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

EcoR1241, EcoDXXI and EcoprrI are the known members of the type IC fam ily of DNA restriction and modification systems. The first three are c arried on large, conjugative plasmids, while EcoprrI is chromosomally encoded. The enzymes are coded by three genes, hsdR, hsdM and hsdS. An alysis 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 s equences present in the bacteriophage P1 genome. The upstream sequence s include functional phd and doc genes, which encode an addiction syst em 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 EcoDXXI and EcoprrI the P1 homology extends for thousands of base pairs while for EcoR1241 an ISI insertion and an associated d eletion have removed most of the P1-homologous sequences. The signific ance of these results for the evolution of DNA restriction and modific ation systems is discussed.