EVOLUTION OF A LYTIC BACTERIOPHAGE VIA DNA ACQUISITION FROM THE LACTOCOCCUS-LACTIS CHROMOSOME

We discovered a phage-host interaction in which the lytic phage u136, in response to pressure exerted by an abortive phage resistance mechan ism, acquired a large DNA fragment from the chromosome of Lactococcus lactis NCK203 to form a new phage, u137. Phage u137 was characterized at morphological, phenotypic, and genotypic levels and was found to be a member of the P335 species. Although it exhibits a high level of DN A homology with u136, phage u137 is resistant to the abortive mechanis m and has a longer tail, a different base plate, and apparently a diff erent origin of replication. The chromosomal DNA implicated in the for mation of new phage u137 was disrupted by site-specific integration in NCK203. This strategy prevented the appearance of u137 during subsequ ent infections with u136.