the P. chrysogenum DNA fragment in the vector pCX3.2 it is concluded that this fragment contains one or more genes that are involved in the ACV synthetase production. Comparison of the restriction map (Figure 6B of EP-A-320272) with the one in this patent application (Figure 4) clearly demonstrates that this fragment of EP-A-320272 encodes an incomplete ACV synthetase lacking the last about 900 nucleotides (about 300 C-terminal amino acids). This is not recognized by the applicants of EP-A-320272. Besides that, a significant amount of ACV synthetase can now be achieved by using the manipulated pcb AB genes of the present invention. Only now it can be envisaged that all catalytic activities required for tripeptide formation (i.e. activation of three amino acids, racemisation of valine, formation of dipeptide AC and tripeptide ACV, release of ACV) can indeed be present in one single polypeptide chain. ACVS hence is a multifunctional enzyme. Multifunctional enzymes are defined herein to be enzymes that consist of one single polypeptide chain and that carry within their structure the ability to perform more than one catalytic reaction. In contrast, in the case of the biosynthesis of Gramicidin S, a peptide antibiotic consisting of two identical units of five amino acids, at least two, and probably three polypeptide chains are involved (e.g. Von Döhren, 1982, In: Peptide Antibiotics , W. de Gruyter & Co., Berlin. pp 169-182; Krätzschmar et al ., Journ. of Bacteriol. (1989) 171, 5422-5429). In vitro synthesis of the ACV tripeptide has been described previously (Adlington and Banko, ( supra ). However, the only available systems up to now have been preparations comprising cell-free extracts of various organisms that possess ACV synthetase activity, followed by an in vitro reaction. The availability of an efficient expression system and the accompanying constructs, designed for high expression of the gene in suitable hosts is highly advantageous. It allows for the design of in vitro systems which do not have the disadvantages of a limited amount of enzyme present within the cell and hence within the cell-free extract, or the presence of an excess of negative factors such as proteases or inhibitors or other similar effects. Increased productivity thus is obtained as a result of the enzyme being present in large quantities in the cell-free extracts prepared from the transformed host organisms as compared to the organisms used by for example Adlington and Banko, supra . Alternatively, large quantities of purified enzyme can be obtained and subsequently incubated in systems devoid of negative components. For preparation of ACV and secondary metabolites, or for preparation of ACV synthetase by recombinant methods, genes encoding ACV synthetase (the pcb AB gene) may be obtained from a variety of sources including Penicillium chrysogenum , Acremonium chrysogenum , Aspergillus nidulans , Flavobacterium or Streptomycetes . The structural genes may be isolated by various techniques. These include isolating mRNA from a host organism which codes for the polypeptide of interest, the mRNA reverse transcribed, the resulting single stranded (ss) DNA used as a template to prepare double stranded (ds) DNA and the dsDNA gene isolated. Another technique is to isolate the chromosomal DNA from the source organism of interest and, using a probe, appropriately degenerate, comprising a region of the most conserved sequences in the gene of interest, identify sequences encoding ACV synthetase in the genome. The probe can be considerably shorter than the entire sequence, but should be at least 10, preferably at least 14, more preferably at least 20 nucleotides in length. Longer nucleotides are also useful, up to about 100 nucleotides of the gene of interest. Both DNA and RNA probes can be used. In use, the probe is typically labeled in a detectable manner (for example with 32 P or biotinylated nucleotides) and are incubated with ss DNA or RNA from the organism in which the gene is being sought after separation and/or immobilization of the ss or ds DNA, typically using nitrocellulose paper or nylon membranes. Hybridization is detected by means of autoradiography. Hybridization techniques suitable for use with oligonucleotides are well known to those skilled in the art. Although probes are normally used with a detectable label that allows for easy identification, unlabeled oligonucleotides are also useful, both as precursors of labeled probes and for use in methods that provide for direct detection of DNA or DNA/RNA. Accordingly, the term "oligonucleotide" refers to both labeled and unlabeled forms. The gene encoding the ACVS can be isolated from various organisms. For the gene encoding the IPNS, it has been shown (See for example, Ingolia and Queener, Medicinal Research Reviews (1989) 9:245-264) that genes isolated from different organisms show a high degree of homology, ranging from about 70% on the DNA level if two fungal or two Streptomyces genes are compared to 60% or more if a fungal and a Streptomycete gene are compared. Homologies on the protein level are 75% and 54%, respectively. Despite the differences on the DNA and protein level, all IPNS proteins catalyze the same reaction in a similar fashion. Therefore, ACVS sequences and enzymes can be identified that share a minimal homology of about 60% on the DNA level or 50% on the protein level. Sequences that are at least substantially identical to the sequence given in sequence listing 1 or sequence listing 2 are of particular interest. By "substantially identical" is intended sequences which can include conservative mutations, where the sequence encodes the same amino acid sequence, but may have as many as 30% different nucleic acid bases, more usually not more than 10% different bases, or mutations which are non-conservative, where fewer than about 10%, more usually fewer than about 5%, and preferably not more than 1% of the encoded amino acids are substituted or deleted, and there are fewer than 5% of inserted amino acids, where the percentage is based on the number of naturally occurring amino acids in the native enzyme. The degree of change which is acceptable can be determined by assessing the enzymatic activity of the expression product of the isolated gene, where the expression product should retain the ability to catalyze the formation of ACV from its constituting amino acids. ACV synthetase genes can be isolated by using, for example, the sequence as given in sequence listing 1, or parts thereof, as a probe in heterologous hybridization experiments. These probes can be restriction fragments derived from the ACVS encoding DNA isolated from P.chrysogenum ; such restriction fragments can easily be selected and isolated using the restriction map as given in Figure 4. Alternatively, synthetic oligonucleotide probes can be made based on the data given in sequence listing 1. In still another variation, oligonucleotides can be designed, based on the data in sequence listing 1, and used in a PCR reaction to generate a larger probe fragment. In this way, for example, the genes of Acremonium chrysogenum and Aspergillus nidulans can be readily isolated; also the genes from Streptomycetes can be isolated using less stringent hybridization conditions. The genes that are isolated in this fashion are also of interest. This procedure is exemplified by the isolation of the pcb AB gene from A.chrysogenum . The data obtained for this gene in their turn can be used for the gene-isolation procedure as described above. For example, using the data of the present invention, regions with a high homology can be identified, such as the domains of Figures 15 and 18, and selected for use as a highly specific probe in the isolation of pcb AB genes from more distantly related organisms. Alternatively, the DNA sequences encoding the ACV synthetase can be synthesized using conventional techniques such as PCR (Polymerase Chain Reaction) or by synthesis of overlapping single strands