PRODUCTION OF VARIANT NISIN

Abstract: A cell which does not contain a natural nisA gene but expresses a nisin comprising a variant nisA gene wherein the variant nisA gene has the same relationship as the natural nisA gene to a gene cluster containing the natural nisA gene and the genes for nisin modification, secretion and immunity. Preferably the natural, chromosomal nisA gene is absent and the cell comprises a variant nisA gene at the chromosomal location of the said natural nisA gene. Methods of making the cells, and processes for producing nisin, in particular variant nisin, are described.

Description: Production of variant ni sin The present invention relates to improved methods and bacterial strains for the production of nisin, in particular protein-engineered nisins. Nisin is a highly modified peptide antibiotic produced, for example, by certain strains of Lactococcus lactis. It is of great interest to the food industry because of its efficient antimicrobial activity against a wide range of gram-positive organisms including many spoilage bacteria and food pathogens, for example, IΛsteήa, Clostridia and Bacillus species (see Fowler & Gasson (1990) in Food Preservatives (eds N.J. Russell & G. W. Goulds) pages 135-152, Blackie and Sons, Glasgow, UK). The chemical structure of nisin is well established (Figure 1). It is a member of the family of antibiotics termed Iantibiotics. These unusual polycyclic peptides share the structural features of dehydro-residues and intrachain sulphide bridges forming lanthionine and 3-methyllanthionine rings. The atypical residues are introduced by post-translational modification of amino acids serine, threonine and cysteine in the primary sequence of a precursor peptide (Iantibiotics are the subject of a recent extensive review by Jung (1991) in Nisins and novel Iantibiotics (eds Jury, G. & Sahl, U.S.) pages 1-34, ESCOM, Leiden, Netherlands). Biosynthesis of nisin thus involves genes for both the inactive precursor of nisin, known as prenisin, (nisA) and also the modifying enzymes responsible for nisin maturation. The mature nisin molecule is based on a sequence of 34 amino acids. The protein encoded by nisA includes a 23 amino acid N terminal signal sequence which is cleaved off during secretion of nisin. The conversion of prenisin, encoded by nisA, into mature nisin involves cleavage of the leader and the modification of individual amino acids. A nisA gene has been cloned and characterised and shown to have a chromosomal location (see Dodd et al (1990) J. Gen. Microbiol. 136, 555-566). A number of additional genes involved in the enzymatic modification of prenisin, translocation and immunity are encoded by nisin producing strains (Kuipers et al (1993) Eur. J. Biochem. 216, 281-291 ; Engelke et al (1994) Appl. Environ. Microbiol. 60, 814- 825). Established protein engineering techniques can be used to introduce changes to the amino acid sequence of nisin. This involves modifying the coding region of the nisin structural gene, nisA, for example by site- directed or random mutagenesis. Expression of these changes is complicated by the fact that nisin is post-translationally modified. Variant nisins may be constructed by the expression of variant nisA genes in a host strain which encodes the necessary maturation machinery, and thus can process the modified precursor peptide. One approach is to transform a nisin producing strain with a recombinant plasmid encoding a variant nisA gene. In this background the host's maturation enzymes are available to process both the resident prenisin and its plasmid-encoded variant. A strategy of this type has been reported for a strain that carries the wild-type nisin transposon (Kuipers et al (1991) in Nisins and novel Iantibiotics (eds Jung, G. & Sahl, H.-S.), pages 250-259, ESCOM, Leiden, Netherlands). However, the disadvantage of this system is that both the host's nisin and the engineered variant are synthesised together, making complex chemical separation procedures necessary prior to analysis of the properties of the novel peptide. Such a procedure would be particularly undesirable for industrial scale production of a variant nisin. WO 93/20213 describes a process for producing a variant nisin from Lactococcus in the absence of natural nisin in which a plasmid-borne variant nisA gene (which encodes the variant nisin) is introduced into a strain of Lactococcus which does not secrete its natural nisA nisin (because the nisA gene has been inactivated) but is capable of expressing genes for nisin modification, immunity and translocation out of the cell. WO 92/18633 discloses plasmid-based systems for the expression of variant nisins from the nisZ gene (or mutants thereof) in Lactococcus strains that do not produce natural nisA nisin. Unexpectedly we have found that by replacing the natural, chromosomal copy of the nisA gene (or at least a part thereof) with a variant nisA gene (or part thereof) we can produce surprisingly high levels of nisin, particularly variant nisins, from Lactococcus. Thus, the present invention provides improved methods and organisms for producing variant nisins with greater efficiency. One aspect of the invention provides a method for making a cell which does not contain a natural nisA gene but expresses a nisin comprising the step of providing a cell with a variant nisA gene and genes for nisin modification, secretion and immunity wherein the variant nisA gene has the same relationship as the natural nisA gene to the gene cluster containing the natural nisA gene and the genes for nisin modification, secretion and immunity. By "providing a cell with a variant nisA gene and genes for nisin modification, secretion and immunity" we include inserting a variant nisA gene into a cell that already contains genes for nisin modification, secretion and immunity as well as inserting into a cell at the same time a variant nisA gene plus genes for nisin modification, secretion and immunity. The gene cluster containing the genes encoding pre-nisin A (which is processed to form nisin A) and the genes for nisin modification, secretion and immunity from Lactococcus lactis (nisABTCIPRK) are described in Kuipers et al (1993) Eur. J. Biochem. 216, 281-291 and Engelke et al (1994) Appl. Environ. Microbiol. 60, 814-825 incorporated herein by reference. The nisA gene is the gene that encodes pre-nisin (pre-nisin includes a 23 amino acid N terminal signal sequence which is cleaved off during secretion); nisB and C are believed to be involved in reactions which modify the pre-nisin formed directly from expression of the nisA gene; nisT is similar to a transport ATPase and is involved in translocation of nisin out of the cell; nisP is involved in the extracellular processing of a fully matured precursor nisin; nisR and K encode regulatory proteins involved in gene expression and nisi is involved in immunity to nisin. The nucleotide sequence of the nisABTCIPRK gene cluster is shown in Figures 7 and 8. Preferably the variant nisA gene occupies the same position as the natural nisA gene in the gene cluster. It is preferred if the cell is a lactococcal cell, most preferably the cell is a Lactococcus lactis cell. Suitable cells, especially Lactococcus cells, are readily available to the skilled person. Clearly it is required that they are a nisin producing cell, preferably a nisin producing, maturing and secreting cell but any such cells can be used. For example, the naturally-occurring nisin-producing strain NCFB894 as deposited in the National Collection of Food Bacteria at the Institute of Food Research, Norwich Laboratory, Norwich Research Park, Colney, Norwich NR4 7UA, UK (and as described in Gasson (1984) FEMS Microbiol. Lett. 21, 7-10) is a suitable Lactococcal cell for use in the methods of the invention. By "natural nisA nisin" we include a peptide antibiotic produced by some naturally occurring nisin-producing strains of bacteria. The mature molecule is based on a sequence of amino acids encoded by a gene, nisA. The chemical structure of a natural nisA nisin is shown in Figure 1. We also include in the term "natural nisA nisin" other naturally-occurring nisins that are based on, but vary from, the nisA nisin shown in Figure 1. For example, we include nisin Z which has the same chemical structure as the nisA nisin shown in Figure 1 except histidine in position 27 has been replaced by asparagine. The gene which encodes nisin Z was found to contain only one nucleotide substitution in comparison with the nisA gene which encodes the nisin A shown in Figure 1. By "elevated level of its natural nisA nisin compared to the natural level" we include a cell modified according to the method which produces at least 5% more, preferably 10% more, more preferably 50% more and most preferably > 100% more natural nisA nisin than an unmodified cell when grown under the same culture conditions. By "variant nisin" we include a protein-engineered variant of a natural nisA nisin in which changes to the amino acid sequence have been made as a result of site-directed or random mutagenesis of a nisA gene. Conveniently, one or more missense mutations are introduced into the protein coding region which result in one or more amino acids being substituted for another. Alternatively, a nonsense mutation can be introduced such that a truncated nisin is produced. In this case, the nisin still retains antibiotic activity. As a further alternative, deletions and/or insertions of the nisA gene can be made so long as the resulting nisin still retains antibiotic activity. Site-directed mutations of the nisA gene may be made, for example, by the oligonucleotide-directed mutagenesis technique of Zoller & Smith (1983) Meth. Enzymol. 