A CO1E1-TYPE PLASMID FROM SALMONELLA-ENTERITIDIS ENCODES A DNA CYTOSINE METHYLTRANSFERASE

The multicopy plasmid pFM366 was isolated from a virulent Salmonella e nteritidis strain and was found to code for DNA methylase activity (Ib anez and Rotger, 1993). The present work was aimed at characterizing t he genetic organization and functional features of this 5.6 kb plasmid . We found pFM366 almost identical to the plasmid P4 isolated from Shi gella sonnei, that encodes the SsoII restriction-modification system ( Karyagina et al., 1993), and related to other ColE1-type plasmids. Exa mination of these plasmids revealed a common organization which sugges ts they were the result of similar recombinational events. The cytosin e methylase of pFM366 is nearly identical to M . SsoII, whereas the ge ne encoding the restrictase homologous to R . SsoII is truncated and i ts product is inactive. The expression of the cytosine methylase encod ed by pFM366 is strongly affected by deletion of regions located upstr eam and downstream of its ORF, and is negatively controlled by the rpo S gene in Escherichia coli. The methylase activity encoded by pFM366 i nduces the SOS response, which could be responsible for the observed d elay in the growth of E. coli. (C) 1997 Elsevier Science B.V.