THE LACTOCOCCAL PLASMID PNP40 ENCODES A 3RD BACTERIOPHAGE RESISTANCE MECHANISM, ONE WHICH AFFECTS PHAGE DNA PENETRATION

The lactococcal plasmid pNP40 mediates insensitivity to phi c2 by an e arly-acting phage resistance mechanism in addition to the previously i dentified abortive infection system, AbiF, in the Lactococcus lactis s ubsp, lactis MG1614 background. A second abortive infection determinan t on pNP40, AbiE, does not confer resistance to phi c2. The early-acti ng mechanism on pNP40 does not prevent phage adsorption nor does it ap pear to operate by restriction/modification. Phage DNA was not detecte d in pNP40-containing cells until 30 min following exposure to phi c2 compared with 5 min in a sensitive host; however, electroporation of p hage DNA into resistant hosts resulted in the release of phage progeny from a dramatically elevated number of cells compared with convention ally infected hosts. It appears therefore that pNP40 encodes a novel p hage resistance mechanism which blocks DNA penetration specifically fo r phi c2.