Mobility of a restriction-modification system revealed by its genetic contexts in three hosts

The flow of genes among prokaryotes plays a fundamental role in shaping bacterial evolution, and restriction-modification systems can modulate this flow. However, relatively little is known about the distribution and movement of restriction-modification systems themselves. We have isolated and characterized the genes for restriction-modification systems from two species of Salmonella, S. enterica serovar Paratyphi A and S. enterica serovar Bareilly. Both systems are closely related to the PvuII restriction-modification system and share its target specificity. In the case of S. enterica serovar Paratyphi A, the restriction endonuclease is inactive, apparently due to a mutation in the subunit interface region. Unlike the chromosomally located Salmonella systems, the PvuII system is plasmid borne. We have completed the sequence characterization of the PvuII plasmid pPvu1, originally from Proteus vulgaris, making this the first completely sequenced plasmid from the genus Proteus. Despite the pronounced similarity of the three restriction-modification systems, the flanking sequences in Proteus and Salmonella are completely different. The SptAI and SbaI genes lie between an equivalent pair of bacteriophage P4-related open reading frames, one of which is a putative integrase gene, while the PvuII genes are adjacent to a mob operon and a XerCD recombination (cer) site.