100, 468-500 and Zoller & Smith (1984) DNA 3, 479-480 which uses mismatched oligonucleotide primers to introduce the mutation. It is convenient to use a method for improving the yield of mutants, for example, the dut-ung method described by Kunkel (1985) Proc. Natl. Acad. Sci. USA 82, 488-492. Alternatively, the polymerase chain reaction (PCR) may be used to generate mutants using mismatched oligonucleotides (Saiki et al (1988) Science 239, 487-491). Random mutants of the nisA gene can be made chemically using, for example, sodium bisulphite or hydroxylamine as the mutagen. Alternatively, random mutations can be introduced into the nisA gene using enzymatic misincorporation using a DNA polymerase with relatively low fidelity, for example AMV reverse transcriptase or Taq DNA polymerase or by using mixtures of oligonucleotides, spiked during synthesis, to incorporate a small amount of each different bases at each position. These methods are well known in the art. By "variant nisA gene" we include fragments of a nisA gene wherein the said fragments vary when compared to the equivalent part of the natural nisA gene. By "variant nisA gene" we do specifically include genes in which the promoter region of the natural nisA gene is replaced by another (heterologous) promoter, preferably one which is known to be a more powerful promoter than the natural nisA gene promoter. Examples of suitable promoters are the inducible lacA promoter (van Rooijen et al (1992) J. Bacteriol. 179, 2273-2280) and the T7 promoter (Wells et al (1993) Mol. Microbiol. 5, 1155-1162), both papers being incorporated herein by reference. We also include in the term "variant nisA gene" genes in which the ribosome binding region of the natural nisA gene is modified, preferably to improve the efficiency of initiation of translation of the nisA coding region. Also included in the term "variant nisA gene" are genes which have silent mutations in the coding region, that is genes in which one or more codons are changed for their synonym, but that the natural nisA nisin is encoded thereby. Efficiency of translation may be improved by using such variant nisin coding regions. We also include genes which comprise a heterologous promoter to drive transcription of a variant coding region, that is, a promoter other than the natural nisA gene promoter. In all cases, it is preferred that the combination of promoter, ribosome binding site and coding region gives optimal expression of the nisin encoded by the coding region. Variant nisins which have improved properties compared with natural nisA nisin are preferred, for example those variant nisins which have more potent antimicrobial activity or that have greater resistance to hydrolysis or degradation when added to foodstuffs. Variant nisins are described in WO 93/20213 and WO 92/18633 (incorporated herein by reference), and in the Examples that illustrate the present invention. A preferred embodiment of the invention provides a method for making a cell which either (a) does not express its natural nisA nisin but expresses a variant nisin or (b) expresses an elevated level of its natural nisA nisin compared to the natural level and, in either case, is capable of expressing genes for nisin modification and immunity comprising the step of substituting a variant nisA gene or part thereof for the natural, chromosomal nisA gene or part thereof at the chromosomal location of the said natural nisA gene. The variant nisA gene or part thereof can be substituted for the natural, chromosomal nisA gene or part thereof at the chromosomal location of the said natural nisA gene in one step by gene replacement. Conveniently, a plasmid containing the variant nisA gene or part thereof is introduced into a host cell containing a chromosomal copy of the natural nisA gene (and preferably the genes for nisin modification, immunity and translocation of nisin out of the cell). A double cross-over recombination event can lead to the natural nisA gene or part thereof being replaced by the variant nisA gene or part thereof. The resulting cell will contain a chromosomal copy of the variant nisA gene and hence produce variant nisin provided that the variant nisA gene comprises a coding region which has been modified. It is not necessary that the whole of the nisA gene is replaced. Rather, it is convenient that a or the part of the nisA gene that encodes the amino acid changes present in the variant nisin or contains the heterologous promoter is replaced. The nisA gene, and other genes necessary for nisin biosynthesis, maturation and secretion are, in nature, located on a transposon which is part of the chromosome. Thus, chromosomal location refers to the presence of the nisA gene in the chromosomal DNA within the nisin gene cluster (nisABTCIPRK) rather than the position of the gene cluster relative to other genetic markers on the chromosome. It is well known that homologous recombination occurs very inefficiently and unpredictably in Lactococcus and, although the above described direct, one-step method is feasible, it is more preferred if the gene replacement is carried out in an indirect, two step process in which it is possible to select for the desired recombinants as now described: A further preferred method comprises the steps of (1) substituting a counter-selectable nisA gene or part thereof for the natural, chromosomal nisA gene or part thereof at the chromosomal location of the said natural nisA gene and (2) substituting a variant nisA gene or part thereof for the counter-selectable nisA gene or part thereof at the chromosomal location of the said natural nisA gene. By "counter-selectable nisA gene" we include a nisA gene modified so that it is readily distinguishable from either the natural nisA gene or from a variant nisA gene. Conveniently, the counter-selectable nisA gene is a nisA gene in which an antibiotic resistance gene (such as that for erythromycin resistance) has been inserted or is a nisA gene in which some or all of the coding region has been deleted. It is preferred, but not necessary, that the counter- selectable nisA gene does not express nisin. It is not necessary that the whole of the nisA gene is replaced. Rather, it is convenient that a or the part of the nisA gene containing the counter- selectable marker is replaced. In these examples the counter-selectable nisA gene can be distinguished from the natural or variant nisA gene by resistance to antibiotic of the counter-selectable gene and/or by size differences. Thus, it is relatively straightforward to determine whether step 1 of the preferred method has been achieved because the resulting cell will, for example, have gained antibiotic resistance or, if the counter-selectable nisA gene has a deletion, specific fragments of the cell's chromosomal DNA will be missing or reduced in size. Whether a specific fragment of a cell's chromosomal DNA is missing or reduced in size can readily be determined using well known molecular techniques such as Southern blotting, polymerase chain reaction (PCR) analysis or restriction fragment length polymorphism (RFLP) analysis. Similarly, it is relatively straightforward to determine whether step 2 of the preferred method has been achieved because the resulting cell will, for example have lost antibiotic resistance or gained a fragment of chromosomal DNA. Conveniently, in this preferred embodiment, there is a selection associated with step 2. For example, it is preferred if the counter-selectable gene in step 1 comprises a deletion of all or part of the nisA coding region (AnisA) and that in step 2 the correct replacement of the variant nisA gene is selected for. Thus, in a preferred method, a Lactococcus lactis strain, containing a AnisA gene (made using step 1) is used in step 2. A thermosensitive shuttle vector (replication-permissive at low temperature but