which may be ligated together to define the desired coding sequences. The termini can be designed to provide restriction sites or one or both termini may be blunt-ended for ligation to complementary ends of an expression vector. For expression of the sequence an initial methionine is provided. Expression vectors are generally available and are amply described in the literature. For preparation of ACVS' and modified secondary metabolites, or for preparation of ACVS' by recombinant methods, genes encoding ACVS' may be obtained from spontaneous pcb AB' mutants from a variety of sources including Penicillium chrysogenum , Acremonium chrysogenum , Aspergillus nidulans , or from Flavobacterium or the Streptomycetes . Also, sequences that encode biologically active mutant ACV synthetase can be derived from sequences that are at least substantially identical to the sequence of a fermentable or known ACV synthetase or from spontaneous mutant pcb AB alleles. Mutant sequences can be "derived" by a variety of genetic and recombinant DNA techniques, such as in vitro mutagenesis and homologous recombination. The gene of interest encodes biologically active mutant ACV synthetase or a biologically active portion of ACV synthetase or a mutant thereof. A spontaneous mutant gene or a wild type gene (from which a mutant structural gene subsequently can be derived) can be isolated by various techniques using host organisms which are ACV synthetase mutants or wild type. These techniques have been described hereinabove. Once the ACV synthetase DNA is obtained, mutations can be introduced by a number of in vitro mutagenesis techniques, either random or site-directed. Precise changes to the amino acid sequence are obtained by site-directed mutagenesis techniques which use synthetic oligonucleotides complementary to the region to be modified, except for the desired nucleotide(s) change. Regions are precisely deleted by "loop-out" mutagenesis techniques using synthetic oligonucleotides. Precise insertions are obtained using synthetic oligonucleotides to generate appropriate restriction sites. A series of mutations localized to a region of the ACV synthetase-encoding DNA sequence on a plasmid are generated by in vitro mutagenesis techniques and subsequently identified by cloning and sequencing the isolated mutagenized plasmids. Once specific mutations have been generated and isolated to individual plasmids, "cassette" mutagenesis is applied to generate a series of new mutants by combining mutations into the same plasmid using appropriate restriction sites. Alternatively, mutagenesis using two or more oligonucleotides directed to different regions of the gene yields mutants with the desired multiple mutations. Mutant proteins are also generated by the insertion, addition, or substitution of coding sequences from other proteins to ACV synthetase coding sequences. The source proteins may be ACV synthetases from other strains or species. Such amino acid sequences introduced into ACV synthetase will impart desirable properties or characteristics which originally belonged to the source protein. Desirable properties are useful properties for activities and functions which include and are not limited to protein stability, secretion, isolation, purification, increased enzymatic activity, resistance to inhibitors, proteases and denaturants, and solubility. Although any region of the protein may be mutated, regions of the ACV synthetase protein which serve as candidates for mutagenesis are defined in order to minimize extensive screening of randomly generated mutants. The nucleotide and deduced amino acid sequences of ACV synthetase provided are a powerful tool with which to delineate such regions. Sequence homology comparisons, at both the DNA and amino acid level, identify protein regions with known function or structure. Hydrophobicity profiles or related biophysical profiles, based on the amino acid sequence, with subsequent comparisons to profiles of other proteins may be used to identify additional protein regions of known structure and function. The finding of sequence or profile homologies indicates that the protein regions share a similar function, activity or enzymatic mechanism. Hydrophobicity or secondary structural profiles can indicate domain "linking" regions, "hinge" regions or "loops" which are candidates for restriction site insertions to generate domain "cassettes". Regions for mutagenesis or replacement are also defined by correlation of a genetic map of ACV synthetase mutant alleles with a physical map, either at the restriction site or sequence level. Preferred regions for mutagenesis, either site-directed or via substitution include the functional domains and sub-domains of ACV synthetase as provided in Figures 13, 14, and 15. Mutant proteins also include functional proteins formed by various combinations or quarternary assemblies of discrete polypeptides. The polypeptides are expressed individually, either from the same or different plasmids, in a host cell or isolated and recombined in vitro . The polypeptides contain one or more enzymatic activities. Preferably the polypeptides are from the domains of ACV synthetase as defined in Figures 13, 14, and 15. Polypeptides from regions of other proteins, as defined in the preceding sections, may also be combined with one or more ACV synthetase polypeptides in the same discrete fashion. Once the desired DNA sequence has been obtained, it may be manipulated in a variety of ways to provide for expression. It is highly desirable that modifications of the nucleotide sequence, other than the modifications which result in the desired mutation(s), retain the three dimensional structure of the expression product, particularly that portion of the structure which may be responsible for the enzymatic activity of the resulting enzyme. Convenient restriction sites may be designed into the DNA sequence of interest; when possible the restriction site(s) leaves the amino acid sequence of the expression product unaltered. However, in some cases, incorporation of new restriction sites may yield an altered amino acid sequence. Where the gene encoding ACVS or ACVS' is to be expressed in a host which recognizes the wild type promoter and terminator of the gene of interest, the entire gene with its wild type 5' and 3'-regulatory regions may be introduced into an appropriate expression vector. Where said gene is to be expressed in a host cell which does not recognize the naturally occurring wild type promoter and terminator, further manipulation may be required. Conveniently, a variety of 3'-transcriptional regulatory regions are known and may be inserted downstream from the stop codons. The non-coding 5'-region upstream from a structural gene may be removed by endonuclease restriction, Bal 31 digestion or the like. Alternatively, where a convenient restriction site is present near the 5'-terminus of the structural gene, the structural gene may be restricted and an adaptor employed for linking the structural gene to the promoter region, where the adapter provides for the lost nucleotides of the structural gene. The biosynthesis of β-lactam antibiotics in general, and of penicillin in particular, is subject to glucose repression (Martin and Liras, TIBS (1985), 3:39-44). This repression by glucose has been unequivocally established for the formation of the tripeptide by the ACV synthetase and for the activity of IPNS (Revilla et al ., J. Bacteriol. (1986), 168:947-952). It is not known at which stage of expression repression by glucose is exerted; this can, for example, be at the transcriptional or at the translational level. If the former applies, constitutive expression of the pcb AB gene will result in an increase in enzyme activity followed by an increase in the production of ACV, and subsequently of the β-lactam antibiotic derived from it. Increased expression of the gene in β-lactam producing bacterial or fungal hosts therefore can be obtained by changing the