Molecular characterization of a phage-encoded resistance system in Lactococcus lactis

A specific fragment of the genome of Tuc2009, a temperate lactococcal bacteriophage, was shown to contain several open reading frames, whose deduced protein products exhibited similarities to proteins known to be involved in DNA replication and modification. In this way, a putative single- stranded binding protein, replisome organizer protein, topoisomerase I, and a methylase were identified. When the genetic information coding for the putative replisome organizer protein of Tuc2009, Rep₂₀₀₉, was supplied on a high-copy-number plasmid vector, it was shown to confer a phage-encoded resistance (Per) phenotype on its lactococcal host UC509.9. The presence of this recombinant plasmid was shown to cause a marked reduction in Tuc2009 DNA replication, suggesting that the observed phage resistance was due to titration of a factor, or factors, required for Tuc2009 DNA replication. Further experiments delineated the phage resistance-conferring region to a 160-bp fragment rich in direct repeats. Gel retardation experiments, which indicated a protein-DNA interaction between this 160-bp fragment and the Rep₂₀₀₉ protein, were performed. UC509.9 strains harboring plasmids with randomly mutated versions of this fragment were shown to display a variable phage resistance phenotype, depending on the position of the mutations.