not at high temperature) is used to introduce the variant nisA gene into the chromosome of the AnisA strain. For example, the AnisA strain is transformed with a plasmid containing the variant nisA gene and a gene for antibiotic resistance, and the cell is incubated at the permissive temperature in the presence of antibiotic. The cell is then transferred to the non-permissive temperature in the presence of antibiotic and a single cross-over event results in the integration of the plasmid in the chromosome at the site of plasmid/chromosome homology (ie at the common regions of the AnisA and variant nisA gene). The cell is then transferred to the permissive temperature to allow plasmid replication. Recombination between homologous sequences flanking the integrated plasmid results in its excision from the chromosome. A second cross-over event occurs resulting in either sequences originating from the integrated plasmid (ie the variant nisA gene) or the original sequences (ie the counter-selectable nisA gene) being retained on the chromosome. As discussed above, the variant nisA gene and counter-selectable nisA gene can be distinguished, and cells containing the variant nisA gene are chosen. Cells are cured of plasmid by culturing in the absence of antibiotic. In this preferred method the entire nisA gene and flanking sequences are effectively replaced with the identical sequences, with the exception of the specifically incorporated mutation. The size of the plasmid DNA fragment, containing the variant nisA gene is limited by the requirement for homologous sequences (on both the plasmid and chromosome) across which recombination can take place to bring about plasmid integration and subsequently, gene replacement. It is preferred for vector construction that there is approximately 1 kb of homology flanking the site of any sequence alteration. As an example our gene replacement vector has approximately 800 bp on one side of the nisA gene and 1 ,200 bp on the other. A reduction in this size would be expected to reduce the incidence of homologous recombination and therefore the chances of detecting the desired gene replacement. Figure 10 illustrates the recombination events which occur during the preferred method. The method of the invention results in cells which produce a variant nisin from a chromosomal copy of a variant nisA gene or a natural nisA nisin from a chromosomal copy of a variant nisA gene. As has been discussed above, it is most preferred if variant nisins have antibiotic activity. Thus, the cells will exhibit a Nis + phenotype because the cells produce a nisin (either natural or variant). We have determined that these nisin-producing cells must necessarily also be immune to the nisin at the level at which they produce this antimicrobial peptide. Thus, a further preferred embodiment of the method comprises a further step of selecting those cells which are immune to nisin, at least to a level of 1000 U/ml. Although it is preferred that the cells produced by the method express a variant nisin, the method also encompasses the making of a cell which can express natural nisA at a high level from a powerful, heterologous promoter. In a less preferred embodiment, the gene cluster comprises a variant nisA gene and the genes for nisin modification and immunity and this gene cluster is carried on an autonomously replicating DNA element. Conveniently, the autonomously replicating DNA element is a plasmid. The host cell for the plasmid is a cell that does not express a natural nisA nisin. For example, a Lactococcal cell in which the natural nisin genes are absent or the natural nisA gene is inactivated. Cloning the entire nisin gene cluster on a plasmid involves the integration of a large segment ( — 1 1 kb) of DNA. A strategy of this type has the advantage of enabling the copy number and therefore gene dosage to be altered and also may facilitate the transfer of nisin determinants to a range of alternative host backgrounds. There are two preferred types of replicons (which use different modes of replication) which can be employed as suitable vectors:- the rolling circle plasmids (for example pTG262, Dodd et al (1990) J. Gen. Microbiol. 136, 555-566) or the theta type plasmids (for example, pIL253, high copy number, and pIL277, low copy number, Simon & Chopin (1988) Biochimie 70, 559-566). Both papers are incorporated herein by reference. When the method of the invention uses a lactococcal cell it is preferred if the plasmid is a shuttle plasmid, that is a plasmid that can replicate in the lactococcal cell and can also replicate in another host cell such as Escherichia coli. A variety of methods have been developed to operably link DNA to vectors via complementary cohesive termini. For instance, complementary homopolymer tracts can be added to the DNA segment to be inserted to the vector DNA. The vector and DNA segment are then joined by hydrogen bonding between the complementary homopolymeric tails to form recombinant DNA molecules. Synthetic linkers containing one or more restriction sites provide an alternative method of joining the DNA segment to vectors. The DNA segment, generated by endonuclease restriction digestion as described earlier, is treated with bacteriophage T4 DNA polymerase or E. coli DNA polymerase I, enzymes that remove protruding, 3 '-single-stranded termini with their 3'-5'-exonucleolytic activities, and fill in recessed 3 '-ends with their polymerizing activities. The combination of these activities therefore generates blunt-ended DNA segments. The blunt-ended segments are then incubated with a large molar excess of linker molecules in the presence of an enzyme that is able to catalyze the ligation of blunt-ended DNA molecules, such as bacteriophage T4 DNA ligase. Thus, the products of the reaction are DNA segments carrying polymeric linker sequences at their ends. These DNA segments are then cleaved with the appropriate restriction enzyme and ligated to an expression vector that has been cleaved with an enzyme that produces termini compatible with those of the DNA segment. Synthetic linkers containing a variety of restriction endonuclease sites are commercially available from a number of sources including International Biotechnologies Inc, New Haven, CN, USA. A desirable way to modify the DNA encoding the polypeptide of the invention is to use the polymerase chain reaction as disclosed by Saiki et al (1988) Science 239, 487-491. In this method the DNA to be enzymatically amplified is flanked by two specific oligonucleotide primers which themselves become incorporated into the amplified DNA. The said specific primers may contain restriction endonuclease recognition sites which can be used for cloning into expression vectors using methods known in the art. A second aspect of the invention provides a cell which does not contain a natural nisA gene but expresses a nisin comprising a variant nisA gene wherein the variant nisA gene has the same relationship as the natural nisA gene to a gene cluster containing the natural nisA gene and the genes for nisin modification, secretion and immunity. The cell of the second aspect of the invention is obtainable by the methods described in the first aspect of the invention. In the most preferred embodiment the natural, chromosomal nisA gene or part thereof is absent and the cell comprises a variant nisA gene or part thereof at the chromosomal location of the said natural nisA gene. Preferably the cell is a Lactococcus, most preferably Lactococcus lactis. It is preferred that the cell expresses a variant nisin, although a cell that expresses an elevated level of natural nisA nisin also forms part of the invention. Conveniently the variant nisA gene contains transcriptional or translational control sequences which enable the cell to either express a variant nisin or, in the case of natural nisA nisin, enable the cell to express it at an elevated level. Thus, in one embodiment the cell comprises a variant nisA gene consisting of a heterologous promoter which drives the expression of a nisin coding region (which may express a natural nisA nisin or a variant nisin). In a less preferred embodiment the cell comprises an autonomously replicating DNA element carrying a variant nisA gene and the genes for nisin modification and immunity. In this case, the cell does not have an active chromosomal nisA gene and preferably no chromosomal nisin genes. A third aspect of the invention provides a process for producing nisin comprising culturing a cell as described in the third aspect of the invention and obtaining the nisin produced thereby. Conveniently, the nisin is a variant nisin. It is preferred if the cells are those of the most preferred embodiment. We have found that using the cells of the most preferred