regulation of gene expression. Thus, the transcriptional regulatory region is preferably one which is not subject to repression by, for example, presence or absence of nutrients such as glucose, or expression products in the growth medium. The transcriptional regulatory region may additionally include regulatory sequences which terminate transcription and which provide sequences or structures which inhibit degradation of the mRNA. Exemplary of changing the regulation of expression is modification of the pcb AB gene. The native sequence generally is replaced by a region which is functional in either the native or heterologous host and wherein expression is either inducible or constitutive. For example, the regulatory sequences can be changed by replacing the pcb AB promoter, which is strongly repressed by glucose, with a promoter which is insensitive to glucose or even is stimulated by it. In the latter situation, antibiotic will be produced during the early stages of the fermentation, when biomass is formed in high-glucose conditions. This modification may further increase the yield of the antibiotic during the fermentation. Expression of the gene may also be brought under control of other promoters, either promoters for which expression can be regulated in a different fashion or promoters that are expressed constitutively. Illustrative promoters which find use in the subject invention, include, for prokaryotic cells, the lac , trp (Sommerville, Biotechnology and Genetic Engineering Reviews (1988), 6:1-41) or tac promoters of E.coli , or aph or tyrosine synthetase promoters of S.lividans . For filamentous fungi, illustrative promoters include the glyceraldehyde phosphate dehydrogenase (gapdh) promoter, the phosphoglycerate kinase ( pgk ) promoter, the nitrate reductase promoter and the like. A preferred embodiment of the present invention which is exemplified herein is the use of the pgk promoter of P.chrysogenum , which has been described in EP-A-354624. In eukaryotic cells, a terminator provides for proper maturation of the mRNA transcript and is necessary for efficient expression. In general, it is preferable to use the native polyadenylation signal associated with the gene of interest. In both eukaryotic and prokaryotic systems, a terminator can also contain sequences or structures which increase the stability of the mRNA species and allow for higher expression. Several examples of prokaryotic sequences are known, for example the trp terminator, the gene 32 (T4) terminator, or synthetic terminators which are similar in sequence to gene 32. For eukaryotes, terminators can be used that are isolated from cloned genes. For yeast, the terminator of the CYC1 gene or the actin gene can be used for example. For filamentous fungi, the terminators isolated from for example the trp C gene, the pgk gene or the pen DE gene are useful. Where it is desired to isolate the ACVS or ACVS', secretion of the enzyme into the media or into the periplasmic space of the transformed microbial host can improve the efficiency of the isolation procedure. Secretion can be accomplished by using DNA expression cassettes as described herein, which further comprise a signal sequence (secretory leader) that is functional in the host cell. The signal sequence will be heterologous to the ACV synthetase gene and may be homologous or heterologous to the host cell, or may be a synthetic signal sequence. The signal sequence provides a peptide sequence that is in-frame with the enzyme sequence, and may be located 5' or 3' to the ACV synthetase sequence. The signal sequence can also be provided by joining, in-frame, an open reading frame of a protein that is secreted by the host cell and the open reading frame of the ACV synthetase. Illustrative secretory leaders include the secretory leaders of penicillinase, α-factor, immunoglobulin, T-cell receptors, outer membrane proteins, glucoamylase, fungal amylase and the like. By fusion in proper reading frame, the mature polypeptide may be secreted into the medium. The promoter and terminator may be homologous (derived from the original host), or heterologous (derived from a foreign source or synthetic DNA sequences). The expression cassette thus may be wholly or partially derived from fermentable or known sources, and either wholly or partially derived from sources homologous to the host cell, or heterologous to the host cell. The various DNA constructs (DNA sequences, vectors, plasmids, expression cassettes) of the invention are isolated and/or purified, or synthesized and thus are not "naturally occurring." The expression cassette may be included within a replication system for episomal maintenance in an appropriate cellular host or may be provided without a replication system, where it may become integrated into the host genome. Integration may be stimulated in yeast and bacteria by the inclusion of (parts of) ribosomal RNA genes or other yeast genes and subsequent linearization in these genes. The DNA may be introduced into the host cell in accordance with known techniques, such as transformation of DNA, transfection by contacting the cells with a virus, micro-injection of the DNA into cells, biolistic transformation and the like. Both prokaryotic and eukaryotic hosts may be employed, which may include bacteria and fungi, particularly filamentous fungi. Prokaryotic cells include Escherichia coli , Flavobacterium and Streptomyces spp . Eukaryotic cells include filamentous fungi such as Penicillium chrysogenum , Acremonium chrysogenum , Aspergillus nidulans , niger and oryzae ; and yeasts such as Saccharomyces cerevisiae , Kluyveromyces lactis . Preferred host cells include P.chrysogenum , A.chrysogenum and A.nidulans . Replacement of the native ACVS gene by the mutant gene may result in in vivo synthesis of β-lactam derivatives. The presence of a wild type gene in these organisms indicates that they allow for efficient expression of the wild type ACVS gene and hence they are inferred to also express the mutant gene without too many difficulties. Other preferred hosts include the Streptomycetes . Several Streptomycetes also synthesize β-lactams (see Figure 1). They can be used for the same reasons as the above-mentioned fungi. On the other hand, a Streptomycete such as S.lividans is very amenable to genetic manipulation (D.A. Hopwood et al ., (1985) Genetic Manipulation of Streptomyces: a Laboratory Manual, The John Innes Foundation, Norwich, U.K.). Moreover, it has been disclosed that β-lactam biosynthetic genes are actively expressed in S.lividans (Chen et al ., supra ). Therefore, S.lividans is a preferred host for expression of the gene, for isolation of the mutant protein, or to use for preparation of cell-free extracts. Expression of foreign genes in E.coli is very well known in the art. One disadvantage of this organism may be that the ACVS could be too large for production of active enzyme; inclusion bodies may be formed or (over)expression may be harmful to the cell. Yeasts, like S.cerevisiae or K.lactis are examples of frequently used hosts for expression of heterologous proteins. Transformed host cells subsequently are grown under conditions that are suitable for the antibiotic production. These conditions have been amply described in the literature (Luengo et al ., J. Gen. Microbiol. (1979), 115:207-211; Barredo et al ., Antimicrob. Agents Chemother. (1988), 32: 1061-1067; Queener and Schwartz, In: Rose AH (ed) Secondary Products of Metabolism , Academic Press, London (1979): 35-122; Queener et al ., In: Biotechnology of Industrial Antibiotics , EJ Vandamme (ed) Marcel Dekker Inc., New York, Basel (1984): 141-170). Generally, media for antibiotic production contain either a slow fermentable carbon source, like lactose, or are limited in the carbon source, e.g. glucose, in a so called fed-batch fermentation procedure. For the production of penicillin G or V the appropriate side chain precursor, as has been described hereinbefore, is added to the medium; for