BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to transformed dairy cultures with a natural 7-8-kb plasmid pSRQ OO which was isolated from Lactococcus lactis subsp. cremoris DCH—-4, a known strain. pSRQ700 encodes a restriction/modification system named lαll. When introduced into a phage-sensitive dairy culture, such as L. lactis , pSRQJOO confers strong phage resistance against the three most common lactococcal phage species: 936, c2 and P335 found in dairy product fermentations. The Llαll endonuclease was purified and found to cleave the palindromic sequence 5VGATC-3'. The low copy plasmid pSRQ OO was mapped and the genetic organization of Llαll localized. Cloning and sequencing of the entire Llαll system allowed the identification of three open reading frames. The three genes (LlαllA, LiαllB, and LZαllC) overlapped and are under one promoter. A terminator was found at the end of LlαllC. The genes LlαllA and LZαllB coded for m ⁶ A-methyltransferases and LlαllC for an endonuclease. The native Llαll R/M system from Lactococcus lactis is also expressed by and conferred strong phage resistance to various industrial S. thermophi lus strains. Resistance was observed against phages isolated from yogurt and Mozzarella wheys. This is the first demonstration of increased phage resistance in S. thermophi lus.

(2) DESCRIPTION OF RELATED ART

Lactococcus lactis and Streptococcus salivarius subsp. thermophi lus cultures are used extensively worldwide in the manufacture of fermented dairy products. The cultures are normally inoculated into pasteurized or heat-treated milk to quickly start and control the fermentation. In this non-sterile milk environment, the added cells come into contact with the wild bacteriophage population that has survived pasteurization. Although natural phage concentration is low, their population increases very rapidly if phage-sensitive cells are present in the starter culture. The consequent lysis of a large number of sensitive cells retards the fermentation process. To cope with this natural phenomenon, the dairy industry has developed a series of solutions including the use of phage resistant Lactococcus lactis strains (Hill,

3C FEMS Microbiol. Rev. 12:87-108 11W*.) ) .Lactococcus lactis

In the last decade, extensive research was conducted interactions between lactococcal phage and their hosts. Lactococc lactis was found to possess many plasmids coding for natural defen mechanisms against bacteriophages. Over 40 plasmids with phage defen barriers have been identified. Phage resistance systems are classifi into three groups based on their mode of action: blocking of pha adsorption, restriction/modification and abortive infection. Phag resistant Lactococcus lactis strains have been constructed by introduci these natural plasmids into phage-sensitive strains (Sanders, M. E. , al., Appl. Environ. Microbiol. 40:500-506 (1980)). The conjugati abilities of some of these plasmids was exploited to construct su resistant strains (Harrington, A., et al., Appl. Environ. Microbio 57=340 -3409 (1991); Jarvis, A. W., et al., Appl. Environ. Microbio 55:1537-1543 (1988); Sanders, M. E. , et al., Appl. Environ. Microbio 52:1001-1007 (1986); and Ward, A. C. , et al., J. Dairy Sci. 75=683-6 (1992)). However, after considerable industrial use of these strain new phages capable of overcoming the introduced defense mechanism ha emerged (Alatossava, T. , et al., Appl. Environ. Microbiol. 57:1346-13 (199D; Hill, C, et al. , J. Bacteriol. 173:4363-4370 U99D; a Moineau, S. , et al. , Appl. Environ. Microbiol. 59:197-202 (1993))- Thu the search for different natural phage barriers is still an ongoi objective for dairy product starter culture manufacturers.

Over the years several studies have established t heterologous nature of the lactococcal phage population (Jarvis, A. W et al., Intervirology 32:2-9 (1991))• Based on electron microscopy a DNA hybridization studies, the Lactococcal and Streptococcal Phage Stu Group, which is part of the International Committee on Taxonomy Viruses, reported the existence of 12 different lactococcal pha species. Recently, this number has been reduced to 10 due to t reclassification of the 1483 and TI87 species into the P335 specie Strong DNA homology is observed among members of the same species but homology is found between species (Braun, V., et al., J. Gen. Microbio 135:2551-2560 (1989); Jarvis, A. W. , et al., Intervirology, 32:2 (1991); Moineau, S., et al., Can. J. Microbiol. 38:875-882 (1992 Powell, I. A., et al.. Can. J. Microbiol. 35:860-866 (1989); and Prevot F., et al., Appl. Environ. Microbiol. 56:2180-2185 (1990)). Althou many species have been isolated, only three appear to be very problemat for the dairy industry. The species 936 (small isometric head) and

(prolate head) have been, by far, the most disturbing lactococcal phage species worldwide. Interestingly, phages from the P335 species (small isometric head) are now being isolated with increasing frequency from North American dairy plants (Moineau, S. , et al., Appl. Environ. Microbiol. 59:197-202 (1993))- Two recent surveys revealed that 1002 of the 45 lactococcal phages isolated from Canadian cheese plants and U.S. buttermilk plants were classified within one of these three species: 22 phages belonged to the 936 species, 18 to the c2 species and 5 to the P335 species (Moineau, S., et al., J. Dairy Sci. 77:18 suppl. 1 (1994); and Moineau, S. , et al., Can. J. Microbiol. 38:875-882 (1992)). Therefore from a practical point of view, industrial Lactococcus lactis strains and in general any bacterium to be used as dairy culture should at least be resistant to the three most common phage species: 936, c2 and P335- Due to the diversity of lactococcal phages, the need for phage defense mechanisms with broad activity (attacking many species) is becoming more meaningful. Because of the characteristics of phages, restriction/modification (R/M) systems have the potential to fulfill this objective.

The phenomenon of R/M was first reported more than 40 years ago (Luria, S. E. , et al., J. Bacteriol. 64:557-569 (1952)) and received a molecular explanation ten (10) years later (Bickle, T. A., et al., Microbiol. Rev. 7:434-450 (1993); and Dussoix, D., et al., J. Mol. Biol. 5:37-49 (1962)). The main biological activity of R/M is believed to be in preventing the entrance of foreign DNA (including phage DNA) into the cell. These gatekeepers are roughly the prokaryotic equivalent of the immune system (Wilson, G. G., Nucleic Acids Res. 19:2539-2566 (1991)). There are currently more than 2400 known restriction enzymes and over 100 have been cloned and sequenced (Raschke, E. , GATA 10:49-60 (1993); and Roberts, R. J., et al., Nucleic Acid Res. 21:3125-3137 (1993))- There are several kinds of R/M systems and they appear to have equivalent biological activities that are however achieved in different ways. At least four types of R/M systems have been identified: I, II, IIs, and IIII (Bickle, T. A., et al., Microbiol. Rev. 57:434-4 0 (1993); Wilson, G. G., Nucleic Acids Res. 19:253 -2566 (199D; and Wilson, G. G. , et al., Annu. Rev. Genet. 25:58 -627 (199D)- Of these, type II is the simplest and the most common. Illustrative patents are European Patent Application 0 316 677, European Patent Application 0 452 224, U.S. Patent Nos. 4,530,904 to Hershberger. et al, 4,883,756 to Klaenhammer et al, 4,931,396 to Klaenhammer et al and 5,019,506 to Dalv et al.