embodiment in the process we can produce an unexpectedly high yield of nisin particularly in comparison to the known processes which rely on plasmid- borne nisA genes to express the nisin (in the absence of plasmid-borne nisin immunity, modification and secretion genes). Further details of this surprising effect are given in the Examples. However, it is worth noting at this point that, in the case of a nisin variant in which both dehydroalanine 5 was replaced by alanine and dehydroalanine 33 was replaced by alanine (known as nisinA/Dha 5A, Dha 33A), the cell of the present invention in which the natural nisA gene is replaced by the variant nisA gene produces more than 100 times the nisin compared with a prior art cell in which the same variant nisin (nisinA/Dha 5A, Dha 33A) was encoded by a plasmid. In addition, all of the variant nisins that have been tested give a higher yield from cells of the present invention compared with the prior art cells containing the variant nisin gene on a plasmid. Further advantages over the prior art methods and cells are obtained using the cells of the most preferred embodiment to produce nisin. For example, because the cells do not contain a plasmid there is no requirement for antibiotic selection during their culture and plasmid loss during culture is not a problem. Thus, for stability there is an advantage in that the variant nisin gene is integrated within the bacterial chromosome albeit as part of the nisin transposon Tn5301. The latter is extremely stable and we have in fact found it difficult to eliminate deliberately. In the laboratory selection for a plasmid marker prevents this being a practical problem, but for industrial use this would be a disadvantage. It may be undesirable to add antibiotics to the fermentation. In a less preferred embodiment, cells carrying the variant nisA gene and the genes for nisin modification and immunity are carried on an autonomously replicating DNA element, such as a plasmid. Clearly, in this embodiment, plasmid selection is required during the culture of the cell. A particularly preferred embodiment of the third aspect of the invention is wherein the cells are cultured in the presence of nisA nisin or a variant nisin which can induce nisin expression. Nisin A wherein Ile30 is replaced by Tip (DOW) is an example of a variant that, as a result of its mutation, does not function well as an inducer of its own biosynthesis. By adding sub-inhibitory concentrations of nisin to the growth medium, during fermentation, higher levels of the variant nisin are produced. Any variants that are less efficient as inducing agents benefit from the inclusion of nisin in the growth medium (ie a nisin induction step in the purification procedure). The amount of induction varies depending on the initial induction capacity of any particular variant (with I30W nisin A production more than doubled as a result of induction). Induction may be routinely included in the method as a means of maximising production levels. Any concerns about contamination with the wild type molecule are minimal as the nisin concentration required for induction is negligible compared to the amount of nisin variant being purified. Conveniently, the nisA nisin is a minimum amount that provides maximal induction of nisin production. This amount can be determined empirically by a person skilled in the art. Suitable nisA nisin concentrations for induction in this embodiment are from 1 nM to 500 nM, preferably 10 nM to 250 nM, more preferably 50 nM to 150 nM, most preferably 100 nM. A reverse-phase HPLC step in any purification would ensure separation of any residual nisA nisin from variant nisins. A fourth aspect of the invention provides a nisin produced by the process of the invention. The presence of unsaturated amino acids in Iantibiotics including nisin and the role they play in the biological properties of these complex molecules is of particular interest in structure/function analyses. It has been proposed that the reactive unsaturated bonds that characterise dehydro- amino acids play a functional role in the antimicrobial activity that subtilin, a lantibiotic, exerts against bacterial spore outgrowth. These residues have also attracted attention as a possible source of molecular instability. It has long been known that the antimicrobial activity of commercial samples of natural nisA nisin deteriorate on storage and that a number of chemical components are found within such samples (Berridge et al, 1952) and Chan et al (1989) have demonstrated that specific cleavage occurs at the dehydroalanine residues in the mature molecule. Cleavage at Dha5 results in the opening of the first lanthionine ring of nisin and is accompanied by a loss of antimicrobial activity. In contrast, the degradation product arising as a result of cleavage at Dha33 retains essentially wild type activity (Chan et al, 1989). In WO 93/20213 we described the construction of L. lactis derivatives expressing nisinA/Dha5A, nisinA/Dha33A and nisinA/Dha5A,Dha33A. We also demonstrated in that work that these engineered nisins retained their antimicrobial activity against sensitive indicator strains. Clearly, as described in detail in the Examples, these nisins can be produced more efficiently by the present process. However, the present process can also be used to produce any further variant nisins which have other, improved properties so long as they are encoded by a variant nisA gene. A fifth aspect of the invention provides the use of a nisin produced according to the process of the invention as an antimicrobial agent. The ability of nisin to inhibit growth of spoilage bacteria and food pathogens has resulted in the extensive use of as a natural preservative in certain food products, particularly dairy products such as soft cheeses. Variant nisins are also used. The invention will now be described in more detail with reference to the following figures and examples wherein: Figure 1 shows the molecular structure of natural nisin A. Changes that have been made to the sequence as a result of protein engineering are indicated by arrows. Figure 2 shows diagrammatically some of the recombinant plasmids constructed and used in this work. Figure 3 shows diagrammatically counter-selectable nisA genes wherein either an erythromycin resistance gene is inserted in a nisA gene or a frame-shift deletion has been made (nisA-fs). The sequences shown are in the sequence listing as SEQ ID Nos. 12 to 17. Figure 4 shows the nucleotide sequence of at least part of the natural nisA gene and expression signals showing changes introduced by PCR-mediated site-specific mutagenesis. BamHl and Bglll sites flanking nisA were engineered into plasmid pFI740 (Figure 2d). The substitution of Ser5 and Ser33 codons for alanine codons in variant nisA genes (Table 1) is shown above the sequence. The sequences shown are in the sequence listing as SEQ ID Nos. 18 and 19. Figure 5 shows an agarose gel electrophoresis of PCR fragment generated with primers P39 and P40. PCR reactions were carried out on colonies of:- track 3, FI5876; 4, FI7990; 5-10, FI7990 (pG hostό derivative) after gene replacement procedure. Size standards:- tracks 1 and 12, λDNA digested with BgH; 2 and 11, λDNA digested with Hindlll. Figure 6 shows a plate diffusion bioassay. 150 μl samples of cell free extracts from strains:- 4, FI5876; 5, FI7990; 6, FI8070; 7, FI8198; 8, FI8199 were loaded into wells bored in MRS agar seeded with the indicator strain Lactobacillus helveticus CH-1. Plates were incubated overnight at 42°C. Standards included on the assay plate are:- 1,50; 2,100; 3,200; 9,300; 10,400 U/ml. Figure 7 shows sequences of the nisA, nisB, nisT, nisC and nisi genes of Tn5276 of L. lactis NIZO R5, and is taken from Kuipers et al (1993) Eur. J. Biochem. 216, 281-291. Putative ribosome-binding sites (RBS) and inverted repeats (-*) are indicated, as is the transcription-initiation site of the nisA gene and its preceding canonical sequences. Positions of restriction sites used are as follows: Accl, 6383-6388; Bell, 2914-2919; EcoRl. 3461-3466; EcoRV, 1805-1810; Haelll, 6509-6512; Ncol, 6218- 6223; Ndel, 4518-4523; Pstl, 7418-7423; Sstl 283-288, 1547-1552 and 2463-2468. Figure 8 shows a nucleotide sequence of cloned 5.0-kb region downstream from nisC with open reading frames nisi, nisP, nisR, and nisK, and is taken from Engelke et al (1994) Appl. Environ. Microbiol. 