the production of cephalosporin the medium may be supplemented with DL methionin. Generally, Corn Steep Liquor or related compounds are supplied as a nitrogen source. For prokaryotic hosts, the culture conditions are known in the art (e.g. T. Maniatis et al ., Molecular Cloning, a Laborarory Manual, Cold Spring Harbor Laboratory (First edition, 1982 or second edition, 1989); and Hopwood ( supra )); in this case it is advisable to maintain selective pressure by addition to the culture medium of the antibiotic that is used as a selective agent. The secondary metabolite is subsequently isolated from the cultured cells. ACVS or ACVS' can be purified from the host cells that have been grown under these conditions or, alternatively, using the conditions that are most suited for the expression of the promoter used; preferably, cells are harvested early in the fermentation (2-4 days) for this purpose. For the isolation of ACVS or ACVS', the procedures as described in Van Liempt et al . ( supra ) or Banko et al . ( supra ) can be used. For isolation of ACVS or ACVS' from transformed E.coli , the cells are grown overnight in e.g. TY or LB medium (Maniatis et al ., supra ) and can be lysed by treatment with lysozyme. Protease inhibitors, such as PMSF or α-2 macroglobulin can be included in the buffers used, in order to avoid degradation of the ACVS or ACVS' to be purified. Cell-free extracts can be prepared from the various cultures using the procedures as they are described in Adlington et al . ( supra ), Banko et al . ( supra ), Jensen et al . ( supra ), Zhang and Demain ( Biochem. Biophys. Res. Comm. (1990) 196:1145-1152), Jhang et al . ( FEMS Microbiol. Lett. (1989) 57:145-150). In general, cells are harvested, washed in a suitable buffer and disrupted in a French press, by grinding in liquid nitrogen or by sonication. The presence of glycerol (40-50%) as a stabilizer is crucial, both for isolation of large quantities of the enzyme and for isolation of an active cell-free extract. The pcb AB genes of the present invention, and more particularly the DNA constructs derived therefrom, can be used to transform suitable host cells, in order to increase the fermentable or known ACV synthetase activity present in the host cells. Cells with an increased level of ACV synthetase find their use in the production of increased amounts of β-lactam antibiotics. This increased production can be the result of the increased ACV synthetase activity by itself, or in combination with an increased activity of other enzymes, known in the art. For example, host cells that have been transformed with the pcb AB gene can be retransformed with a construct containing other β-lactam-biosynthetic genes, such as the construct pGJ02 that has been described before (EP-A-357119). In this way the entire pathway leading to penicillin G or V formation can be amplified. Combined use of these two constructs can also confer the ability to synthesize penicillin to host organisms that by nature lack this ability. Cells with increased ACV synthetase activity moreover find their use in the isolation of increased quantities of the ACV synthetase enzyme; the isolated enzyme can be used, for example, in in vitro reactions or for the further unraveling of the reaction mechanism. Cells with increased ACV synthetase activity also find their use in the preparation of cell-free extracts with increased ACV synthetase activity, leading to improved yields -and hence an improved economy- of in vitro synthesis of β-lactam antibiotics and their precursors. Amplification of the mutant pcb AB gene, particularly one with a phenotype of increased enzyme activity, will result in a further enhancement in intracellular enzyme activity, which in turn may be accompanied by an increase in productivity of the antibiotics of both the cephalosporin and penicillin pathways. Cells with increased ACVS' activity also find their use in the isolation of ACVS' enzyme or in the preparation of cell-free extracts with ACVS' activity. The following examples are offered by way of illustration and not by way of limitation. EXPERIMENTAL General Methods In all examples described herein, the experimental techniques mentioned are known in the art: these have been described e.g. in Maniatis ( supra ). Deposits The following organisms have been deposited with Centraal Bureau voor Schimmelcultures (CBS), Oosterstraat 1, 3742 SK Baarn, Netherlands: E.coli WK6 containing plasmid pPCV01, Accession No. CBS 142.90, was deposited February 28, 1990; Cosmid HM193, Accession No. CBS 179.89, was deposited on April 3, 1989 as a DNA sample (cosmid clone in E.coli unstable); P.chrysogenum strain Wisconsin 54-1255 npe 5 containing an ACV synthetase gene mutation, Accession No. CBS 178.89, was deposited on April 3, 1989. Example 1 Characterization of the pcbAB gene from P.chrysogenum A. Isolation of the pcbAB gene The isolation of DNA fragments encoding the ACV synthetase enzyme is described in EP-A-357119. Cosmid HM193 contains one such DNA fragment. Using parts of cosmid clone HM193 as a probe, cosmid clones containing similar or overlapping inserts have been isolated, using techniques known in the art. The presence of the pcb AB gene on the DNA fragments has been demonstrated in various ways. The DNA fragments were able to complement a mutation in the gene encoding ACV synthetase, which is present in the strain Wisconsin 54-1255 npe 5, deposited as CBS 178.89. This is a mutant ACV synthetase-negative P.chrysogenum strain derived from Wis54-1255. Restoration of penicillin production by complementation was accompanied by the reappearance of in vitro ACV synthetase activity in cell-free extracts. Moreover, restoration of penicillin production was also accompanied by reappearance of a large protein on SDS-PAGE gels. B. Localization of the pcbAB aene Northern hybridizations were performed using mRNA that was isolated from penicillin-producing cultures as described in EP-A-354624. The subcloned Sal I fragments indicated in Figure 3 were used as probes. Using probes I, II, III and IV a large mRNA (≥ 10 x 10 3 nucleotides) was detected in the Northern blot hybridizations. Probe V detected the mRNAs of the pcb C and pen DE genes (about 1.5 x 10 3 nucleotides). Using smaller probes (A-D and E-H in Figure 4), the positions of the putative 5'- and 3'-ends of the gene were located. The gene encoding the ACV synthetase is present on the chromosome of P.chrysogenum as indicated by the shaded region in the schematic of Figure 3. C. Polarity of the gene The direction of transcription was determined by Northern hybridization of mRNA of P.chrysogenum using as a probe the following synthetic oligonucleotides that have been designed based on the nucleotide sequence surrounding the two Hind III sites located within the ACVS coding region (Figure 4). Only probes AB1549 and AB1550 hybridized to the ACV synthetase mRNA and the deduced direction of transcription is indicated in Figure 5. The direction of transcription of the ACV synthetase gene is in the opposite direction to the genes encoding IPNS and AT. D. Determination of the nucleotide sequence The nucleotide sequence of a DNA fragment contained in cosmid HM193 containing the ACV synthetase gene was determined by the dideoxy method (Sanger et al ., Proc. Natl. Acad. Sci. U.S.A. (1977) 74:5463-5467) using the Sequenase system 2.0 (U.S. Biochemicals, Cleveland, OH). Double sequencing reactions with dGTP and dITP were used in some clones to avoid errors (Barnes et al ., (1983) Methods in Enzymology 101, 98-122). The sequencing strategy is given in Figure 4. The nucleotide sequence of the 12,364 nucleotide DNA fragment is given in sequence listing 1. In this sequence a long open reading frame (ORF) of 11,337 nucleotides was found from which a protein sequence of 3778 amino acids was deduced. E. Determination of partial protein sequence of ACV Synthetase To further confirm that the gene cloned is indeed the gene encoding the P.chrysogenum ACV Synthetase, amino