Plasmid pC present in Salmonella enterica serovar Enteritidis PT14b strains encodes a restriction modification system

Salmonella enterica serovar Enteritidis (S. Enteritidis) possesses plasmids of different sizes and roles. Besides the serovar-specific virulence plasmid present in most field strains, S. Enteritidis can harbour plasmids of low molecular mass whose biological role is poorly understood. We therefore sequenced plasmid pC present in S. Enteritidis strains belonging to phage type PT14b. The size of plasmid was determined to be 5,269 bp and it was predicted to encode four open reading frames (ORFs). The first two ORFs were found (initial 3,230 bp) to be highly homologous to rom and mbeA genes of ColE1 plasmid of Escherichia coli. Proteins encoded by the other two ORFs were 99% homologous to a restriction methylase and restriction endonuclease encoded by plasmid pECO29 of a field strain of E. coli. Using insertional mutagenesis we confirmed experimentally that the plasmid pC-encoded restriction modification system was functional and could explain the high resistance of S. Enteritidis PT14b strains to phage infection.