60, 814-825. Possible ribosome-binding sites (RBS), restriction sites, and inverted repeats are underlined. Open reading frames are designated by a one- letter code. Arrows indicate the putative signal peptide cleavage sites of Nisi and NisP; the putative membrane anchor sequence of NisP is underlined. Conserved, functional, and active-site amino acids are written in boldface letters and marked by asterisks. Figure 9 illustrates a gene replacement vector. Sizes of the cloned fragments that make up the nisA cassette and flanking sequences are given in base pairs. Figure 10 describes diagrammatically a gene replacement protocol. Figure 11 shows the double-stranded nucleotide sequence of nisA gene and pre-nisin amino acid sequence. The -35 and -10 regions and the transcription initiation site are indicated together with restriction enzyme sites used in the nisA gene cassette (see Figure 2) above the DNA sequence. The location of primers (5 '-end) employed in amplification of the cassette fragments and PCR-mediated mutagenesis are shown, above and below the sequence, as horizontal black arrows indicating the direction of DNA synthesis. Specific amino acid substitutions, as a result of the mutagenesis, are shown below the pre-nisin sequence. Figure 12 is a representation of the organisation of the nis genes. Example 1: Construction of Lactococcal cells in which the natural. chromosomal nisA gene is replaced bv a variant nisA gene Methods Microbiological techniques and strains used. The Lactococcal strains used in this study and their derivation are given in Table 1. Table 1. Lactococcal strains used in this studv:- Strain nisA Activity Immunity Reference mutation (U/ml x 10 3 ) MG1614 0.01 Gasson (1983) J. Bacteriol. 154, 1-9 FI5876 wild type + > 1 Dodd et al (1990) J. Gen. Microbiol. 136, 555-566; Horn et al (1991) Mol. Gen. Genet. 228, 129-135 FI7847 nisA-(fs) - 0.5-0.75 This work FI7990 AnisA - 0.25-0.5 This work FI8070 nisA/S5A + > 1 This work FI8198 nisA/S33A + > 1 This work FI8199 nisA/S5A,S33 + > 1 This work A FI7893 nisA + > 1 This work FI8003 nisA - 0.25-0.5 This work Unless stated otherwise, cultures were grown at 30°C in M17 medium (Terzaghi & Sandine (1975) Appl. Environ. Microbiol 29, 807-813) supplemented with 0.5% (wt/vol) glucose (GM17 medium). Screening strains for resistance to antibiotics was carried out at the following levels: erythromycin, (Em 1 ) 5 μg/ml; streptomycin, (Sm 200 μg/ml. Escherichia coli MCI 022 (Casadaban & Cohen (1980) J. Mol. Biol. 138, 179-207 was the host strain for construction and molecular analysis of recombinant plasmids derived from the vectors pMTL23p (Chambers et al (1988) Gene 68, 139-149), pGEM-3Z (Promega), pCR™II (Invitrogen) and pG+host6 (Appligene). Recombinant plasmids used, and constructed during the course of this study, are shown in Figure 2. E. coli cultures were propagated at 37°C in L broth (Lennox (1955) Virology 9, 190-206. Selection for ampicillin resistance (Ap 1 ) was carried out at 100 μg/ml, chloramphenicol (Cm*) at 15 μg/ml and erythromycin, (Em at 400 μg/ml. Nisin activity in Lactococcal strains was assayed by both deferred and direct means. Plate diffusion bioassays were performed as previously described (Dodd et al (1992) Appl. Environ. Microbiol. 58, 3683-3693. Colonies growing on the surface of a GM17 plate were directly assayed by inverting over chloroform for 12 minutes and overlaying with agar seeded with the nisin sensitive L. lactis strain MG1614. Plates were incubated overnight and zone sizes around colonies compared with those of controls. Nisin immunity was determined by streaking cultures on a series of GM17 agar plates containing an increasing concentration of nisin and assessing the degree of growth at the different nisin levels. Control cultures (FI5876, positive) and MG1614 (negative) were included on each plate. Molecular techniques. Total DNA, plasmid DNA was carried out as described by Dodd et al (1990) J. Gen. Microbiol. 136, 555-566 and Horn et al (1991) Mol. Gen. Genet. 228, 129-135. Restriction enzyme and other DNA modifying enzymes from various sources were used according to the suppliers recommendations. Recombinant plasmids were recovered by transformation of E. coli as described previously (Dodd et al (1992) supra or electroporation of L. lactis according to Holo and Nes (1989) Appl. Environ. Microbiol. 55, 3119-2123 with the modifications of Dodd et al (1992) supra. Conditions used for polymerase chain reaction (PCR) were as described in Horn et al (1991) Mol. Gen. Genet. 228, 129-135. Primers were synthesised on an Applied Biosystems DNA synthesizer (model 381 A) and are listed in Table 2. Fragments generated for the construction of gene-replacement vectors were amplified using Dynozyme (Flowgen) and cloned into pCR™II prior to nucleotide sequence confirmation. For routine PCR screening of recombinant clones AmpliTaq-DNA polymerase (Perkin Elmer) was used. Direct nucleotide sequence determination of purified PCR-generated templates was carried out on an Applied Biosystems DNA Sequencer (model 373A) using the manufacturers' Taq "Dyedeoxy" terminator cycle sequencing kit. Table 2. Primers used in this study:- P13 (SEQ ID No 1 5'-AACGGATCCGATTAAATTCTGAAGTTTG-3' BamHI P17 (SEQ ID NO 2 5'-TCAGAGCTCCTGTTTTACAACCGGGTGTACATA GTGCAAT-3' P18 (SEQ ID No 3 5'-TAGTATTCACGTAGCTAAATAACC-3' P19 (SEQ ID No 4 5'-TTGGTTATTTAGCTACGTGAATAC-3' P25 (SEQ ID No 6 5'-AATCGGATCCGTTTATTATGCTCGC-3' BamHI P26 (SEQ ID No 6 5'-ATAGTTGACGAATATTTAATAATTTT-3' Hindi P27 (SEQ ID No 7 5'-CTTGGTCGACACCATATTTT-3' Sail P28 (SEQ ID No 8 5'-GTTAGATCTGACATGGATAC-3' Bglll P32 (SEQ ID No 9 5'-CCATGTCAGATCTAACAAAATAC-3' Bglll P39 (SEQ ID No 10) 5'-GACTTTCCATTATGCTTGGATTTTT-3 ' P40 (SEQ ID No 11)5 , -GCTCCTATGCCAAATGTAGAATC-3' Construction of nisA gene replacement vectors. The thermosensitive shuttle vector pG+host6 was employed for carrying out gene replacement. Homologous plasmid DNA originated from pFI172 (Dodd et al (1990) supra) which contains a 2.1kb region of the FI5876 chromosome (Fig 2a) including the nisA gene. The entire fragment was subcloned into pG+host6 to generate the nisA gene-replacement vector pFI690 (Fig 2b). Subsequent manipulation of this region, resulting in inactivation or mutagenesis of the nisA gene (see below), was carried out in either vector pGEM-3Z or pMTL23P. The final step in the construction of each gene-replacement vector was cloning the modified 2.1kb fragment into pG+host6. The derivatives of pG+host6 were established in E. coli and plasmid DNA, from this host, used to transform L. lactis FI7990 (Table 1). nisA frame shift mutation - nisA-(fs):- Insertional inactivation of the nisA gene, by cloning an Em r gene into the internal Sacl site, has been described previously (Dodd et al (1992) supra). In this plasmid the Em r gene is flanked by a short multiple cloning site (Fig 3). Digestion with the restriction enzyme Smal, followed by ligation to recircularise the vector sequences, resulted in deletion of the Em r gene. Residual sequences from the multiple cloning site leave a 20bp insertion within the Sacl site and cause a frameshift mutation to occur in codon 16 of the nisA gene. The first 38 amino acids encoded by this mutated gene [designated nisA-(h)] are unaffected. However, the predicted translation product would be a truncated prenisin (45 residues) including the on-nisin amino acid sequence RYPGTEL at its COOH-terminus (Fig 3). The nisA-(fs) mutation was subcloned into pG+host6 to generate the gene-replacement vector pFI674 (Fig 2c). nisA deletion - AnisA:- - Inactivation of the nisA gene was also achieved by deletion of the coding region. In order to confine the deletion to just nisA it was necessary to engineer additional restriction enzyme sites on either side of the gene. Primers were designed, for PCR amplification of this region of the chromosome, that incorporated a BamHl site (P13, Table 2) 80bp upstream of the start of nisA and a BgHl site (P32, Table 2) 25bp beyond the stop codon, as a result of 2bp changes in each case (Fig 3). The flanking fragments (shown in Fig 2d) were also generated using PCR amplification. Primers P26 and P25 were employed for amplification of the upstream 211 bp Hindi/ BamHl fragment and primers P28 and P27 employed for the downstream 1.1 kb Bglll/ Sail fragment (Table 2). The template used for these PCR reactions was pFI 172 DNA. The resulting plasmid (pFI740) contained an intact nisA gene flanked by an engineered BamHl and BgHl sites, all contained within 2.1 kb of sequences homologous to the chromosome (Figure 2d). Digestion of pFI740 with these two enzymes, followed by ligation of their compatible ends, resulted in the generation of plasmid pFI751 in which the nisA gene has been deleted, designated as AnisA (Fig 2e). PCR amplification of this part of the plasmid and nucleotide sequence analysis of the region spanning the deletion in the amplified fragment confirmed that fusion of the BamHl and Bglll sites had occurred. nisA site-specific mutations:- The construction of the plasmid pFI877 (Fig 2f) allowed a cassette mutagenesis strategy to be employed for the introduction of site-specific mutations