acid sequences have been obtained from ACV synthetase that has been purified from Aspergillus nidulans . The similarity between protein and nucleotide sequence data confirm the identity of the cloned gene. ACV Synthetase was purified from A.nidulans , strain G-69, as has been described in Van Liempt et al ., ( supra ). The enzyme containing fractions from the DEAE column were applied to a Mono Q column on a FPLC apparatus (Pharmacia). ACV synthetase was eluted using a 0-300 mM NaCl gradient in Tris/HCl buffer (pH 7.5). The peak fraction, having a protein concentration of 0.5 mg/ml was shown to contain almost pure (over 90%) ACV synthetase; this was demonstrated by SDS-PAGE. For digestion with protease, 1.25 ml (0.625 µg) of ACV synthetase from the Mono Q fraction was incubated with 62.5 µl (6.3 µg) of Subtilisin (Sigma) for 60 min. at room temperature. The reaction was terminated by the addition of trichloroacetic acid (10% final concentration). The precipitated protein was recovered by centrifugation, dissolved in approximately 200 µl of Laemmli sample buffer, and the mixture was neutralized by the addition of 4M Tris. The protein was dissolved and incubated for 5 min. at 95°C. A slab gel of 140 x 170 x 1.5 mm of polyacrylamide (separating gel 5% T), with a 3% stacking gel (Laemmli) was allowed to polymerize overnight at 4°C. The digested protein was applied in several slots, and electrophoresis was carried out with 0.02% thioglycolic acid in the upper buffer compartment. After the Bromophenol Blue marker had migrated up to 70% of the gel length, the electrophoresis was terminated and the proteins were transferred electrophoretically onto a PVDF-membrane (Immobilon) in a semidry blotting apparatus (Sartorius). The transfer buffer was 25 mM Tris/HCl (pH 8.5), 0.5 mM dithioerythritol. After transfer (approximately 2 hrs.) the membrane was washed with water, stained in 0.5% Coomassie R 250, 50% Methanol for 5 min., destained in 50% Methanol, 10% Acetic Acid, washed with water and air dried. From the complex pattern of protein bands present on the membrane, the bands that were the most pronounced and least contaminated with other nearby bands were excised. The amino acid sequence of the peptides in the excised bands was determined using a gas-phase sequenator (Applied Biosystems model 470a). The following sequence was determined (in this notation amino acids separated by a slash indicates ambiguity of interpretation at this position while an amino acid in parentheses indicates uncertainty in the interpretation; Xxx indicates the presence of an unidentified amino acid): F. Comparison of protein and nucleotide sequences The amino acid sequences from Example 1E ( A.nidulans ) were compared with the deduced amino acid sequence from Example 1D ( P.chrysogenum ), which is shown in sequence listing 1. For this comparison the MicroGenie™ 6.0 program (Beckman) was used. The following results have been obtained: The degree of homology found between A.nidulans and P.chrysogenum varies between 50 and 61%. This is very similar to the degree of homology between the IPNS proteins derived from both organisms (e.g. G. Cohen et al ., Trends in Biotechnology (1990), 8:105-111); hence the protein data confirm the conclusion that the gene cloned is indeed the P.chrysogenum ACV synthetase gene. G. Identification of distinct domains in the ACVS enzyme which are used to obtain ACVS' enzyme From the nucleotide sequence of the pcb AB gene an amino acid sequence has been deduced, as indicated in sequence listing 1. Upon matrix comparison of this protein sequence with itself, three distinct regions of homology are found: these regions are defined herein as domains. These domains are located between amino acids 301 and 1068 (domain I), 1392 and 2154 (domain II) and between amino acids 2474 and 3295 (Domain III; Figures 13 and 15). Similar domains were found in the pcb AB gene of A.chrysogenum . Within these domains, several even more conserved elements can be distinguished. A summary is given in Figure 15. Since the ACVS enzyme synthesizes a tripeptide, which most probably requires the activation of three amino acids, a role of these three domains in the amino acid activation reactions seems likely. Therefore, the said domains are candidate regions for in vitro mutagenesis. Comparison of the deduced amino acid sequence with the protein sequences known for other multifunctional enzymes reveals a significant homology with the Bacillus brevis tyrocidin synthetase I (hereinafter referred to as TYI; Weckermann et al ., Nucleic Acids Research (1988), 16:11841) and gramicidin synthetase I (hereinafter referred to as GSI; Krätzschmar et al ., ( supra ); compare Figure 13). Since both TYI and GSI are involved in activation and racemisation of the amino acid phenylalanine, this homology supports the notion that these conserved sequences may represent centers involved in ATP-mediated activation of amino acids. If the mechanism of peptide synthesis by ACVS or Gramicidin Synthetase is comparable, the expected order of the domains in the enzyme is in the same order as that of the amino acids in the tripeptide (Krätzschmar, supra ). Upon comparison of the ACVS protein sequence with other known protein sequences of large enzymes, a significant homology is found with the Fatty Acid Synthetases (long chain) from rat and chicken (Figure 14; M. Schweizer et al ., Nucleic Acids Research (1989) 17:567-586; Z. Yuan et al ., PNAS (1988), 85:6328-6331). The homology is found between the COOH-terminal part of the ACVS protein (domain IV in Figure 15) and the thioesterase domain of the long chain Fatty Acid Synthetase proteins. Even the active site of the thioesterase, viz. G.X.S.X.G. (e.g. Krätzschmar, supra ), is present in the ACVS protein sequence. A similar homology has been described for a subunit of gramicidin Synthetase; in this case the homology is found between the grsT subunit and the type II fatty acid synthetases. The demonstration of the said homology suggests that ACVS most probably also contains within its structure the ability to release the tripeptide, once it has been formed and bound to the enzyme by thioester bond formation, by the action of its thioesterase domain. This activity may form another target for in vitro mutagenesis, e.g. by increasing the efficiency of the release of the tripeptide, in case this part of the reaction is the rate determining step. However, in experiments designed to change substrate specificity, this domain preferably remains unchanged. The functional domains of the ACVS protein being identified, it now is possible to construct modified peptide synthetases by the exchange of functional domains. This can be achieved e.g. by interchanging the three domains that have been identified within the ACVS protein. An alternative way is the exchange of ACVS specific domains with domains from other proteins known to have a similar function. Selected domains can be isolated by restriction enzyme digestion of clones containing the genes encoding the said proteins or domains thereof. However, suitable restriction sites at useful positions are seldom encountered. Therefore, the Polymerase Chain Reaction offers a good alternative for the isolation of suitable DNA fragments. In general, DNA fragments containing functional domains are prepared as follows: for each domain two oligonucleotides are designed, one starting at the N-terminal end of the selected domain, in the direction of the C-terminal end. The other oligonucleotide is derived from the C-terminal end of the domain and is designed in the opposite direction; consequently this oligonucleotide is derived from the other DNA strand. At the 5'-end of each oligonucleotide a suitable restriction site can be included in the oligonucleotide, in order to facilitate ligation of the domains after amplification. The DNA fragments between the two oligonucleotides are amplified using the polymerase chain reaction, thereby following the protocols known in