The bacteriophage resistance plasmid pTR2030 forms high-molecular-weight multimers in lactococci.
Lactococcus lactis ME2 can transfer a 46-kb plasmid, pTR2030, which encodes abortive phage infection (Hsp) and restriction/modification (R/M) activities. pTR2030 can be detected as a monomeric plasmid in transconjugants at low copy number, but not in ME2. pTR2030-specific probes were cloned and used to determine the location of the element in ME2. No homology was observed between these pTR2030-specific probes and the CsCl-purified plasmid content of ME2. However, probes specific for pTR2030 hybridized strongly to a high-molecular-weight moiety, and not to chromosomal DNA, in total DNA isolated by a gentle lysis procedure. The absence of junction fragments indicates that pTR2030 forms high-molecular-weight multimers in lactococci. A phage-sensitive derivative of ME2, L. lactis N1, is cured of pTR2030 and no longer possesses the high-molecular-weight species. When pTR2030 was reintroduced to N1 via conjugation, an ME2-like phage-insensitive phenotype was restored. pTR2030 could remain as a detectable monomeric plasmid in the N1 transconjugants or could revert to the high-molecular-weight structure.
Hill
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Department of Food Science, Southeast Dairy Foods Research Center, North Carolina State University, Raleigh 27695-7625.
Miller
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LA
Klaenhammer
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TR
D001435:Bacteriophages / Q000502:physiology; D003227:Conjugation, Genetic; D004269:DNA, Bacterial / Q000235:genetics; D005798:Genes, Bacterial; D010641:Phenotype; D010957:Plasmids
D004269:DNA, Bacterial