Many R/M systems have been characterized at the protein leve Restriction enzymes are very dissimilar, suggesting an independe evolution and not development from a common ancestor (Bickle, T. A., al., Microbiol. Rev. 7:434-450 (1993); Wilson, G. G., Nucleic Acids Re 19:2539-2566 (1991); and Wilson, G. G., et al., Annu. Rev. Genet. 25:58 627 (1991))• In contrast, extensive similarities occur among t methyltransferases (Bickle, T. A., et al., Microbiol. Rev. 57:434-4 (1993); Klimasauskas, S., et al.. Nucleic Acids Res. 17:9823-9832 (1989 Lauster, R. , J. Mol. Biol. 206:313"321 (1989); McClelland, M. , et al Nucleic Acids Res. 20:2145-2157 (1992); Wilson, G. G., Nucleic Acids Re 19:2539-2566 (1991); and Wilson, G. G. , et al., Annu. Rev. Genet. 2 :58 627 (1991))- They can be grouped into three classes corresponding to t modification types: m^C, m ⁵ C and m ⁶ A (Wilson, G. G. , Nucleic Acids Re 19:2539-2566 (1991); and Wilson, G. G., et al., Annu. Rev. Genet. 25:58 627 (1991)). m^C and m ⁶ A can be further divided in two (α and ξ>) a three (α, β, and γ) subclasses respectively, based on their amino ac sequences (Klimasauskas, S. , et al., Nucleic Acids Res. 17:9823-98 (1989); and Lauster, R. , J. Mol. Biol. 206:313-321 (1989)).

A number of plasmids encoding R/M have been identified Lactococcus (Hill, C, FEMS Microbiol. Rev. 12:87-108 (1993) Surprisingly, only a handful have been partially characterized. The Ll R/M system encoded on the conjugative plasmid pTR2030, isolated fr Lactococcus lactis subsp. lactis ME2, was the first to be analyzed at t sequence level (Hill, C, et al.. J. Bacteriol. 173:4363-4370 (199D The methylase gene of pTR2030 system has been sequenced and the deduc protein was found to share similarities with the type-I methyltransferase (m ⁶ A) , M. Fokl (Hill, C. L. , et al., J. Bacterio 173:4363-4370 (1991)). The endonuclease genes have also been sequenc and four open reading frames were identified (O'Sullivan, D. J., et al FEMS Microbiol. Rev. 12:P100 (1993)). Recent data have provided eviden for a new class of multisubunit endonucleases (O'Sullivan, D. J., et al FEMS Microbiol. Rev. 12:P100 (1993))- The restriction complex, howeve has yet to be purified and its recognition sequence is unknown.

ScrFI was the first classical type II restriction enzy isolated from Lactococcus lactis and is the only one commercial available (Fitzgerald, G. F. , et al.. Nucleic Acid Research. 10:8171-81 (1982)). ScrFI recognizes the sequence 5'-CCNGG-3' where N is a nucleotide. Two methylase genes from the Lactococcus lactis subs lactis UC503 chromosome have been cloned and sequenced (Davis, R.,

al., Appl. Environ. Microbiol. 5 :777-785 (1993); and Twomey, D. P., et al., Gene 136:205-209 (1993)). They both coded for a m ⁵ C MTase. The endonuclease gene has yet to be identified. Mayo et al (Mayo, B. , et al., FEMS Microbiol. Lett. 79:195-198 (1991) isolated a type II endonuclease (also named Llαl) from L. lactis subsp. lactis NCD0497 which recognized the sequence 5'-CCWGG-3 (W is A or T) but the R/M genes have not been cloned.

Recently Nyengaard, N. , et al, Gene 136, 371-372 (1993) described Llαl and LlαBI, which are type II restriction endonucleases from Lactococcus lactis subsp. cremoris W9 and W56. These endonucleases recognize DNA sequences 5'/GATC-3 and 5'-C/TRYAG3' , respectively. The plasmids from these strains were transformed into a plasmid free and endonuclease negative Lactococcus lactis subsp. lactis by electroporation to produce a transformed strain which resisted phage attack. The DNA was not isolated and sequenced and the natural plasmid was used for the transformation. Further, the authors did not indicate if the plasmids encoded methyl transferase. Strains W9 and W56 were not tested. In Journal of Bacteriology, July 1991. P 4363-4370 Hill C. et al. describe the Llal as being a protein of 72,5 kDa with organisational similarities to the type Ila methylase Fokl. It is atypical of other type II proteins which generally have a molecular weight of 30-50 kDa. The use of such a sequence for rendering lactobacilli or streptococci resistant to the large group of the three most common phage species: 936, c2 and P335 is not disclosed. They merely describe a plasmid comprising the nucleic acid encoding the 421 amino acids of the amino domain of a truncated protein and illustrated this was sufficient to encode a functional methylase enzyme. Streptococcus thermophi lus

Similar information on phage and phage resistance is still very limited for Streptococcus thermophi lus despite sustained phage infections in the yogurt and Mozzarella cheese industry (Mercenier et al. Genetic engineering of lactobacilli, leuconostocs and Streptococcus thermophi lus . In M. J. Gasson and W. M. DeVos (ed.), Genetics and biotechnology of lactic acid bacteria. Blackie Acad. Prof. Glaskow, UK p. 253-293 (1994)). Fortunately, S. thermophi lus phages are much more closely related to each other than the L. lactis phages. It appears that there is only one S. thermophi lus phage species (Mercenier et al Genetic engineering of lactobacilli, leuconostocs and Streptococcus thermophi lus , In M. J. Gasson and W. M. DeVos (ed.). Genetics and biotechnology of lactic acid

8

bacteria. Blackie Acad. Prof. Glaskow, UK p. 253-293 (1994)). Only few phage defense mechanisms have been reported for S. thermophi Four chromosomally-encoded type II R/M systems have been identified i thermophi lus . Solaiman and Somkuti (Solaiman, D.K.Y., et al., Microbiol. Lett. 67:261-266 (1990); and Solaiman, D.K.Y., et al., Microbiol. Lett. 80:75-80 (1991)) have isolated the endonuclease Sth and Sthll7I which are isoschizomers of Hpall and EcoRII, respectiv Benbadis et al (Benbadis, L., et al., Appl. Environ. Microbiol. 57:3 3678 (1991)) and Guimont et al (Guimont, C, et al., Appl. Microb Biotechnol. 39:216-220 (1993)) have isolated the endonucleases ssll Sth455Iι respectively. Both are also isoschizomers of EcoRII. addition, S. thermophi lus might possess abortive-like phage def mechanisms (Larbi et al. J. Dairy Res. 59:349~357 (1992)), alth definitive proof has yet to be demonstrated. None of the R/M system far identified in S. thermophi lus have been cloned, sequenced, or use commercial strains for improvement of phage resistance. There believed to be no report on improvement of phage resistance of thermophi lus strains. OBJECTS It is therefore an object of the present invention to pro an isolated DNA encoding only restriction and modification enzymes impart phage resistance. Further, it is an object of the pre invention to provide transformation vectors and transformed bact incorporating the DNA which are particularly useful in the d industry. These and other objects will become increasingly apparent reference to the following description and the drawings. BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is an electrophoresis gel showing a plasmid anal of Lactococcus lactis strains wherein Lane 1 is supercoiled DNA la (GIBC0/BRL) ; Lane 2 is Lactococcus lactis DCH-4; Lane 3 is Lactoco lactis SMQ-17 (pSA3 and pSRQJOO) ; Lane 4 is Lactococcus lactis SM

(PSA3) .

Figure 2 is an endonuclease restriction map of lactoco plasmid pSRQ700. Site positions are indicated in kb. Figure 3 s a map showing cloning of Llαll from pSRQJOO pSA3. Clones were electroporated into LM0230. Transformants were te for phage resistance against Ψp2.

Figure 4 is a nucleotide sequence of the 3~kb Wrul-E fragment from pSRQJOO. The deduced amino acid sequence of the 3 ORF

presented. The putative promoter, terminator and ribosome binding site are underlined. The first codon of each ORF is in bold. The amino acids are in single letter code.

