Sites of contact between λ operators and λ repressor

DNA bearing λ operator sequences was methylated by dimethyl sulfate (DMS) in the presence or absence of λ repressor. Under the experimental conditions, DMS methylates only the purine residues. The presence of λ repressor affects only the methyl-ation of certain G residues in the operators. Repressor blocks the methylation of certain G's and enhances the methylation of other G's. Since the reactive ring-nitrogen of G lies in the major groove of double-stranded DNA, and the reactive ring-nitrogen of A lies in the minor groove, the above results imply that the repressor makes contacts in the major groove of the helix. The repressor effect on G-methylation is sharply confined to the three 17 base pair units within each λ operator previously proposed as the repressor-binding sites.