into the nisA gene. This pGEM-3Z derivative contains the equivalent sequences to those in pFI690 (Fig 2b), but includes the engineered Bglll site downstream of nisA in pFI740 (Fig 2d). In pFI877 a Hindi/ Sacl fragment encoding the amino-terminal region of nisA and upstream expression signals, replaced the PCR- generated fragment of pFI740 that contains the engineered BamHl site. Thus, the only difference between sequences in pFI877 and the equivalent chromosomal wild-type sequences is the presence of an additional BgHl site downstream of the nisA gene. The construction of this nisA cassette is such that site-specific mutations could be readily incorporated into the gene. PCR-mediated mutagenesis was used to amplify either the Hincll/Sacl or Sacl/BgHl fragments containing the amino-or COOH- terminal regions of the nisA gene respectively (Fig 2f). These fragments, containing a specific mutation, were then substituted for the wild-type fragment of pFI877. Mutations were incorporated in either the primers used to amplify the cassette fragments or, if the desired site of mutation was internal, the technique of spliced overlap extension was used (Ho et al (1989) Gene 77, 51-59) with the specific mutations incorporated on two complementary primers spanning the mutation site (Dodd et al (1992) supra). Gene replacement protocol. L. lactis FI7990 transformants containing derivatives of pG+host6 were established at 28 °C and grown overnight at this temperature in GM17 containing Em at 5 μg/ml (GM17-Em). Approximately 10 5 cells were used to inoculate 100 ml of fresh, prewarmed GM17-Em and the cultures were incubated at 28 °C for 4 hours. Incubation was continued overnight at the elevated temperature of 37 °C. This temperature is non-permissive for pG+host 6 replication (Biswas et al (1993) J. Bacteriol. 175, 3628- 3635) and the presence of Em in the growth media ensures selection for those cell lines in which a single cross-over results in the integration of the derivative in the chromosome at the site of plasmid/chromosome homology (Leenhouts et al (1989) Appl. Environ. Microbiol. 55, 394-400; Leenhouts et al (1990) Appl. Environ. Microbiol. 56, 2726-2735; Chopin et al (1989) Appl. Environ. Microbiol. 55, 1769-1774). Prewarmed GM17 (with no Em) was inoculated with approximately 10 5 cells from the overnight culture and incubated overnight at 28 °C. At this temperature plasmid replication is possible and recombination between homologous sequences flanking the integrated plasmid results in its excision from the chromosome. Depending on where the second cross-over occurs either sequences originating from the integrated plasmid, or the original sequences will be retained in the chromosome. The lengths of DNA homology are shown in Figure 9. These sequences originated as an Accl/Sall fragment making up part of a Sail fragment that is cloned and sequenced in the plasmid pFI 172 (Dodd et al (1990) J. Gen. Microbiol. 136, 555-566). Cultures were diluted and spread, for single colonies, on GM17 agar plates. In order to cure the cells of plasmid, the plates were incubated at 37°C. Colonies (approximately 50) were screened for loss of the pG+host6 derivative by patching onto GM17 plates containing Em (5 μg/ml). When using this technique to disrupt the nisA gene colonies were also screened for loss of nisin activity and PCR analysis of the relevant region of the chromosome used to confirm any changes at the molecular level. The gene replacement protocol is illustrated diagrammatically in Figure 10. RESULTS Inactivation of chromosomally encoded FI5876 nisA gene. In order to identify cell lines that have acquired variant nisA genes it was convenient to first construct a Nis host strain, by inactivating the resident nisA gene. The well characterised nisin-producing strain L. lactis FI5876 was selected for this purpose (Dodd et al (1990) supra: Horn et al (1991) supra). The nisin biosynthesis genes from this strain have been cloned and sequenced (Dodd et al (1992) supra). Gene-replacement was used to substitute the wild-type nisA gene of FI5876 with the plasmid pFI674-encoded nisA-(fs) gene (Fig 2c, Fig 3). Of fifty colonies screened five were both Em 1 and Nis ' suggesting that in these FI5876 derivatives gene replacement had occurred. Furthermore, this result indicated that the modified nisA gene, was defective and did not express a precursor molecule that could be matured to an active form. One of the Nis " strains, designated FI7847, was analysed further. To test the system it was demonstrated that the nisA-(fs) mutation in FI7847 could be reverted back to wild-type by carrying out the equivalent experiment using the nisA gene replacement vector pFI690 (Fig 2b). Recovery of nisin production by the resulting gene-replaced strain, FI7898, indicated that the Nis ' phenotype exhibited by FI7847 was due solely to disruption of the nisA gene. The other nisin biosynthesis determinants appeared to have been unaffected by the switching of nisA genes. An alternative approach was to generate a Nis ' host by deletion of the entire chromosomal nisA gene in FI5876. Plasmid pFI751 (Fig 2e) was constructed for this purpose and gene replacement used to incorporate the approximate 300 bp deletion AnisA (Fig 4) in place of nisA. Nis " strains were recovered at about the same frequency as was found with the nisA- (fs) gene-replacement. In this case the AnisA containing strains could be readily distinguished from the parent strain by PCR analysis. Primers P39 and P40 (Table 2, Fig 2a) amplified a 1.8kb fragment in the gene-replaced strains (Fig 5, track 4) compared to a 2.1 kb fragment generated from the equivalent region of FI5876, encoding wild-type nisA (Fig 5, track 3). In one of the Nis " strains, designated FI7990, nucleotide sequence analysis of the PCR-generated 1.8kb fragment confirmed that AnisA was incorporated in the correct region of the chromosome. Again the system was tested and it was shown that nisin production could be restored in a FI7990 derived strain by gene replacement with pFI690 (Fig 2b). PCR analysis of these Nis + colonies demonstrated that the AnisA mutation (1.8kb fragment) had been replaced by the wild-type nisA gene (2.1kb fragment). As this system has the advantage of being able to readily identify gene-replacement on the basis of PCR analysis (see Fig 5) further characterisation and mutant construction was carried out using FI7990 as the host strain. Nisin immunity The effect of disruption of the nisA gene on immunity of the host strain to nisin has been described previously (Dodd et al (1992) supra). As would be expected both FI7847 [nisA-(is)] and FI7990 (AnisA) displayed reduced immunity to nisin (Table 1). The nisA deleted strain FI7990 was sensitive to nisin at a concentration of between 250 and 500 U/ml compared to the parent strain, FI5876, which will continue growing in the presence of nisin to over 1000 U/ml. Interestingly, FI7847, encoding a truncated nis A gene, exhibited intermediate levels of immunity to nisin (an upper limit of between 500 and 750 U/ml with poor growth continuing to 1000 U/ml, Table 1). A possible explanation for the difference in nisin sensitivity of these two Nis " strains came from gene replacement studies involving the vector pFI740 (Fig 2d). Strain FI8003, generated by substitution of the defective nisA gene with the intact plasmid pFI740-encoded nisA, had a Nis phenotype. This result contrasts with that of the equivalent gene replacement experiment, involving pFI690 (Fig 2b), in which a Nis + phenotype was recovered (see above). The only difference between the two sequences involved is that pFI740 has an additional BamHl site incorporated 80bp upstream of the ATG start codon of nis A, and a Bglll site immediately downstream of the coding region (Fig 4). Examination of these sequences revealed that the BamHl site overlaps with the proposed -35 region of the promoter identified by Kuipers et al (1993) Eur. J. Biochem. 