Figure 5 is a chart showing a comparison of the amino acids between A) M.M. LlαllA (SEQ ID NO. 2), M. DpnII (SEQ ID NO. 5), K.Hboh (SEQ ID NO. 6) and E. coli Dam (SEQ ID NO. 7) methylases; B) M.LlαllB (SEQ ID NO. 3), DpnA (SEQ ID NO. 8), M.ΛboC (SEQ ID NO. 9) and M.Hfn/I (SEQ ID NO. 10); C) R.Llαll (SEQ ID NO. 4), R.DpnII (SEQ ID NO. 11) and R.WioI (SEQ ID NO. 12) . The asterisk (*) indicates conserved amino acids. Bars show gaps in the aligned sequences.

Figure 6 is an electrophoresis gel showing restriction patterns of ΦQl , øQ3 and ΦQ5- Lane 1 and 5, 1-kb ladder (Bethesda Research Laboratories); Lane 2, ΦQl DNA cut with EcoRV, Lane 3. ΦQ2- cut with EcoRV; Lane 4, ΦQ5 cut with EcoRV; Lane 6, ΦQl cut with Wool; Lane 7, ΦQ7 cut with Λbol; Lane 8, ΦQ5 cut with Mbol.

Figure 7 is a schematic flow sheet showing the construction of the plasmids used in this study. DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to an isolated nucleic acid encoding only a protein, polypeptide or enzyme which is sufficiently duplicative of a member selected from the group consisting of LZαllA, LlαllB and LlαllC and mixtures thereof to restrict or modify a phage. The nucleic acid according to the invention does not comprise the nucleic acid sequence encoding the amino domain of the truncated Llall protein described in Journal of Bacteriology 1991 already cited as this is a Llal derivative and not a Llall derivative. A Llal protein is approximately 72,5 kDa. A Llall protein is less than 70 kDa, generally between 30 and 60 kDa, most preferably between 30 and 50 kDa. Preferably the nucleic acid sequence according to the invention is considered food grade i.e. is derived from a food grade organism or encodes a product that occurs in a food grade organism. Suitable food grade organisms are organisms used in dairy culture. A suitable organism from which a nucleic acid sequence according to the invention can be derived is a Lactobacillus or a

Streptococcus. Preferably the expression product of a nucleic acid sequence according to the invention exhibits the phage restriction or modification activity under circumstances present during dairy processing. Preferably a nucleic acid sequence according to the invention will correspond to a naturally occurring sequence in a food grade organism.

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8

The nucleic acid according to the invention encodes a protei polypeptide or enzyme wherein "sufficiently duplicative" implies havi activity selected from m ⁶ A methyl transferase activity and endonuclea activity. Suitably the endonuclease activity of the "sufficient duplicative" protein, polypeptide or enzyme is directed at t palindromic recognition site 5'/GATC-3.' A polypeptide, protein or enzy that can be considered suficiently duplicative of Llall will generally less than 70 kDa. A protein, polypeptide or enzyme that can be consider "suitably duplicative" is any expression product of an allelic derivati of the encoding nucleic acid sequences ORF 1, ORF 2 and ORF 3 of Sequen Id. No. 1 and sequence Id. Nos 2, 3 and 4 which expression produ exhibits the aforementioned phage restriction or modification activit In particular the expression product may exhibit m ⁶ A α or β meth transferase or endonuclease activity or a combination thereof. A nucleic acid sequence encoding an amino acid sequence encoded by any the ORF 1, ORF 2 and ORF 3 nucleic acid sequences of Sequence id no. 1 Sequence id nos 2-4 is included within the scope of protection by virt of the term "suitably duplicative'On the basis of the degeneracy of t genetic code. Modified nucleic acid sequences of ORF 1, ORF 2 and ORF or Sequence id nos 2-4 encoding expression products with essentially t same degree of phage restriction or modification activity as t expression products of ORF 1, ORF 2 and ORF 3 of Sequence id. no. 1 Sequence id nos 2-4 or even better activity are also consider "sufficiently duplicative" within the terms of the invention. In a specific embodiment the present invention relates to isolated nucleic acid with a nucleotide sequence essential corresponding to that set forth in SEQ ID NO. 1 selected from the gro consisting of 0RF1 (positions 97 to 948), 0RF2 (positions 9 l to 174 and 0RF3 (positions 1740 to 2651) or Sequence id nos 2-4 and combinatio thereof. Any nucleic acid sequence capable of hybridising to any of t aforementioned group of sequences under stringent hybridisati conditions, said nucleic acid sequence further encoding an expressi product capable of exhibiting phage restriction or modification activi is included within the term "essentially corresponding" and thus considered "substantially duplicative". A definition of stringe hybridisation conditions can be found in molecular cloning handbooks a other food related patent applications of applicant and is well known persons skilled in the art (Maniatis, T., E.F. Fritsch and J. Sambro 1982, Molecular Cloning: a laboratory manual, Cold Spring Harb

Laboratory, Cold Spring Harbor, N.Y.).

The coding nucleic acid of SEQ ID N0:1 can have modifications in sequence and still be sufficiently homologous to still encode enzymes which have the necessary phage resistance properties. Generally within 75- 002 homology is sufficient to be considered "substantially duplicative".

The isolated nucleic acid according to the invention is preferably operatively linked to promoter and/or enhancer sequences such that expression of the polypeptide, protein or enzyme or combination of the polypeptides, proteins or enzymes encoded by the nucleic acid sequence is possible in a host cell. In particular a nucleic acid sequence operatively linked such that it is capable of being expressed in a host cell used in dairy cultures (such as Lactobacilli and Streptococci) forms a preferred embodiment of the invention. The phage to be modified or restricted is preferably a phage that occurs in dairy cultures. In particular a phage belonging to any of the categories of lactococcal phages 936, c2 or p335 is to be restricted or modified by the expression products of the nucleic acid according to the invention. Most preferably the phages to be restricted or modified fall within one of the categories 936 or p335-

The present invention also relates to a plasmid containing nucleic acid encoding an enzyme sufficiently duplicative of a member selected from the group consisting of LZαllA, LlαllB and LlαllC and mixtures thereof to restrict or modify a phage i.e. a plasmid comprising nucleic acid sequence according to the invention as described above falls within the scope of the invention. The plasmid according to the invention is preferably capable of expression of the nucleic acid sequence. In particular said plasmid is capable of expression of the nucleic acid sequence in a host cell used in dairy cultures such host cell for example being a Lactobacillus or Streptococcus. The plasmid according to the invention can be a recombinant plasmid or an isolated naturally occurring plasmid. Preferably the plasmid according to the invention will be food grade. The phage to be modified or restricted is preferably a phage that occurs in dairy cultures. In particular a phage belonging to any of the categories of lactococcal phages 936, c2 or p335 is to be restricted or modified by the expression product(s) of the plasmid according to the invention, with a preference for phages in the category 936 or p335-

Further the present invention relates to a recombinant bacterium harboring a nucleic acid sequence and/or a plasmid containing

10 the nucleic acid sequence according to the invention as described abov In particular a recombinant bacterium harboring a nucleic acid sequen encoding a polypeptide, protein or enzyme sufficiently duplicative of member selected from the group consisting of LlαllA, LlαllB and Llαll and mixtures thereof such that upon expression of the nucleic acid t bacterium can restrict or modify a phage falls within the scope of t invention. Recombinant bacteria capable of expressing the nucleic aci sequence under conditions present in dairy processes are preferred. preferred group are recombinant bacteria where the non recombina bacteria is useful in dairy processes but is not resistant to a pha that occurs in dairy processing prior to incorporation of the nuclei acid according to the invention. A recombinant bacterium according to t invention can quite suitably comprise a recombinant plasmid according the invention. A surprising embodiment of the invention is thus an improve recombinant Streptococcus, in particular a Streptococcus thermophi lus said improvement residing in the presence of a natural plasmid comprisi the natural Lactobacillus Llall R/M system as disclosed herein. Naturall an improved Streptococcus according to the invention may comprise any the nucleic acid sequences according to the invention or plasmi according to the invention as disclosed above in various suitable a preferred embodiments for rendering the Streptococcus phage resistant.