the art (described e.g. in 'PCR-Technology', supra ). Preferably, Taq DNA polymerase is used for the amplification reaction. Example 2 Expression of the pcbAB gene from P.chrysogenum in P.chrysogenum Synthesis of pPCV01: This vector was derived from pBluescript II KS M13(+) (Stratagene, La Jolla, CA) and contains the phleomycin resistance gene under control of the P.chrysogenum pgk promoter. It moreover contains a synthetic multiple cloning site, including a unique Spe I site. The P.chrysogenum pgk gene has been isolated from a genomic cosmid library (EP-A-357119) using the corresponding gene of Saccharomyces cerevisiae (Dobson et al ., Nucleic Acid Research (1982) 10:2625-2637) as a probe (Van Solingen et al., Nucleic Acid Research (1988) 16:11823). The sequence of part of the promoter is disclosed in EP-A-354624. The promoter and a small part of the coding region can be isolated as a 1.5 kb Hind III fragment. Synthesis of PCV02: The pcb AB gene was isolated from cosmid clone HM193 as a 1.2 x 10 4 nucleotide Spe I fragment and was subcloned into the vector pPCV01, using the unique Spe I site. Conditions used were as disclosed in Maniatis et al . ( supra ). The resulting construct was named pPCV02 (Figure 7). The orientation of the gene in the vector has been determined by digestion with restriction enzymes. Transformation The plasmid pPCV02 has been transformed into P.chrysogenum Wis54-1255 npe 5 (CBS 178.89) using the procedure described in EP-A-260762. Strain npe 5 is a non-producing mutant of Wis45-1255; the npe phenotype is caused by the absence of ACV synthetase activity. Transformants have been selected for resistance against 30 µg/ml of phleomycin. Isolated transformants have been tested in a bioassay, as described in EP-A-354624, for a restoration of penicillin production. In a representative experiment penicillin production has been restored in 80% (8 out 10) of the pPCV02-transformants analysed; in transformants having received the vector pPCV01, without the ACV synthetase insert, a restoration of the penicillin production has not been demonstrated (0 out of 26). The construct pPCV02 is also transformed into wild type P.chrysogenum . Selected transformants are assayed for an increased ACV synthetase activity, using cell-free extracts as described in EP-A-357119, or for an increased penicillin productivity, using shake flask experiments as have been described in EP-A-354624. Example 3 Expression of the pcbAB aene from P.chrysogenum under control of the P.chrysogenum pgk promoter Transcription of the pcbAB gene is subiect to glucose repression mRNA preparations, isolated from Penicillium cultures grown on either glucose- or lactose-containing media (EP-A-354624) are transferred to Genescreen-plus® (NEN/DuPont) and hybridized with the 1.5 x 103 nucleotide Hind III fragment, which is internal to the pcb AB gene (Figure 4). In glucose-grown cultures, no pcb AB mRNA is detected, while in the lactose grown cultures, a large mRNA (≥ 10 x 10 3 nucleotides) is detected. Construction of pPCV03 The region surrounding the ATG start codon of the pcb AB gene is isolated as a 1.7 x 10 3 nucleotide Sal I- Dra I fragment. The vector pTZ18R (US Biochemical Corporation, Cleveland, OH) is digested with Sal I and Sma I restriction enzymes. The digested vector and the 1.7 x 10 3 fragment are ligated. A construct containing the pTZ18R vector bearing the 1.7 x 10 3 fragment insert is isolated. Into this construct, the P.chrysogenum pgk promoter is ligated as a 1.5 x 10 3 nucleotides Hind III fragment (EP-A-354624). A construct containing the pgk promoter in the desired orientation (same polarity as the pcb AB fragment) is isolated. From this construct single stranded DNA is isolated by superinfection with the helper phage M13K07, (U.S. Biochemical Corporation, Cleveland, OH) using techniques known in the art or as prescribed by the supplier of the pTZ cloning vector. By in vitro mutagenesis using a synthetic oligonucleotide having the following sequence: a DNA fragment containing the mature pgk region and the region upstream of the pcb AB ATG is looped out. This mutagenesis introduces an Nde I site at the position of the ATG (underlined in the oligonucleotide sequence). The construct is named pTZpgk::acvsl. This construct is partially digested with Hind III and Xba I and the promoter-gene fusion is isolated on a DNA fragment of 2.9 x 10 3 nucleotides as described by Maniatis et al ., ( supra ). This fragment is ligated with the 16.3 kb DNA fragment containing the pcb AB gene, isolated after partial Xba I digestion of pPCV02, the remaining Hind III- Xba I sticky ends are filled in with T4 DNA polymerase and the construct is circularized by the addition of ligase. The resulting construct, pPCV03 (Figure 8), is isolated as follows. The ligation mixture is transformed into E.coli HB101 (ATCC 33694) using standard techniques. Plasmid DNA is isolated from several transformants and analysed by restriction enzyme-digestion and agarose gel electrophoresis. Transformants containing the correct DNA constructs are grown on large scale (500 ml) and plasmid DNA is isolated using methods as disclosed in Maniatis et al ., ( supra ) and transformed into P.chrysogenum . Expression of the pcb AB gene, enzyme activity and penicillin production of transformants is analyzed after growth in shake flasks on both glucose- and lactose-containing media; the data obtained are compared with those obtained for transformants containing pPCV02. In contrast to transformants containing pPCV02, transformants containing pPCV03 express the pcb AB gene in glucose-containing media: both an ACV synthetase-specific mRNA and ACV synthetase enzyme activity are detected. Example 4 Expression of the pcbAB gene in E.coli For efficient production of ACV synthetase enzyme in E.coli it is necessary to place the pcb AB gene under control of a promoter which allows efficient gene expression in E.coli . Examples of such efficient promoters are the trp promoter, the lac promoter and the tac promoter. In this example the trp promoter is described, but it will be obvious to those skilled in the art that the experiments can be easily repeated with the lac and tac promoters, leading to similar results. If desired, a runaway replicon can be included in the construct; this will allow for controlled amplification of the plasmid copy number after a temperature shift. Synthesis of pMA-ACVS : pMAtrp (Figure 9), having the trp promoter region between -113 and the ATG transcription start flanked by an Nde I site at the position of the ATG (Sommerville, supra ), is digested with Sma I and Nde I. From pPCV03 a 1.5 x 10 3 nucleotide Nde I- Dra I fragment, containing the 5'-part of the pcb AB gene, is isolated and ligated into the Sma I, Nde I digested pMAtrp. Into the Xba I sites of the resulting construct, the 3'-part of the pcb AB gene is inserted as a 1.0 x 10 4 nucleotide Xba I fragment isolated from HM193 or pPCV02. Constructs are selected for the correct orientation of the inserted Xba I fragment; the construct containing the fragment in the correct orientation is named pMA-ACV synthetase (Figure 10). Orientation is determined by digestion with various restriction enzymes. pMA-ACV synthetase is isolated and used to transform a suitable E.coli host, such as, for example E.coli HB101, E.coli C600 or E.coli JM101. Transformants are analyzed by determination of ACV synthetase activity in cell-free extracts, by electrophoresis of cell-free extracts in 5% polyacrylamide gels (SDS-PAGE) and by immunoblotting, using a polyclonal antiserum which had been raised against purified ACV synthetase. Example 5 Expression of the pcbAB gene from P.chrysogenum in Streptomyces lividans For expression in a Streptomyces host, several options are available. S.lividans is a preferred host because of the ease of transformation of this host, as compared to several other Streptomycetes . Expression of the P.chrysogenum ACV synthetase gene under control of the aph promoter The