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. Prefers double-stranded DNA over single-stranded DNA as substrate. 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] In contrast to some bacterial and mammalian enzymes, MGT1 is not induced by alkylating agents. Present with 150 molecules/cell in log phase YPD medium, but not detectable in stationary phase cells. 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. Belongs to the MGMT family.