In particular the present invention relates to a recombinan bacterium, preferably isolated and purified, selected from the grou consisting of Streptococcus salivarius subsp. thermophilus a Lactococcus lactis naturally lacking in phage resistance which bacteri contains a heterologous nucleic acid sequence encoding a polypeptide protein or enzyme sufficiently duplicative of a member selected from t group consisting of LlαllA, LlαllB and LlαllC and combinations thereof t impart phage resistance. In particular an embodiment wherein furthermo the nucleic acid sequence for the member is essentially as set forth i any of ORF 1, ORF 2 and ORF 3 of SEQ ID N0:1 or Sequence Id nos 2-4 t impart phage resistance is included. In general a Streptococcus suitabl for use in food processes, in particular dairy processes can be improv by rendering it phage resistant through incorporation of a heterologo nucleic acid sequence, said heterologous nucleic acid sequence encodi an endonuclease and optionally one or more methy1transferases. Th heterologous nucleic acid sequence can encode enzymes with an amino aci sequence derived from a R/M phage resistance system of lactobacilli suc

11 as Lactococcus lactis . It is possible to incorporate a naturally occuring R/M plasmid from lactobacillus in a Streptococcus to achieve a phage resistant Streptococcus.

Further still, the present invention relates to a recombinantly produced purified protein, polypeptide or enzyme containing a sequence of amino acids "sufficiently duplicative" of that of a member selected from the group consisting of ORF 1, ORF 2 and ORF 3 and combinations thereof in SEQ ID NO. 2, 3 or 4 such that restriction or modification of a phage can be performed with the enzyme. In particular a suitable embodiment is a protein or polypeptide that has been produced from isolated nucleic acid corresponding to that of the SEQ ID N0:1. The protein or polypeptide can be used for assays as described hereinafter. Preferably the recombinant protein or polypeptide will exhibit a larger homology at amino acid level than is illustrated by ORF 1, ORF 2 and ORF 3 with the amino acid sequences of Figure 5 i-e. the amino acid sequences of M. DpnII, M. MboA, Dam, M. DpnA, M. MboC, M. Hinfl, R. DpnII and R. Mbol. The recombinant polypeptide, protein or enzyme will exhibit m ⁶ A methyl transferase activity and/or endonuclease activity, preferably at the palindromic sequence 5'\GATC-3'. The recombinant polypeptide, protein or enzyme with m ⁶ A α methyltransferase activity according to the invention will preferably possess more than 88,9 2 amino acid homology with the amino acid sequence of the m ⁶ A α methyl transferase DpnII as indicated in Figure 5A for M. LlallA. The recombinant polypeptide, protein or enzyme with m6A α methyltransferase activity will possess at least the consensus sequence indicated in Figure A for M. LlallA if it is to be considered "sufficiently duplicative" of LlallA. Preferably the degree of conserved amino acids will be higher than 20$,• The degree of conserved tryptophan residues will be higher than 502, preferably higher than 702- The recombinant polypeptide, protein or enzyme with m6A α methyltransferase activity will preferably possess more than 75.4 2 amino acid homology with the amino acid sequence of the m6A β methyl transferase DpnII as indicated in Figure 5B for M. LlallB if it is to be considered "sufficiently duplicative" of LlallB. The recombinant polypeptide, protein or enzyme with m6A β methyltransferase activity will possess at least the consensus sequence indicated in Figure 5B for M. LlallB. Preferably the degree of conserved amino acids will be higher than 282. The degree of conserved tryptophan residues will be higher than 502, preferably higher than 702-

The recombinant polypeptide, protein or enzyme with endonuclease activity will preferably possess more than 32 amino acid homology with the amino acid sequence of the Mbol endonuclease and more than 32 amino acid homology with the amino acid sequence of DpnII as indicated in Figure 5C for R. Llall if it is to be considered "sufficiently duplicative" of LlallC. The recombinant polypeptide, protein or enzyme with endonuclease activity will possess at least the consensus sequence indicated in Figure 5C for R. Llall.

Suitably such enzymatic activity will be exhibited under dairy processing conditions. In particular the activities will be exhibited at temperatures of at least 30 ^C C and can be exhibited at 38 ^β C. Note that nucleic acid sequences, plasmids and recombinant bacteria comprising such nucleic acid sequences as heterologous nucleic acid sequences, wherein the nucleic acid sequences encode a recombinant protein, polypeptide or enzyme according to the invention are also included within the scope of the invention.

Further, the present invention relates to a method of imparting phage resistance to a bacterium which is sensitive to the phage which comprises incorporating nucleic acid according to the invention e.g. recombinant DNA encoding a polypeptide, protein or enzyme sufficiently duplicative of a member selected from the group consisting of LlαllA, LlαllB and LlαllC and mixtures thereof into the bacterium to impart the phage resistance. Preferably a nucleic acid sequence encoding an expression product with m6 methyltransferase activity and an expression product with endonuclease activity is incorporated. More preferably the methyltransferase activity being m6 A methyl transferase activity, with a preference for both α and β activity. The endonuclease activity is directed against GATC. Suitably the nucleic acid sequence encoding the member is that contained in strain Lactococcus lactis SMQ-17 deposited as NRRL-B-21337. Preferably the bacterium is a dairy culture.

Finally, the present invention relates to a method for fermenting a dairy product, the improvement which comprises using a dairy culture of bacteria (for example selected from the group consisting of Lactococcus lactis and Streptococcus saliυaτius subsp. thermophi lus) in which a nucleic acid sequence according to the invention is present or is incorporated such that it can be expressed in the fermentation process i a manner known per se for the fermentation process, said nucleic aci sequence encoding a polypeptide, protein or enzyme sufficientl duplicative of a member selected from the group consisting of LlαllA,

LlαllB and LlαllC to impart phage resistance. In general terminology said nucleic acid sequence encoding a polypeptide, protein or enzyme sufficiently duplicative of a member selected from the group consisting of m6A α methyl transferase, m6A β methyl transferase and GATC endonuclease in any of the embodiments according to the invention disclosed above for the nucleic acid sequences and/or recombinant polypeptides. A particular embodiment comprises application of a dairy culture of bacteria in a fermentation process wherein the nucleic acid sequence imparting the phage resistance is that contained in strain Lactococcus lactis SMQ-17 deposited as NRRL-B-21337. The nucleic acid sequence can be introduced in a manner known per se for incorporating nucleic acid sequences in such types of bacterium e.g. using general transformation protocols.

The DNA of SEQ ID N0:1 and Figure 4 (Appendix I) is contained in Lactococcus lactis SMQ-17 deposited under the Budapest Treaty on September 29, 1994 as NRRL-B-21337. The strain is available upon request by name and deposit number. The isolated DNA can be obtained by means of EcoRV or Nrul-TcoRV digestion of pSRQ700 as described hereinafter.

The art of DNA isolation and cloning is well known to those skilled in the art. Further, the terminology of this art is well developed, see for instance EP 0316677 A2. As used herein, the term "transformed" means to transfer DNA from one bacterium to another in related bacterium. The term "recombinant" as used herein means nucleic acid in a form not existing in nature or in an environment it is not normally associated with in nature. In general the recombinant nucleic acid sequence according to the invention contains DNA encoding only one or more of the sequence of amino acids for LlαllA, LlαllB and LlαllC as set forth in SEQ ID N0:1 Sequence id nos 2-4 or substantially duplicative versions thereof. The recombinant enzymes that are claimed are considered to exclude known naturally occurring isolated enzymes exhibiting either m6A methyl transferase activity or endonuclease activity against the restriction site 5'\OATC-3'. The recombinant enzymes resemble the naturally occurring enzymes with regard to their phage restriction or modification activity but can have different physical configurations.

