Most B,K, and w strains of E.coli also contain two site specific DNA methylases, Which are not part of the RM system and are encoded by the dum and dcm genes. The dam methylase transfers a methyl group from SAM to the N6 position of the adenine residue in the sequence GATC, while the dcm methylase transfers a methyl group from SAM to the internal cytosine residues in the sequences CCAGG or CCTGG
Dam methylation regulates post-repilcation mismatch repairs. All DNA isolated from E.coli is not methylated to the same extent. For instance, the pBR322 plasmid DNA purified from E.coli after amplification with chloramphenicol is resistant to the digestion by Mbo I,Which does not cut at methylated sites Furthermore only 50% of the ʎ