216, 281-291. A single base pair change introduced as a result of engineering the BamHl site has the effect of converting the -35 sequence from CTGATT to CCGATT (Fig 4). These results suggest that in pFI740 the natural nisA promoter has been disrupted and hence, those strains, such as FI8003, which have incorporated the BamHl site by gene replacement will also have acquired the defective promoter. The increased nisin sensitivity of these strains (approximately 50% that of wild-type), despite an intact nisA gene, suggest that these potentially promoter active sequences play a role in nisin immunity. The preferred protocol uses nisin immunity as a means of directly selecting Nis + strains that have undergone gene replacement and relies on the fact that inactivation of the nisA promoter in FI7990 results in a sufficiently high sensitivity to nisin that the parent strain will not grow on the selective plates. Nis " strains that retained the upstream promoter sequences (eg FI7847) were unsuitable for this procedure as they grew well at the levels of nisin that were found to be optimal for selection of Nis + recovery, ie 500 U/ml (Table 1). Gene replacement - identification of variant nύΛ-encoding strains. From the preliminary gene-replacement experiments carried out in the construction and testing of the Nis strains FI7847 and FI7990 it was known that substitution of chromosomal sequences for the equivalent homologous region carried by the pG+host6 derivative, occurred at low frequency. The subsequent restoration of an intact nisA gene in these hosts, by gene replacement, would be expected to lead to the recovery of nisin activity. Any Nis + strains within the population would then be at a selective advantage over the original Nis " parent. However, initial attempts to recover an activate nisA gene again resulted in the majority of colonies screened retaining the defective nisA parental sequences. The restoration of a Nis + phenotype necessitates a functional nisin immunity mechanism and this requires the expression of the nisA gene. The gene-replacement protocol, employed for the construction of FI7847 and FI7990, was modified to facilitate the identification of derivatives that had acquired nisA or variant nisA genes that resulted in nisin production. The recovery of a Nis + colonies hinged on our interpretation that these cells must necessarily also be immune to nisin at the level at which they were producing this antimicrobial peptide. In the modified gene replacement protocol the final step included the addition of nisin to the GM17 agar plates, at a level of 500 U/ml. Nisin immune colonies that grew on this media were screened for Em r and assayed for nisin production. PCR analysis was also used to determine the organisation of genomic sequences. Figure 5 shows the fragments generated by PCR (using primers P39 and P40, Fig 2a) from six colonies that had been through the gene replacement procedure. All were found to have acquired a functional copy of a nisA gene (in this case nisA/SSA) as shown by the 300bp increase in size of the PCR fragment. This procedure was found to be a very reliable means of identifying Nis + derivatives of FI7990 as this host strain was itself sensitive to the levels of nisin employed in the selection plates. The majority of colonies (approximately 90%) screened in this way were found to have undergone gene-replacement and to be expressing a functional nisA gene or variant in place of the chromosomal lesion AnisA. This strategy has been successfully employed to select for several derivatives of FI7990 that are now exclusively expressing engineered nisins in place of nisin A. As described above, the protocol involves the integration of the thermo- sensitive plasmid, p+Ghost6 in the chromosome, followed by its excision. Assuming that cross-overs occur with equal frequency between homologous sequences on either side of the mutation, it would be predicted that the number of cells now carrying the mutation would be the same as those cells identical to the parent strain. This did not prove to be the case and the majority of colonies screened retained the genetic organisation of the parent strain FI7990. The reason for this is not clear, but it suggests that the immediate effect of integration of a functional nisA gene is detrimental to the host cell. It has been reported that expression of the nisA gene precedes that of the adjacent nisB gene by 30 minutes (Engelke et al (1994) supra) and transcription of other determinants in the nisin gene cluster may be similarly delayed, with respect to prenisin production. Those strains that acquire a nisA gene by gene-replacement may not have recovered full immunity before the nisin molecule exerts its antimicrobial action. Such strains would not be viable. However, we have been able to restore a Nis + phenotype by gene replacement when nisin production has been delayed allowing full nisin immunity to be established. Conversion of Dha to Ala residues. The dehydroalanine (Dha) residues at positions 5 and 33 (Fig 1) were initially targeted for engineering changes in the nisin molecule. The aim was to substitute the serine residues, from which the Dhas are derived, for alanines which lack a potentially unstable unsaturated side chain. The mutation in nisA,S5A was generated by PCR using primers P26 and PI 7 (Table 2). Amplification resulted in a 404bp Hindi/ Sad fragment containing the amino-terminal end of nisA and including the substitution of a Ser codon (coordinate 173, Fig 4) for CGT which specifies alanine. The 90bp Sacl/BgUl fragment containing the COOH-terminal end of nisA was generated by PCR using primers P10 and P32 (Table 2) and included a spliced overlap extension step with primers 18 and 19 (Table 2). This latter pair of complementary primers contain an alanine codon, CGT, in place of the serine codon at coordinate 257 (Fig 4). Subcloning these PCR generated fragments, either separately or together, into the appropriate gene-replacement vector resulted in an uninterrupted coding region specifying either a nisA/S5A, nisA/S33A or m ' _iyl/S5A,S33A gene. Transformation of FI7990 with plasmid DNA followed by the gene- replacement procedure generated a number of colonies the majority of which were found to be Em s and Nis + . The relevant region of the chromosome was investigated by PCR using the primer combination P39 and P40 (Fig 2a) and in each case a 300bp increase in fragment size, compared to FI7990 (see Fig 5), indicated that gene replacement had occurred. Nucleotide sequence analysis of these PCR generated fragments confirmed that, in each case, the three variant nisA genes were incorporated in the chromosome, in place of the AnisA lesion. A representative of each gene-replaced strain, FI8070 (nisA/S5A), FI8198 (nisA/S33A) and FI8199 (W ' Λ /S5A,S33A) was characterised further. Expression of all three mutated nisA genes resulted in the production of an active molecule as determined directly by colony overlays. Plate diffusion bioassays on cell extracts demonstrated that the levels of antimicrobial activity against Lactobacillus helveticus were comparable to that of the parent strain FI5876 (Fig 6). FI8070, encoding nisAIS5A. generated a zone of inhibition similar in size to that of the parent strain FI5876. This suggests that the mutation in the nisin variant (nisinA/Dha5A) does not significantly affect its antimicrobial properties against this indicator organism. A cell extract from FI8198 (producing the variant nisin A/Dha33A) was consistently found to generate a zone of inhibition larger than that of the parent strain (Fig 6). This corresponds to an increase of approximately 50% of nisin A levels, under the conditions used here (Table 1). This higher level of production was not found in the extracts of FI8199, producing the double mutant nisin A/Dha5A,Dha33A. The inhibitory effect of this nisin variant was equivalent that of nisin A containing the single mutation and also the wild type molecule (Table 1). In all cases the yield for a particular nisin variant was higher when using this gene replacement strategy than when the equivalent plasmid-encoded gene was employed in a plasmid complementation system (Table 3, Dodd et al (1992) (1993) supra). Table 3. Comparison of nisin activity from Lactococcal expression systems. nisin variant nisin activity* (% of wild type) complementation 11 gene replacement 0 nisin A (wild-type) 50 100 nisin A (Dha5A) 25 100 nisin A (Dha33A) 10 150 nisin A (Dha5A,Dha33A) < l d 100 determined from plate diffusion bioassays antimicrobial activity achieved by plasmid-encoded nisA genes complementing nisA deficiency in host strain FI7332 antimicrobial activity achieved by gene replacement. Functional nisA gene incorporated in the chromosome of FI7990 in place of nis A deletion. d activity was below the level of detection of the bioassay The system developed here is producing variant nisins at yields equivalent to that of the nisin A-producing parental strain FI5876 (Table 1). In the case of the wild-type nisA gene, this is about 50% higher than nisin levels previously achieved using an analogous plasmid complementation approach (Dodd et al (1992) (1993) supra). A further comparison of these two systems reveals that the difference between the levels of production is surprisingly more pronounced for the nisin variants (Table 3). The gene replacement approach increases nisin A/Dha5A yields approximately 4 fold and for nisin A/Dha33A the yield is over ten times higher. The increased efficiency of production of the double mutant nisin A/Dha5A,Dha33A is particularly striking (Table 3). When the gene that specifies this variant nisin is plasmid encoded and used to complement the host strains nisA deficiency antimicrobial activity was only detected in the more sensitive colony overlay assay. Cell extracts from this strain did not display any activity in plate diffusion bioassays (Table 3). However, when the gene is incorporated into the chromosome using the gene replacement strategy the activity levels were equivalent to that of nisin A representing an increase in production of over 100% . This unexpected finding is of relevance to the subsequent chemical and biochemical analysis of the engineered molecules. Considerable amounts of purified peptides are required to fully characterise the novel nisins and to produce amount on a scale suitable for satisfying the market of a food preservative and the system described here appears to ensure that relatively high yields are achieved. We have produced a variety of variant nisin-producing strains using the methodology described in this example. These are described in Table 4. Suitable oligonucleotide primers for effecting the specific mutations were designed from the sequence given in Figure 4 and as shown in Figure 11. Suitable oligonucleotide primers for effecting the specific mutations were designed from the sequence given in Figure 4 and as shown in Figure 11. Table 4. Nisin producing strains generated by gene replacement. Strain Number nisA mutation Activity * MICH" (% of t) (μg ml 1 ) MG1614 - - - FI5876 wt 100 0.13 FI7990 AnisA ' - - FI8070 S5A 100 0.25 FI8198 S33A 150 0.25 FI8199 S5A,S33A 100 1.00 FI8167 H27W < 1 nd c FI8122 S5A,H27W 10 nd FI8307 II27K 100 0.13 FI8328 H31K 25 nd FI8330 H27K,H31K 10 nd FI8256 K12L 10 0.13 FI8290 ΔM21 < 1 nd FI8289 BOW < 1 0.16 Antimicrobial activity in culture supernatants determined in plate diffusion bioassays. Minimum inhibitory concentrations (MICs) were determined against the sensitive L. lactis strain MG1614, nd, not determined. In some circumstances it is desirable to add nisA nisin as an inducer. Table 5 shows the results of using various inducing agents. Table 5 Strain Inducing agent Induction* MIC° (nisin variant) 100 ng ml '1 1 mg ml '1 (mg ml 1 ) MG1614 - 2 4 - FI5876 - 94 100 - FI7847 - 3 3 - FI7847 A (wild type) 104 104 0.13 FI7847 Dha5A 114 107 0.25 FI7847 Dha33A 17 101 0.25 FI7847 Dha5,33A 34 106 1.00 FI7847 H27K 86 nt 0.13 FI7847 K12L 81 nt 0.13 FI7847 I30W 41 nt 0.16 Example 2: Purification of a variant nisin Strains FI8070 (nisA/SSA) is cultured and the variant nisin (in which Dha5 is replaced with alanine) is secreted into the culture medium. The variant nisin is purified using a method based on that described by Mulders et al (1991) Eur. J. Biochem. 201, 581-584. 1 litre cultures were incubated at 30°C for 16 hours. The pH of cultures was reduced to 2-3 with HC1 before centrifugation at 10,000 rpm for 10 minutes. The cell- free supernatants were retained and the pH increased to 5-6 with 10 mM NaOH. To each 10 ml of supernatant 0.99 g of (NH 4 ) 2 S0 4 was added. This solution was then filtered (Millipore, 0.45 μm) prior to running on a Fractogel TSK Butyl 650S (Merk) column, bed volume 5 x 20 cm, previously equilibrated with 0.8 M (NH 4 ) 2 S0 4 . The column was washed with ~ 1 litre of 0.8 M (NH 4 ) 2 S0 4 until the absorbance at 220 dropped to below 0.5. The bound nisin was eluted with 5 mM HCl and 10 ml fractions were assayed for nisin activity. Active fractions were pooled and freeze dried. Reverse phase HPLC was carried out on the resuspended samples using μBondapak C, g column 3.9 x 300 mm run at room temperature. Solvents used were 0.06% (v/v) trifluoroacetic acid and 0.06% (v/v) trifluoroacetic acid in 90% (v/v) aqueous acetonitrile. Absorbance was measured at 220 nm. Example 3: Addition of variant nisin to cheese The variant nisin produced by strain FI8070 (in which Dha5 is replaced with alanine) is added at a concentration of 12.5 mg per kg to soft cheese spread in order to prevent the growth of food-spoilage or pathogenic bacteria.

Claims: ________ 1. A method for making a cell which does not contain a natural nisA gene but expresses a nisin comprising the step of providing a cell with a variant nisA gene and genes for nisin modification, secretion and immunity wherein the variant nisA gene has the same relationship as the natural nisA gene to a gene cluster containing the natural nisA gene and the genes for nisin modification, secretion and immunity. 2. A method according to Claim 1 wherein the variant nisA gene encodes a variant nisin. 3. A method according to Claim 1 or 2 wherein the variant nisA gene comprises a regulatory region other than the natural nisA gene regulatory region and a nisin coding region. 4. A method according to any one of Claims 1 to 3 comprising substituting a variant nisA gene for the natural, chromosomal nis A gene at the chromosomal location of the said natural nisA gene. 5. A method according to any one of Claims 1 to 3 comprising providing a gene cluster comprising a variant nisA gene and the genes for nisin modification and immunity on an autonomously replicating DNA element. 6. A method according to any one of the preceding claims wherein the cell is a Lactococcus. 7. A method according to Claim 1 or 2 comprising the steps of (1) substituting a counter-selectable nis A gene for the natural, chromosomal nisA gene at the chromosomal location of the said natural nisA gene and (2) substituting a variant nisA gene for the counter-selectable nisA gene at the chromosomal location of the said natural nis A gene. 8. A method according to any one of the preceding claims comprising a subsequent step of selecting a cell that is immune to nisin. 9. A method according to Claim 7 or 8 wherein the counter-selectable nisA gene comprises an antibiotic resistance gene. 10. A method according to Claim 9 further comprising the step of selecting a cell that is sensitive to the said antibiotic. 11. A method according to any one of the preceding claims wherein the variant nisA gene contains a modification to the transcriptional or translational control sequences of the natural nisA gene and, as a consequence, the cell expresses an elevated level of its natural nisA nisin compared to the natural level. 12. A cell obtainable by the method of any one of the preceding claims. 13. A cell which does not contain a natural nisA gene but expresses a nisin comprising a variant nisA gene wherein the variant nisA gene has the same relationship as the natural nisA gene to a gene cluster containing the natural nisA gene and the genes for nisin modification, secretion and immunity. 14. A cell according to Claim 13 wherein the variant nis A gene encodes a variant nisin. 15. A cell according to Claim 13 or 14 wherein the variant nisA gene comprises a regulatory region other than the natural nisA gene regulatory region and a nisin coding region. 16. A cell according to any one of Claims 13 to 15 wherein the natural, chromosomal nisA gene or part thereof is absent and the cell comprises a variant nisA gene at the chromosomal location of the said natural nisA gene. 17. A cell according to any one of Claims 13 to 15 comprising an autonomously replicating DNA element carrying a variant nisA gene and the genes for nisin modification, secretion and immunity. 18. A cell according to any one of Claims 13 to 17 wherein the cell is a Lactococcus. 19. A cell according to any one of Claims 13 to 18 that is immune to nisin. 20. A cell according to any one of Claims 13 to 19 wherein the variant nisA gene contains a modification to the transcriptional or translational control sequences of the natural nisA gene and, as a consequence, the cell expresses an elevated level of its natural nisA nisin compared to the natural level. 21. A process for producing nisin comprising culturing a cell according to any one of Claims 12 to 20 and obtaining the nisin produced thereby. 22. A process according to Claim 21 wherein the nisin is a variant nisin. 23. A process according to Claim 21 or 22 wherein the cell is cultured in the presence of nisA nisin or a variant nisin which can induce nisin expression. 24. A process according to Claim 23 wherein the amount of nisA nisin is a minimum amount that provides maximal induction of nisin production. 25. A nisin produced according to any one of Claims 21 to 24. 26. The use of a nisin according to any one of Claims 21 to 24 as an antimicrobial agent.

Inventors: DODD HELEN MAIR, GASSON MICHAEL JOHN, DODD, HELEN, MAIR, GASSON, MICHAEL, JOHN
Applicants: BIOTECH & BIOLOG SCIEN RES, DODD HELEN MAIR, GASSON MICHAEL JOHN, BIOTECHNOLOGY AND BIOLOGICAL SCIENCES RESEARCH COU, DODD, HELEN, MAIR, GASSON, MICHAEL, JOHN
Application year: 1995
Main classification: C12N15
IPC codes: C07K14, C12N1, C12N15, C12P21, C12R1