Various shuttle vectors can be used. pSA3 from Dao, M. , et al., Applied Environ. Microb. 49:115-119 (1985) was used.

The recombinant bacterium can be for instance Escherichia coli , a Lactococcus sp. or a Streptococcus sp. used in dairy fermentations.

The E. coli are used to produce the enzymes of SEQ ID N0:2, 3 and/or which can be used to produce a DNA or RNA probe in a known manner or c be used to produce antibodies to the enzymes in a well known manner f use in assays for the enzymes. Purification of the enzymes can achieved in a manner known per se using affinity chromatography and/ molecular filtration.

The preferred use of the transformed cultures containing t recombinant DNA of SEQ ID N0:1 is in dairy product fermentations. Su fermentations are well known to those skilled in the art. The preferr strains are transformed Lactococcus lactis and Streptococcus salivari sp. thermophi lus which are used in the dairy product fermentations.

EXAMPLE 1 Bacterial strains, plasmids, and media. The strains a plasmids and enzymes used in this invention are listed in Tables 1 and

(Table 1)

Bacteria, plasmids and phages Relevant characteristics Source φc2 Prolate headed, c2 species, Sanders, M.E., et al., Appl. Environ.

20.7 kb Microbiol. 40:500-506 (1980) φml3 Prolate headed, c2 species, W.E. Sandine 20.2 kb

10 φebl Prolate headed, c2 species, L.L. McKay 19.6 kb φul36 Small isometric headed, P335 species, Moineau, S., et al., Can. J. Microbiol. 38: 28.8 kb 875-882 (1992)

15 φq30 Small isometric headed, P335 species, Moineau, S., et al., J. Dairy Sci. 77:18 37.0 kb Suppl.l (1994) φq33 Small isometric headed, p335 species, Moineau, S., et al., J. Dairy Sci. 77:18 29.6 kb Suppl. 1 (1994)

20

L.L McKay, University of Minnesota; W.E. Sandine, Oregon State University; Lac, Lactose-fermenting ability; Ap', ampiciUin resistance; Cm', chloramphenicol resistance; Em', erythromycin resistance.

Table 2. Plasmids used in this study

s

25 Ap'j ampiciUin resistance: Cm' chloramphenicol resistance; Cm', sensitive to chloramphenicol; Em', erythromycin resistance; I tetracycline resistance; Tc', tetracycline resistance; R7M\ active restriction/active modification enzymes;

Escherichia coli was grown at 37°C in Luria-Bertani (Sambrooke, J., e al.. Molecular cloning: a laboratory manual. 2nd ed. Cold Spring Harbo Laboratory, Cold Spring Harbor, N.Y. (1989)). Lactococcus lactis strain were grown at 30°C in M17 (Terzaghi, B. E., et al., Appl. Microbiol 29:807-813 (1975)) supplemented with 0.5 glucose (GM17) or 0.5% lactos (LM17)- When appropriate, antibiotics were added as follows: for E coli , 50μg/ml of ampiciUin (Ap) , lOμg/ml of tetracycline (Tc) , an 20μg/ml of chloramphenicol (Cm); for L. lactis , 5 Ug/ml of erythromyci (Em). Bacteriophage propagation and assays. Bacteriophages used i this invention are listed in Table 1. Bacteriophages were propagated an titrated by the method of Jarvis (Jarvis, A. W., Appl. Environ Microbiol. 36:785-789 (1978)). Efficiency of plaquing (EOP) assays wer performed as described by Sanders and Klaenhammer (Sanders, M. E., e al., Appl. Environ. Microbiol. 40:500-506 (I98O)). Bacteriophages c2 p2, ski and jj50 were supplied by T. R. Klaenhammer (North Carolina Stat University) .

DNA Isolation and manipulation. Plasmid DNA from E. coli wa isolated as described previously (Moineau, S., et al., Appl. Environ Microbiol. 60:1832-1841 (1994)). Large quantities of E. coli plasmid DN was isolated by using plasmid MIDI or MAXI kit (Qiagen Inc., Chatsworth CA) . Plasmid DNA from L. lactis was isolated as described by O'Sulliva and Klaenhammer (O'Sullivan, D. J., et al., Appl. Environ. Microbiol 59:2730-2733 (1993)). A large quantity of lactococcal plasmid DNA wa obtained using the Leblanc and Lee procedure (Leblanc, D. J., et al., J Bacteriol. 140:1112-1115 (1979)) as modified by Gonzalez and Kunk (Gonzalez, C. F. , et al., Appl. Environ. Microbiol. 46:81-89 U983)) Restriction endonucleases (Gibco/BRL, Grand Island, NY) and T4 DNA ligas (Boehringer Manheim, Indianapolis, IN) were used according t manufacturer's instructions. When needed, DNA fragments were obtaine from low-melting agarose using a QIAEX gel extraction kit (Qiagen, Inc. Chatsworth, CA) .

Electroporation. E. coli was grown, electroporated, incubated and plated as described previously (Moineau, S., et al., Appl. Environ Microbiol. 60:1832-1841 (1994)). L. lactis was grown in GM1 supplemented with 0.5M sucrose (SGM17) and 1% glycine and electroporate as described by Holo and Nes (Holo, H. , et al., Appl. Environ. Microbiol 55:3119-3123 (1989)). The Gene Pulser apparatus (Bio-Rad Laboratories Richmond, CA) was set at 25μF and 2.45 kV, and the Pulse Controller wa

set at 200Ω. Plasmid DNA was mixed with 4θμl of cells in a chilled cuvette (0.2 cm) . After electroporation, L. lactis cells were resuspended in SGM17. incubated for 2 h at 30°C, plated on GM17 supplemented with erythromycin (5μg/ml) and incubated for 2 days at 30°C. Sequencing. The entire LZαll system (3 kb Nrul-ECORV fragment from pSRQ700) was cloned into E. coli pBluescript. The resulting clone was named pSRQ708. Nested deletions were made in both orientations from pSRQ708 using the ERASE-A-BASE kit (Promega, Madison, WI) . For the first set of deletions, the endonuclease Sstl was used for protection and Xbal was used to start the deletion. The restriction pairs Kpnl-Drαll were used to obtain the nested deletions in the other orientation. Plasmid DNA was extracted from the nested clones with QIAGEN and directly used for sequencing. The sequencing reactions were performed using the DYEDEOXY TERMINATOR TAQ sequencing kit for use on the 373A automated DNA sequencing system (Applied Biosystems, Foster City, CA) . The T7 and T3 primers were used for annealing.

Restriction enzyme purification. I. lactis SMQ-17 was grown in 2L, concentrated by centrifugation (10,000 rpm, 15 min.) and washed twice in saline. The cells were then resuspended in 30 ml of PME buffer (10 mM NaH ₂ P0i, pH 7.4, 0.1 mM EDTA and 10 mM β-mercaptoethanol) . Cells were lysed by 15 bursts (30 seconds each followed by one minute rest) with glass beads and a bead beater (BI0SPEC, Bartlesville, OK). After centrifugation to remove cell debris and glass beads, the supernatant was used for ion exchange chromatography. Successive chromatographies were performed on phosphocellulose (Whatman Pll, Maidstone, England) and dimethylaminoethyl cellulose (Whatman DE5, Maidstone, England) using a salt gradient in PME buffer. Restriction endonuclease activity was found in the fractions around 0.5 M NaCl. Lactococcal phage ul36 DNA was used as substrate and the digestions were performed at 37°C for 2-4 h using the buffer system #2 from GIBC0/BRL (50 mM Tris-HCl pH 8.0, 10 mM MgCl ₂ , 50 mM NaCl). DNA samples were analyzed as described by Sambrooke et al in Molecular cloning: a laboratory manual, 2nd ed. Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (1989) in 0.1% agarose gels in TAE.

