Characterization of two mobilizable plasmids isolated from enterobacter cloacae

We isolated two plasmids, pS51A and pS51B which were 5782 bp and 4854 bp in size, respectively, from the third generation cephalosporin-resistant E. cloacae suspected to express metallo-beta-lactamase, and analyzed their structures. These two plasmids encode RNA I/RNA II genes for replication origin, relaxase genes of mobABCD for plasmid transfer, and several open reading frames. According to the classification of mobilizable plasmids by gene organization of the relaxases, pS51A and pS51B belong to the ColE1 superfamily of mobilizable plasmids, commonly detected in Enterobacteriaceae. The metallo-beta-lactamase gene was not identified in either pS51A or pS51B by homology search of the putative open reading frames. Open reading frames encoded in pS51A include E. coli protein L-like, E. coli heat shock protein-like, and E. coli plasmid replication initiation protein-like, and those encoded in pS51B include helix-turn-helix protein-like, E. coli plasmid replication initiation protein-like, and Salmonella replication initiation protein-like. These plasmids are stably maintained in one strain of E. cloacae, thus, the encoded gene functions may confer growth advantage to the host cell.