P.chrysogenum ACV synthetase gene is isolated as the 1.2 x 10 4 nucleotide Spe I fragment described in Example 2. The sticky ends of this fragment are made blunt by treatment with T4 DNA polymerase using the procedures known in the art. The vector pIJ61 has been described by C.J. Thompson et al ., (1982) Gene 20, 51-62); reviewed in. Hopwood et al . ( supra ); the vector can be obtained from D.A. Hopwood. The vector pIJ61 was digested with Bam HI, and the ends were made blunt using T4 DNA polymerase. Subsequently, the blunt-ended 1.2 x 10 4 nucleotide Spe I fragment is inserted into the blunt-ended Bam HI site via ligation, and the mixture is used to transform S.lividans 66, by the method of Hopwood et al. (D.A. Hopwood et al. , 1985, supra ). Transformants are selected for resistance to thiostreptone (50 µg/ml) and are subsequently analyzed for the orientation of the pcb AB gene within the aph gene. A construct having the pcb AB gene in the same orientation as the aph gene is named pSLACV-01 (Figure 11). Selected transformants containing the plasmid pSLACV-01 were cultured as described in Chen et al ., ( supra ). Cell-free extracts are prepared and analyzed by SDS-PAGE or immunoblotting for the presence of a large (>250 kDa) protein; the ACV synthetase activity in the extracts is also determined using the procedure as described by Van Liempt ( supra ). Expression of the P.chrysogenum pcbAB aene in S.lividans under control of the tyrosinase promoter The vector pIJ702 is digested with either Bgl II or Ss tI. pIJ702 is described by E. Katz et al ., Journ. Gen. Microbiol. (1983) 129:2703-2714; reviewed in: D.A. Hopwood et al ., supra ; obtainable from D.A. Hopwood. Bgl II digested pIJ702 is made blunt-ended by treatment with T4 DNA polymerase; Sst I digested pIJ702 is treated with Mung bean nuclease in order to obtain blunt ends. The blunt-ended Spe I fragment containing the P.chrysogenum pcb AB gene as described hereinabove is inserted into both blunt-ended vectors via ligation. Thiostreptone resistant transformants are screened for a melanin-negative phenotype by application of a soft agar overlay containing tyrosine (0.1 mM) as described in Hopwood et al . ( supra ). Melanin-negative transformants contain an interrupted tyrosinase gene (by insertion of the ACV synthetase gene) and their colonies remain colorless upon addition of tyrosine, while wild type colonies turn brown. Analysis by restriction enzyme digestion and agarose gel electrophoresis for the correct orientation of the inserts yielded plasmids pSLACV-03A ( Sst I site; Figure 12A) and pSLACV-03B ( Bgl II site; Figure 12B). Analysis of transformants is as described hereinabove. Induction of the tyrosinase promoter is established by the addition of methionine (10 mM) to the culture medium. Example 6 Isolation and characterization of the pcbAB gene from A.chrysogenum Isolation of the A.chrysogenum pcbAB gene A gene library of A.chrysogenum C10 (ATCC 48272) has been constructed in the lambda cloning vector EMBL3. To this end, A.chrysogenum DNA has been partially digested with Sau 3A and fragments ranging in size from 13 to 17 x 10 3 nucleotides have been isolated by sucrose gradient ultracentrifugation. The vector EMBL3 has been digested with Bam HI and arms have been separated and purified by sucrose gradient centrifugation. About 0.4 µg of vector arms have been ligated with 0.5 µg of partially digested, purified A.chrysogenum DNA; the ligation mixture has been packaged in vitro using the lambda phage packaging system of Amersham (Buckinghamshire, UK), following the procedure provided by the supplier. The packaged mixture has been used to infect E.coli Q-359 (ATCC 47019); about 70,000 plaques have been obtained. In order to isolate phages containing the A.chrysogenum pcb AB gene, the gene library first was screened using as a probe the isolated pcb C gene from P.chrysogenum , namely a 1 x 10 3 nucleotides Nco I fragment, carrying the promoterless gene (Barredo et al ., (1989)b, Mol. Gen. Genet. 216:91-98); conditions have been described in Barredo et al ., 1989b ( supra ). Five phages, showing a positive hybridization signal have been purified and a restriction map has been constructed. The position of these phages (F1, F3, F4, F5 and F6) relative to a restriction map of the genomic region is given in Figure 16. To identify the presence of another gene on the phages, the isolated DNA has also been hybridized with a 6.0 x 10 3 nucleotides Sal I fragment (III in Figure 3) derived from the P.chrysogenum pcb AB gene as it is present in, for example, HM193. All five phages showed a strong hybridization signal with this latter probe. This finding indicates that in A.chrysogenum the pcb C and pcb AB genes are linked, as they are in P.chrysogenum and as indicated in Figure 16. To isolate phages with an insert containing more upstream sequences, as compared to the isolated phages (which extend a maximal 11.5 x 10 3 nucleotides upstream from the 5'-end of the pcb C gene) the library has been rescreened using as a probe a 0.9 x 10 3 nucleotides Xho fragment (probe P6 in Figure 17) isolated from, for example, phages F1, F3, F5 or F6. This rescreening has yielded another seventeen phages. Eight of these phages have been further purified and characterized; the position of the phages F12, F17, F18, F20, F21, F23, F24 and F25 relative to a restriction map of the genomic region is given in Figure 16. Localization of the pcbAB gene of A.chrysogenum . Northern blot hybridizations have been performed using mRNA that has been isolated from A.chrysogenum following the procedure described in Barredo et al ., 1989b ( supra ). As probes, the fragments P1-P8 as indicated in Figure 17 have been used. Probes P3 and P4 hybridize to a transcript of about 1.15 x 10 3 nucleotides; this transcript is derived from the pcb C gene and encodes the IPNS enzyme. Probes P1, P2, P5, P6 and P7 hybridize to a large mRNA, size >11 x 10 3 nucleotides; this indicates the presence of a large gene in the region covered by these probes. Moreover, the ends of the large transcript have been more accurately mapped by the results using the small probes P5-P8: absence of hybridization using probe P8 indicates that the distal end of the putative pcb AB gene is located within the 0.5 x 10 3 nucleotides AccI fragment preceding P8 (namely in region P7 in Figure 17) while the hybridization patterns of probes P4 and P5 indicate that the proximal end of the putative pcb AB gene has to be present outside region P4, most probably within region P5. The orientation of the gene has been established by homology with the P.chrysogenum gene; it is transcribed in the opposite direction to the pcb C gene (Figure 16), which also occurs in P.chrysogenum . Determination of the nucleotide sequence of the pcbAB gene from A.chrysogenum . The nucleotide sequence of the region encoding the putative pcb AB gene of A.chrysogenum has been determined. Five subclones have been constructed in pBluescript KS(+) (Stratagene, LaJolla, U.S.A.), starting from phage F12 (Figure 16). The subclones contain the following fragments: A (3.6 x 10 3 nucleotides Bam HI), B (1.7 x 10 3 nucleotides Sal I- Kpn I), C (3.2 x 10 3 nucleotides Sal I), D (2.4 x 10 3 nucleotides Sal I- Pst I) and E (2.0 x 10 3 nucleotides Pst I- Kpn I). Fragment A has been cloned in both orientations in pBluescript KS(+) which had been digested with Bam HI; subsequently the fragment has been subcloned into 23 smaller, overlapping fragments, which have been sequenced using the dideoxy chain termination method (Sanger et al ., Proc. Natl. Acad. Sci. (U.S.A.) (1977) 74:5463-5467), using either Sequenase (U.S. Biochemicals, Cleveland, OH) or Taq polymerase (Promega, Madison, Wisc). The sticky ends of fragment B have been made blunt, using standard techniques, and the blunt end fragment has been cloned into EcoRV digested pBluescript KS(+). Subsequently, the fragment has been subcloned into fourteen overlapping smaller fragments. These fragments