DNA and protein analysis. The DNA sequence was analyzed with DNA strider 1.2. The SwissProt database (release 29, June 1994) was searched for homology to all three lαll amino acid sequences of SEQ ID N0:1.

Isolation of pSRQ700. For many years, Lactococcus lactis subsp. cremoris DCH-4 has performed very well during the industrial

buttermilk and sour cream production. One reason for continued goo performance is the natural resistance of DCH-4 to lactococca bacteriophages. One objective of this invention was to identify an transfer the DNA responsible for the phage resistance of DCH-4. Th total plasmid DNA of DCH-4 was isolated and co-electroporated with th vector pSA3 into the phage sensitive-plasmid free L. lactis LM0230. Th latter strain was selected because it can propagate phages from tw species, 936 and c2. The DNA ratio of DCH-4:pSA3 used fo electroporation was about 10:1. Em-resistant colonies were tested fo phage resistance by spot assay (10 ³ - 10 pfu of Ψp2/spot) . A few phag resistant colonies were obtained, analyzed, and found to contain pSA3 an a 7«8 kb low copy plasmid which was named pSRQ700 (Figure 1). Th transformant containing pSRQ700 was named L. lactis SMQ-17 (NRRL-B 21337)- Plasmid pSRQ700 was also electroporated into L. lactis UL8 whic can propagate phages from the P335 species. The transformant was name L. lactis SMQ-87.

Effectiveness of pSRQ700 on lactococcal phage species. L lactis SMQ-17 and SMQ-87 were tested for phage resistance against a tota of 9 phages belonging to 3 species (3 phages/species) . Phages p2, sk and jj50 were selected as representatives of the 93 species (Table 1) The lactococcal phage species c2 was represented by the phages c2, ml and ebl. These six phages were individually tested on SMQ-17 and thei EOPs are presented in Table 3-

Table 3

Comparison between the efficiency of plaquing of lactococcal phages on L lactis SMO-17 and the number of Mbol sites in the phage genome.

EOP on SMQ-17 Number of Mbol sites ^*

936 species φp2 1.7 x 10 ^"6 11

10 φskl 2.5 x 10 ^"6 9 φjj50 2.0 x 10 ^"6 10 I

c2 species φc2 . 1.0 x 10^ 3

15 φml3 6.1 x 10 ^"3 2 φebl 5.5 x 10 ^'3 2

P335 species φul36 2.7 x 10 ^"7 13

20 φQ30 5.2 x 10" 12

ΦQ33 1.3 x 10 ^"7 15

Only number of fragments > 0.5 kb were determined.

The new emerging P335 species was represented by the phage ul36, Q30 and Q33. They were tested separately on SMQ-87 and their EOP are also presented in Table 3- All three 936 phages had similar EOPs i the range of 10" ⁶ . More variability was observed with the c2 species where EOPs range from 10 ^~3 to 10 ^" ''. The P335 phages were the most affected by pSRQ70 since EOPs of 10 ^"7 were observed (Table 3) • Identical results wer obtained when phage resistance was tested at 21, 30 and 3δ°C (data no shown) . These results indicated that the phage resistance mechanis encoded on pSRQ700 is temperature insensitive.

Identification of the phage resistance mechanism on pSRQ700 Phages capable of overcoming the defense mechanism encoded on pSRQ70 were isolated. These phages had EOPs of 1.0 on L. lactis SMQ-17. Whe these resistant (modified) phages were propagated back on their origina host, they became sensitive (restricted) to pSRQ700 at the same previou level (data not shown). This temporary host specific immunity demonstrates the presence of a classical R/M system encoded on pSRQ700 The R/M system was named Llαll.

Isolation of the restriction endonuclease. The non-specifi nucleases were removed after ion exchange chromatographies performed o phosphocellulose (Whatman Pll) and dimethylaminoethyl cellulose (Whatma

DE5) using a salt gradient in PME (10 mM NaH ₂ P0<, pH 7.4, 0.1 mM EDTA an

10 mM β-mercaptoethanol) buffer. DNA from the well-characterize lactococcal phage ul36 (Moineau, S., et al., Can. J. Microbiol. 38:875 882 (1992: Moineau, S. , et al., Appl. Environ. Microbiol. 59:197-20

(1993); and Moineau, S. , et al., Appl. Environ. Microbiol. 60: ;l832-l84

(1994)) was digested with Llαll. The digestions were conducted overnigh at 37°C since the R/M encoded on pSRQ700 is temperature-insensitive (u to 38°C) . Defined DNA fragments were identified on agarose gels (dat not shown) . No attempts were made to determine the number of activit units in the collected fractions nor the percentage of recovery from th crude supernatant. Unexpectedly, the restriction patterns obtaine corresponded to Mbol restriction patterns. Attempts to cut pSRQ700 wit

Mbol were unsuccessful. It was concluded that the R/M system present o pSRQ700 was similar to the Mbol system which recognized the sequence 5'

GATC-3' and cleaved it before the guanine.

Happing of pSRQ700. Single, double and triple digestions wer performed with endonucleases to obtain a map of pSRQ700. The results ar presented in Figure 2. The following endonucleases did not cut pSRQ700

ipαl, Aval , Avail, Ball , BαmHI, Hpαl, rtbol, PstI, PvuII , Sail, Smal, SphI , Xbal , Xhol .

Localization of the Llαll system on pSRQ700. The Llαll R/M system was entirely cloned into E. coli using the E. coli-L. lactis shuttle vector pSA3 (Figure 3)- Since appropriate unique restriction sites were present on PSA3 and PSRQ700, total plasmid DNA from L. lactis SMQ-17 was directly used for cloning. Plasmid DNA from SMQ-17 was digested with selected endonucleases, phenol extracted, ethanol precipitated, ligated and the ligation mixture electroporated in E. coli DH5α. This strategy was very effective because expected clones were rapidly obtained. The clones were electroporated into L. lactis LM0230 and phage resistance was determined. The relevant clones are presented in Figure 3- The entire R/M system of PSRQ700 was localized on a 3~kb NruI-EcoRV fragment. The PSA3 clone containing this 3kb fragment was named pSRQ706. Similar EOPs were obtained with PSRQ700 and PSRQ706 (Figure 3) • This is due to the similar low copy number of PSA3 and PSRQ700 (Figure 1).

