Specific binding of SsoII DNA methyltransferase to its promoter region provides the regulation of SsoII restriction-modification gene expression

The regulation of the Ssoll restriction-modification system from Shigella sonnei was studied in vivo and in vitro. In lacZ fusion experiments, Ssoll methyltransferase (M.Ssoll) was found to repress its own synthesis but stimulate expression of the cognate restriction endonuclease (ENase). The N-terminal 72 amino acids of M.Ssoll, predicted to form a helix-turn-helix (HTH) motif, was found to be responsible for the specific DNA-binding and regulatory function of M.Ssoll. Similar HTH motifs are predicted in the N-terminus of a number of 5-methylcytosine methyltransferases, particularly M.EcoRll, M.dcm and M.Mspl, of which the ability to regulate autogenously has been proposed. In vitro, the binding of M.Ssoll to its target DNA was investigated using a mobility shift assay. M.Ssoll forms a specific and stable complex with a 140 bp DNA fragment containing the promoter region of Ssoll R-M system. The dissociation constant (K-d) was determined to be 1.5 x 10(-8) M. DNasel footprinting experiments demonstrated that M.Ssoll protects a 48-52 bp region immediately upstream of the M.Ssoll coding sequence which includes the predicted -10 promoter sequence of M.Ssoll and the -10 and -35 sequences of R.Ssoll.