The type IC hsd loci of the enterobacteria are flanked by DNA with high homology to the phage P1 genome: implications for the evolution and spread of DNA restriction systems.

EcoR124l, EcoDXXl and Ecoprrl are the known members of the type IC family of DNA restriction and modification systems. The first three are carried on large, conjugative plasmids, while Ecoprrl is chromosomally encoded. The enzymes are coded by three genes, hsdR, hsdM and hsdS. Analysis of the DNA sequences upstream and downstream of the type IC hsd loci shows that all are highly homologous to each other and also to sequences present in the bacteriophage P1 genome. The upstream sequences include functional phd and doc genes, which encode an addiction system that stabilizes the P1 prophage state, and extend to and beyond pac, the site at which phage DNA packaging begins. Downstream of the hsd loci, P1 DNA sequences begin at exactly the same place for all of the systems. For EcoDXXl and Ecoprrl the P1 homology extends for thousands of base pairs while for EcoR124l and IS1 insertion and an associated deletion have removed most of the P1-homologous sequences. The significance of these results for the evolution of DNA restriction and modification systems is discussed.