DNA Sequence Analysis of the Llαll. The 3-kb NruI-EcoRV fragment containing the Llαll genes was sequenced in both directions and found to contain 2,987 bp (Figure 4; SEQ ID N0:1). This fragment was 65 - 1% A+T rich, typical of lactococcal genes (Van de Guchte, M. , et al., FEMS Microbiol. Rev. 88:73-92 (1992)). Three overlapping open reading frames (orfs) were found and the genes were named LlαllA, LlαllB and LlαllC. In reference to Figure 4 and SEQ ID N0:1, the gene LlαllA was localized from position 97 to position 948 and coded for a protein of 284 amino acids with an estimated weight of 33.031 Da. The gene LlαllB was localized from position 94l to position 1747 and coded for a protein of 269 amino acids with an estimated weight of 30,904 Da. The gene LlαllC was localized from position 1740 to position 2651 and coded for a protein of 304 amino acids with an estimated weight of 34,720 Da. Phage p2 E0P of 1.0 on L. lactis harboring pSRQ702 or pSRQ703 suggested that LlαllC coded for the endonuclease (Figure 3) • No putative ribosome binding site (RBS) was found for LlαllA and LlαllB. A putative RBS (GGAG) was found preceding LlαllC. Atypical RBS have been identified for the DpnII methylases which are similar to Llαll (Figure 5). They were not found in the Llαll system. Atypical RBS may be related to translational control of the methylase gene expression (Lacks, S. A., et al., In:Genetics and Molecular Biology of Streptococci, Lactococci and Enterococci, Dunny, G. M., P. P. Cleary and L. L. McKay, (eds) ASM, Washington , D. C. p.71-76

(1991))- All three genes appear to be under the control of the sa promoter. However, no definite consensus E. coli-10 and -35 promot sequences could be identified. Because EOPs were the same in PSRQ70 pSRQ701 and PSRQ703 (Figure 3) . it is believed that the promoter w present in the 3-0-kb fragment. The putative promoter sequences upstre of LlαllA is of interest. A putative -35 region was localized position 27, followed by a 18 bp spacer, and a putative -10 region position 51 (Figure 4). A search for palindromic sequences identifi two perfect inverted repeats of 19 bp, typical of a strong rh independent terminator, at the very end of LlαllC (Figure 4) Interestingly, the stop codon of LlαllC was within the beginning of t stemloop structure.

Protein analysis. Homology searches showed that the deduc protein coded by LlαllA was 75-4% identical to DpnII methyla (Mannarelli. B. M. , et al., Proc. Natl. Acad. Sci. 82:4468-4472 (1985)) 41.52 identical Mbol methylase (Ueno, T, , et al., Nucleic Acids Re 10:2309-2313 (1993)) and 30.12 to the Dam methylase of £. coli (Brook J. E. , et al., Nucleic Acids Res. 11:837-851 (1983))- It was conclud that LlαllA codes for a methylase and was named M. lαllA. All thr methylases (M. pnII, M.MboA and Dam) homologous to LlαllA are N-6 adeni ethyltransferase (m ⁶ A-MTases) . The most conserved amino acid sequen motifs among the m ⁶ A-Mtases are F-G-G (motif I) and DPPY (motif II) Their organization in the protein allowed the division of the A-Mtas in three subclasses (α, β and γ). In the m A-Mtase subclass α, the mot I is found close to the N-terminal followed by a variable region of 10 200 aa and the motif II close to the C-terminal. The reverse situati is found in the subclass fi, where the motif II appears before the mot I. M. lαllA has all the characteristics of a m ⁶ A-Mtase subclass α:F-G- motif, a 146 aa variable region and a DPPY motif (Figure 5). The F-G motif probably contained the S-adenosylmethionine binding site and DP might be involved in the methylation of exocyclic amino aci (Klimasauskas, S., et al., Nucleic Acids Res. 17:9823-9832 (1989)).

The deduced protein coded by LlαllB was found to be 88. identical to the second methylase of DpnII (Cerritelli, S., et al., Pro Natl. Acad. Sci. USA, 86:9223-9227 (1989)). 50.2% identical to the seco methylase of Mbol (Ueno, T. , et al., Nucleic Acids Res. 10:2309-23 (1993)) and 43.62 identical to the Hinfl methylase (Chandrasegaran, S. et al.. Gene 70:387-392 (1988)). It was concluded that LlαllB also cod for a methylase and was named M. lαllB. All three methylases (M.Dpn

M.MboC and Hinfl) homologous to LlαllB are m ⁶ A-Mtases but subclass β. M. lαllB has all the subclass characteristics: a DPPY motif, a 175 aa variable region and a F-G-G motif. Interestingly, Figure 5 also showed the amino acid comparison between two sets of four m A-Mtases isolated from two Gram-positive and two Gram-negative bacteria. This enzyme methylates the same 5'~GATC-3' sequence. Despite the various origins, about 202 and 282 of the amino acids are respectively conserved in the four α and ξ> methylases studied. Interestingly, almost all tryptophan residues are conserved in the methylases studied (Figure 5) • The deduced protein coded by LlαllC was 34 and 312 identical to Mbol (Ueno, T. , et al., Nucleic Acids Res. 10:2309-2313 (1993)) and DpnII (de la Campa, A. G. , et al., J. Biol. chem. 263:14696-14702 (1987)) endonucleases, respectively. These results confirmed that LlαllC coded for an endonuclease and was named R. lαll. Conserved aa motifs were observed among the three isoschizomers but their functionality is unknown.

It was thus found that Lactococcus lactis subsp cremoris DCH-4 harbors a 7.8-kb low copy plasmid (PSRQ700) coding for a temperature- insensitive R/M system similar to DpnII (Lacks, S. A., et al.. In: Genetics and Molecular biology of Streptococci. Lactococci and Enterococci. Dunny, G. M. , P. P. Cleary and L. L. McKay, (eds) ASM, Washington , D. C. p-71-76 (1991)) and Mbol (Ueno, T. , et al.. Nucleic Acids Res. 10:2309-2313 (1993))- These systems recognize the non- methylated DNA sequence 5'-GATC-3' where the endonuclease cleaved before the guanine (Lacks, S. A., et al.. In: Genetics and Molecular biology of Streptococci, Lactococci and Enterococci. Dunny, G. M., P. P. Cleary and L. L. McKay, (eds) ASM, Washington , D. C. p-71-76 (1991); and (Ueno, T. , et al.. Nucleic Acids Res. 10:2309-2313 (1993))- The plasmid PSRQ700 is probably one reason for the strong phage resistance shown by DCH-4 over the years. Any phage containing the non-methylated GATC sequence in its genome will be restricted when infecting a L. lactis strain containing PSRQ700.

Members of the three most common lactococcal phage species were strongly restricted by PSRQ700 as shown by their reduced EOPs (Table 3)- The small isometric-headed phages of the P335 and 936 species were particularly affected by PSRQ700. This is due in part to their larger genomes. The average genome size for the P335. 936 and c2 phages used in this study was 31.8, 29.7 and 20.2-kb, respectively. However, the most important factor was the number of Llαll sites in the phage genome.

Three Llαll sites in the prolate Φc2 genome were enough to restrict i EOP by 4 logs on L. lactis SMQ-17 (Table 3)- Two Llαll sites in the øm and Φebl genomes were still enough to reduce the EOP by 3 logs. The data are in agreement with the single hit kinetic of R/M system and sho that restriction at one site is enough to prevent phage proliferati (Wilson, G. G., et al., Annu. Rev. Genet. 2 : 85-627 (1991)). For t small isometric phages which had more Llαll sites in their genome, t presence of 9 to 12 sites gave a 6 log reduction in EOP, whereas 13 to sites were needed for a 7 log reduction. As reported previously, the E decreases logarithmically as the number of sites in the phage geno increases (Wilson, G. G., et al., Annu. Rev. Genet. 25:585-627 (1991)).

Thus, phage resistance conferred by PSRQ700 was substanti against members of the 3-lactococcal phage species tested.

Close gene linkage is a feature of all R/M system a accordingly Llαll genes are adjacent (Wilson, G. G. , Nucleic Acids Re 19:2539-2566 (1991); and Wilson, G. G. , et al. Annu. Rev. Genet. 25:58 627 (1991)). The Llαll system is highly related to DpnII (Lacks, S. A. et al., In:Genetics and Molecular Biology of Streptococci, Lactococci a Enterococci. Dunny, G. M. , P. P. Cleary and L. L. McKay, (eds) AS Washington, D. C. p. 71-76 (1991))- They share the same genet structure: two methylases followed by an endonuclease (de la Campa, G., et al.. J. Mol. Biol. 196:457-469 U987)). There is also ge overlapping in both systems. The most striking similarity is the methylases (Cerritelli, S., et al., Proc. Natl. Acad. Sci. USA. 86:922 9227); and Mannarelli, B. M., et al., Proc. Natl. Acad. Sci. 82:4468-44 (1985)). Amino acids comparison showed 752 identity between M.LlαllA a M.DpnII and 882 between M. lαllB and M.DpnA (Figure 5).

