Summary: 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, Em). Conjugal matings were performed to mobilize pTK6-encoded markers from subsp. MMS362 and MG1363. Em transconjugants were recovered at 10 per input donor and harboured pTK6 or recombinant plasmids not found in either parental strain. The recombinant plasmids (pTRK78 and pTRK79) encoded Em, Hsp and R/M, and transferred at high frequency in second-round matings. Mobilization of pTK6 from the otherwise plasmid-free donor, 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 LM2345 transconjugants, a reduction in plaque size was accompanied by a significant decrease in the efficiency of plaquing for phages c2 (10 to 10) and p2 (< 10). NCK203 transconjugants containing pTRK78 or pTRK79 exhibited an additional 100-1000-fold reduction in the plaquing efficiency of ø48 (10 to 10) over pTK6 imposed restriction (10). 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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