Involved in the cellular defense against the biological effects of 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. This is a suicide reaction: the enzyme is irreversibly inactivated. The methylated ADA protein acts as a positive regulator of its own synthesis, as well as that of other proteins. The transcription-activating function of the ADA protein resides in its N-terminus. It activates the transcription of alkA, alkB and aidB. 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. 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.