Despite the strong homology between Llαll and DpnII methylase the endonucleases are still divergent. Only 312 of the amino acids a identical. In fact, the endonuclease of Llαll is more homologous to Mb than to DpnII. One might suggest that the methylase had a comm ancestor whereas endonucleases evolved independently (Bickle, T. A., al., Microbiol. Rev. 57:434-450 (1993); (Wilson, G. G.. Nucleic Aci Res. 19:2539-2566 (1991); and Wilson, G. G., et al. Annu. Rev. Gene 2 :585-627 (1991)). Many type II R/M system appear to have form partnerships with miscellaneous genes that were initially separate They became linked due to a persistent selective advantage (Bickle, A., et al., Microbiol. Rev. 7 3 -450 (1993); (Wilson, G. G., Nucle Acids Res. 19:2539-2566 (1991); and Wilson, G. G., et al. Annu. Re

Genet. 25: 85-627 (1991)).

Finally, from a culture manufacturer standpoint, the introduction of the natural low copy number PSRQ700 into industrial Lactococcus lactis strains can confer strong phage resistance against phages of the 936 species and the newly emerging P335 species. Its effectiveness against c2 species will be variable. The temperature insensitivity nature of Llαll (up to 3δ°C) makes this phage resistance mechanism amenable to various types of high-temperature dairy fermentations, especially cheese. The use of PSRQ700 as part of a starter rotation scheme (to avoid the build up of modified phages) can improve the overall phage resistance of starter cultures.

EXAMPLE 2 The native Llαll R/M system from Lactococcus lactis was expressed by and conferred strong phage resistance to various industrial S. thermophi lus strains. Resistance was observed against phages isolated from yogurt and Mozzarella wheys. Bacteria, bacteriophages, and media. The strains used in this study are listed in Table 4. S. thermophi lus strains were confirmed by Rapid ID 32 Strep (BioMerieux Vitek, Inc., Hazelwood, MO). Streptococcus thermophi lus strains were grown at 42°C in GM17. When needed, antibiotics were added at 5 Pg of chloramphenicol per ml. Bacteriophages used in this study are listed in Table 4.

Streptococcal phages were propagated by the method of Jarvis et al (Jarvis, A.W., et al., Intervirology 32:2-9 (1991)). EOP assays on S. thermophi lus hosts were performed as follows: strains were grown in GM17 overnight at 37°C, 500μl of cells and lOOμl of diluted phages were mixed with 2.5 ml of soft agar (GM17 supplemented with lOmM CaCl ₂ ) and layered onto bottom agar (GM17 + CaCl ₂ ) . Plates were incubated overnight 42«C in an anaerobic jar (BBL GasPaK Plus, Beckton Dickinson, Cockeysville, MD) . DNA isolation and manipulation. Plasmid DNA from S. thermophi lus was isolated by using the method of O'Sullivan and Klaenhammer (O'Sullivan, D. J., et al., Appl. Environ. Microbiol. 59:2730-2733 (1993)). Phage DNA was isolated as described previously (Moineau, S., et al., Appl. Environ. Microbiol. 6θ:l832-l84l (1994)).

Electroporation. S. thermophi lus cells were electroporated as follows: cells were grown in GM17 until mid-exponential phase, centrifuged, washed twice with SG buffer (0.5M sucrose and 10# glycerol) and put on ice until use. Plasmid DNA (lμg) was mixed with 40 μl of cells in a chilled Gene Pulser cuvette (0.2 cm). The Gene Pulser apparatus (Bio-Rad Laboratories, Richmond, CA) was set at 25 μF and 2.45 kV, and the Pulse Controller at 200 Q. After electroporation, the S. thermophi lus cells were immediately resuspended in the rescue broth used for L. lactis cells (Hill, C, FEMS Microbiol. Rev. 12:87-108 (1993)) and incubated for 2 hours at 42°C before they were plated on GM17 supplemented with the appropriate antibiotic. Phage isolation. Phages ΦQl and ΨQ3 were recently isolated from yogurt samples whereas ΨQ5 and øQ6 were isolated also in our laboratory but from Mozzarella whey samples. Phages ΦQl and øQ3 were then propagated on S. thermophi lus SMQ-119. ΦQ5 on SMQ-173 and ΦQβ on SMQ-174. The genomic DNAs of these streptococcal phages were compared after digestion with EcoRV and Mbol (Fig. 6). All four S. thermophi lus phages had different restriction patterns (Fig. 6) and consequently they were different.

Expression of Llαll in Streptococcus thermophi lus. To verify if Llαll system could be functional in S. thermophi lus , the LZαll genes were cloned into a vector with an origin of replication functional in L. lactis and S. thermophi lus . The lactic acid bacteria shuttle vector pNZ123 (2.5 kb) (DeVos, W. M. , FEMS Microbiol. Rev. 46:281-295 (1987)) was selected. A 7.0-kb EcoRI fragment from pSRQJOO was cloned into the unique EcoRI site of pNZ123 (Fig. 7). The ligation mixture was electroporated directly into the phage sensitive strain L. lactis LM0230. Cm-resistant transformants were obtained and tested for resistance to

30 øp2. A phage-resistant transformant thus obtained was named SMQ-15 The resulting pNZ123 clone containing the 7.0 kb fragment from pSRQJ was named pSRQ707. This plasmid was electroporated into S. thermophi l SMQ-119 and a Cm ^r -transformant was named SMQ-154. This clone was test for resistance against two S. thermophilus phages (ΦQl and 0Q3) • Bo phages were severely restricted on SMQ-154 since they had EOPs of 1 (Table 5).

Table 5- Efficiency of Plaquing of S. thermophi lus phages on various hosts.

32

Plasmid pSRQ707 was also electroporated into S. thermophi l SMQ-173 and SMQ-174 which are commercially used for Mozzarella chee production. Transformants were obtained for both strains, and named SM 211 and SMQ-212, respectively. Both transformants were tested for pha resistance. Phage Q5 had an EOP of 10 ^"6 on SMQ-211 whereas 0Q6 and an E of 10 ^"5 on SMQ-212 (Table 5) • The phage resistance observed again Mozzarella phages was slightly weaker than with the yogurt phages, b still significant. These results show that the Llαll R/M system functional in various S. thermophi lus strains and can confer strong pha resistance in this lactic acid bacteria. This is the first report increased phage resistance in S. thermophi lus.

Thus, in general the present invention relates to an isolat and purified Streptococcus thermophi lus naturally lacking in at least o phage resistance and containing recombinant DNA encoding an endonuclea from a Lactococcus lactis to impart the phage resistance.

Further, it relates to a method for fermenting a dairy produc the improvement which comprises using a dairy culture of Streptococc thermophi lus lacking in at least one phage resistance for t fermentation incorporating recombinant DNA encoding an endonuclease Lactococcus lactis to impart the phage resistance.

Still further, it relates to a method of imparting pha resistance to a Streptococcus thermophi lus which is lacking in at lea one phage resistance which comprises incorporating recombinant D encoding an endonuclease of Lactococcus lactis into the Streptococc thermophi lus to impart the phage resistance.

The foregoing description is only illustrative of the prese invention and the present invention is limited only by the hereinaft appended claims.