have been sequenced using the dideoxy chain termination method. Fragments C, D and E have been sequenced by generating sets of ordered deletion mutants using the "erase-a-base" system (Promega, Madison, Wisc). To this end, fragment C has been subcloned into Sal I-digested pBluescript KS(+) in both orientations; the clones have been opened by digestion with Bst XI and Xba I to obtain appropriate ends to generate the deletions. Fragment D has been subcloned in both orientations into Eco RV-digested pBluescript KS(+), after filling in the sticky ends of the fragment. For the generation of sets of deletion mutants, the clones have been opened by digestion with Pst I and Eco RI. Fragment E (after filling in the sticky ends) has been subcloned into Eco RV digested pBluescript KS(+) in both orientations; clones have been opened for generation of sets of deletion mutants by digestion with Eco RI and Pst I. The digested clones of the fragments C, D and E have been treated with exonuclease III, followed by deletion of the remaining sticky ends with exonuclease S1. The gaps that have been introduced by both enzymes have been repaired with Klenow DNA polymerase, followed by ligation of the linear molecules and transformation into E.coli . For all treatments, the conditions recommended by the supplier of the "erase-a-base" system have been used. The resulting fragments have been sequenced using the dideoxy chain termination method. Clones overlapping the junctions of the five fragments have also been sequenced; the entire region has been sequenced in both strands. The nucleotide sequence of a 11.8 x 10 3 nucleotides DNA fragment is shown in sequence listing 2. In this sequence a long open reading frame (ORF) of 11,139 bp has been demonstrated. From this ORF a protein sequence of 3712 amino acids has been deduced (sequence listing 2). Similarity with the P.chrysogenum pcbAB sequence The nucleotide sequence of the A.chrysogenum pcb AB gene has been compared with the nucleotide sequence that has been determined for the P.chrysogenum pcb AB gene (Example 1; sequence listing 1). A 62.9% homology has been determined at the nucleotide level. At the protein level the similarity is 54.9% (based on the deduced amino acid sequences); a comparison between the deduced amino acid sequences for the P.chrysogenum and A.chrysogenum ACVS is given in Figure 18. The ACV synthetase enzyme activities reside on a single gene in P.chrysogenum and A.chrysogenum . Elevated activity of these enzymes can be obtained in host cells using the recombinant compositions of the instant invention. Heterologous expression of ACV synthetase can provide a means of using more efficient and robust hosts for the commercial production of ACV synthetase and its secondary metabolites. Finally, the availability of large amounts of the ACV synthetase-enzyme, for example, by expression of the gene in a suitable host, will allow for better prospects for commercial application of in vitro synthesis of β-lactam antibiotics and their precursors. All publications and patent applications cited in this specification are herein incorporated by reference as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Sequence Listing No. 1 SEQ. No.: 1 SEQUENCE TYPE : Nucleotide with corresponding protein SEQUENCE LENGTH: 12364 base pairs STRANDEDNESS: single TOPOLOGY: linear MOLECULE TYPE: genomic ORGANISM: Penicillium chrysogenum FEATURES: from 264 to 11600 bp peptide PROPERTIES: gene encoding ACV Synthetase Sequence Listing No. 2 SEQ. SEQ. ID No.: 2 SEQUENCE TYPE: Nucleotide with corresponding protein SEQUENCE LENGTH: 11601 base pairs STRANDEDNESS: single TOPOLOGY: linear MOLECULE TYPE: genomic ORGANISM: Acremonium chrysogenum FEATURES: from 388 to 11526 bp peptide PROPERTIES: gene encoding ACV Synthetase

Claims: A DNA fragment of fewer than about 15 kbp comprising: an open reading frame encoding δ-(L-α-aminoadipyl)-L-cysteinyl-D-valine synthetase, wherein the sequence of said open reading frame is included in the complete sequence shown in sequence listing 1 or in sequence listing 2 or wherein the sequence of said open reading frame includes conservative and/or non-conservative mutations of the sequence shown in sequence listing 1 or in sequence listing 2, said mutations producing a biologically active expression product. An expression cassette comprising: as components, in the direction of transcription, a promoter functional in a host cell, an open reading frame encoding δ-(L-α-aminoadipyl)-L-cysteinyl-D-valine synthetase, as defined in claim 1, and a terminator functional in a host cell, wherein expression of said synthetase is under regulatory control of said promoter and terminator. The expression cassette of claim 2, wherein said promoter is functional in a filamentous fungus, and is preferably a Penicillium chrysogenum pgk promoter. The expression cassette of claim 2, wherein said promoter is active in the presence of glucose. The expression cassette of claim 2, wherein said promoter is functional in a prokaryotic cell, and is preferably an E.coli trp promoter, a Streptomyces aph promoter or a tyrosinase promoter. Plasmid pPCV02, having a restriction site and functional map of Figure 7; plasmid pPCV03, having a restriction site and functional map of Figure 8; plasmid pMA-ACVS, having a restriction site and functional map of Figure 10; plasmid pSLACV-01, having a restriction site and functional map of Figure 11; plasmid pSLACV-03A, having a restriction site and functional map of Figure 12A, or plasmid pSLACV-03B, having a restriction site and functional map of Figure 12B. A transformed host cell comprising: an expression cassette as defined in any one of the claims 2-5. The cell of claim 7, wherein said cell is a eukaryotic cell, preferably a filamentous fungus, and more preferably Penicillium chrysogenum, Acremonium chrysogenum or Asperqillus nidulans . A transformed Penicillium chrysogenum, Acremonium chrysogenum or Asperqillus nidulans host cell comprising plasmid pPCV02 or pPCV03, as defined in claim 6. A transformed E.coli cell comprising plasmid pMA-ACVS, as defined in claim 6. A transformed Streptomyces cell comprising plasmid pSL-ACV01, pSL-ACV03A or pSL-ACV03B, as defined in claim 6. Use of a transformed host cell as defined in claims 7-9 in an industrial β-lactam production process. A method for obtaining or enhancing ACVS production in a microbial host cell, said method comprising: growing the host cell of claims 7-9 under conditions whereby said ACVS is produced and optionally isolated. A method for enhancing production of β-lactam antibiotics in a microbial host cell, said method comprising: transforming a host cell capable of producing β-lactam antibiotics with the expression cassette of claim 2-5 whereby transformed cells are obtained; cloning said transformed cells by means of a selection marker on said expression cassette; identifying transformed cells producing increased amounts of β-lactam antibiotics as compared to untransformed cells (hereinafter "enhanced expressers") and isolating said enhanced expressers; growing said enhanced expressers whereby enhanced production of β-lactam antibiotics in a microbial host is obtained; and optionally isolating said β-lactam antibiotics. The method according to claim 14, wherein said host cell is a filamentous fungus, and preferably a Penicillium chrysogenum, Acremonium chrysogenum or Aspergillus nidulans cell, or a Streptomyces cell.

Inventors: BARREDO JOSE LUIS, DOEHREN HANS VON, GARCIA BRUNO DIEZ, GUTTIEREZ SANTIAGO, LIEMPT HENK VAN, MARTIN JUAN FRANCISCO, MONTENEGRO PRIETO EDUARDO, PALISSA HARRIET, VEENSTRA ANNEMARIE EVELINE, BARREDO, JOSE LUIS, GARCIA, BRUNO DIEZ, GUTTIEREZ, SANTIAGO, MARTIN, JUAN FRANCISCO, MONTENEGRO PRIETO, EDUARDO, PALISSA, HARRIET, VAN LIEMPT, HENK, VEENSTRA, ANNEMARIE EVELINE, VON DOEHREN, HANS
Applicants: GIST BROCADES NV, GIST-BROCADES N.V.
Application year: 1991
Main classification: C12N15
IPC codes: C12P37, C12R1, C12N9, C12N15, C12N1, C12P35, C12P21