Affinity modification of EcoRII DNA methyltransferase (M-EcoRII) by DNA duplexes containing oxidized 2'-O-beta-D-ribofuranosylcytidine (Crib*) or 1-(beta-D-galactopyranosyl)thymine (Tgal*) residues was performed. Cross-linking yields do not change irrespective of whether active Crib* replaces an outer or an inner (target) deoxycytidine within the EcoRII recognition site. Chemical hydrolysis of M-EcoRII in the covalent cross-linked complex with the Tgal*-substituted DNA indicates the region Gly(268)-Met(391) of the methylase that is likely to interact with the DNA sugar-phosphate backbone. Both specific and non-specific DNA interact with the same M-EcoRII region. Our results support the theoretically predicted DNA binding region of M-EcoRII.