ME2 exhibits at least three mechanisms which confer resistance to virulent bacteriophage. These include plasmid-induced interference with phage adsorption, host-controlled restriction and modification activities, and a heat-sensitive mechanism which suppresses development of virulent phage. Conjugal mating experiments were done with ME2 to determine if phage-defence mechanisms present in this strain could be mobilized, associated with plasmid DNA elements and phenotypically characterized in transconjugants. Agar-surface matings of ME2 with LM0230 demonstrated that lactose-fermenting ability (Lac) was transferred in a conjugation-like process at frequencies of 10 per donor cell and was associated with a 40 MDal plasmid designated pTR1040. Resistance to nisin (Nis) was acquired or lost simultaneously with Lac, indicating that pTR1040 carried determinants for both phenotypes. Lac Nis transconjugants that carried a 30 MDal plasmid (pTR2030) exhibited a heat-sensitive phage-defence mechanism (Hsp) which limited the burst size and plaque size of phage c2 without altering the efficiency of plaquing (e.o.p.) or the level of adsorption. The ability of phage c2 to initiate plaquing at an e.o.p. of 1.0 indicated that DNA injection and early viral gene expression are not affected in the Hsp transconjugants. We suggest, therefore, that the Hsp phenotype may result from plasmid-induced abortive infection of phage dependent on the presence of pTR2030. Hsp transconjugants carrying pTR2030 also promoted high-frequency conjugal transfer of Lac Nis associated with pTR1040 (> 10 per donor cell). It was concluded that Hsp and determinants for conjugal transfer ability (Tra) are located on pTR2030.


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