Abortive phage infection and restriction/modification activities directed by pTR2030 determinants are enhanced by recombination with conjugal elements in lactococci

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Abstract
The recombinant plasmid pTK6 is composed of a 13.6 kb fragment from pTR2030 encoding phage resistance determinants for restriction/modification (R+/M+) and abortive phage infection (Hsp+) cloned into shuttle vector pSA3 (erythromycin resistance, EMr). Conjugal matings were performed to mobilize pTK6-encoded markers from Lactococcus lactis subsp. lactis MMS362 and MG1363. EMr transconjugants were recovered at 10-6 per input donor and harboured pTK6 or recombinant plasmids not found in either parental strain. The recombinant plasmids (pTRK78 and pTRK79) encoded EMr, Hsp+ and R+/M+, and transferred at high frequency in second-round matings. Mobilization of pTK6 from the otherwise plasmid-free donor, L. lactis MG1363, confirmed the presence of a conjugal element in this strain. Phage resistance in transconjugants containing pTRK78 and pTRK79 was markedly enhanced over pTK6-directed Hsp+ and R+/M+. In L. lactis LM2345 transconjugants, a reduction in plaque size was accompanied by a significant decrease in the efficiency of plaquing for phages c2 (10-2 to 10-6) and p2 (<10-9). L. lactis NCK203 transconjugants containing pTRK78 or pTRK79 exhibited an additional 100-1000-fold reduction in the plaquing efficiency of .vphi.48 (10-4 to 10-5) over pTK6 imposed restriction (10-2). Increased resistance to phage was a consequence of the physical interaction of pTR2030-derived sequences on pTK6 with a conjugal element resident in the donor strains.

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