The Escherichia coli Ada protein repairs O6-methylguanine residues and methyl phosphotriesters in DNA by direct transfer of the methyl group to a cysteine residue. This domain contains four conserved cysteines that form a zinc binding site [1,2]. One of these cysteines is a methyl group acceptor. The methylated domain can then specifically bind to the ada box on a DNA duplex [2].

1: Zinc binding by the methylation signaling domain of the Escherichia coli Ada protein. Myers LC, Terranova MP, Nash HM, Markus MA, Verdine GL; Biochemistry 1992;31:4541-4547. PMID:1581309

2: Folding topology and DNA binding of the N-terminal fragment of Ada protein. Sakashita H, Sakuma T, Ohkubo T, Kainosho M, Sakumi K, Sekiguchi M, Morikawa K; FEBS Lett 1993;323:252-256. PMID:8500619