Analysis of phage resistance mechanisms encoded by lactococcal plasmid pAJ2074

Abstract

Lactococcal plasmid pAJ2074 is a 74-kb plasmid that confers phage resistance at 30 °C against all lactococcal phages with prolate heads (referred to as prolate phage), and most small lactococcal phages with isometric heads (referred to as small isometric phage) that have been tested. The presence of pAJ2074 had no effect on phage adsorption or injection of phage DNA. Replication of prolate phage c2 DNA could not be detected in bacterial cells containing the plasmid up to 60 min after phage infection, whereas phage c2 DNA replication could be demonstrated at 20 min in the control strain. With pAJ2074 present there was no detectable growth of phage c2 and an 87% reduction in burst size for the small isometric phage sk1. Infective centres were reduced in the presence of pAJ2074 by 99% for phage c2 and by 93% for phage sk1. Plasmid pAJ2074 differed from pTR2030, in that the major effect of pAJ2074 was on prolate phage c2, rather than on the small isometric phage sk1, and no restriction and modification system could be detected. In addition, no DNA homology was detected between pAJ2074 and pTRK67 (derived from pTR2030). A recombinant plasmid pAJ88 containing an 8.4-kb insert from pAJ2074 conferred an intermediate level of phage resistance. The DNA region that encoded reduced phage sensitivity was further defined by the subcloning of a 5.6-kb EcoRV fragment that conferred resistance similar to pAJ88. The possibility of two phage-resistance mechanisms being encoded by pAJ2074 is discussed.Key words: phage resistance, lactococcal phages, lactococcal plasmids, lactococci.