Involved in the adaptive response to alkylation damage in DNA caused by alkylating agents. Repairs O6-methylguanine (O6-MeG) and O4-methylthymine (O4-MeT) in DNA. Repairs the methylated nucleobase in DNA by stoichiometrically transferring the methyl group to a cysteine residue in the enzyme (Cys-321). Also specifically repairs the Sp diastereomer of DNA methylphosphotriester lesions by the same mechanism, although the methyl transfer occurs onto a different cysteine residue (Cys-38). Cannot demethylate the other diastereomer, Rp-methylphosphotriester. This is a suicide reaction: the enzyme is irreversibly inactivated. The methylation of Ada by methylphosphotriesters in DNA leads to its activation as a transcriptional regulator that activates the transcription of its own gene, ada, and other alkylation resistance genes, alkA, alkB and aidB. (2'-deoxyribonucleoside 5'-methylphosphotriester)-DNA + L-cysteinyl-[protein] = 2'-deoxyribonucleotide-DNA + H(+) + S-methyl-L-cysteinyl-[protein] a 6-O-methyl-2'-deoxyguanosine in DNA + L-cysteinyl-[protein] = a 2'-deoxyguanosine in DNA + S-methyl-L-cysteinyl-[protein] a 4-O-methyl-thymidine in DNA + L-cysteinyl-[protein] = a thymidine in DNA + S-methyl-L-cysteinyl-[protein] Binds 1 zinc ion per subunit. Up-regulated by methylated Ada itself in response to the exposure to alkylating agents. Consists of two domains. The 20 kDa N-terminal domain repairs the Sp diastereomer of methylphosphotriesters and, in its methylated form, binds DNA in a sequence-specific manner. The 19 kDa C-terminal domain repairs the mutagenic lesions O6-methylguanine. Each domain retains its activity when separated form the other. This enzyme catalyzes only one turnover and therefore is not strictly catalytic. According to one definition, an enzyme is a biocatalyst that acts repeatedly and over many reaction cycles. In the C-terminal section; belongs to the MGMT family.