<p>Synonym(s): 6-O-methylguanine-DNA methyltransferase, O-6-methylguanine-DNA-alkyltransferase, Methylated-DNA-[protein]-cysteine S-methyltransferase.</p><p>This entry represents the active site of 6-O-methylguanine-DNA methyltransferases.</p> <p>The repair of DNA containing O6-alkylated guanine is carried out by DNA-[protein]-cysteine S-methyltransferase (<db_xref db="EC" dbkey="2.1.1.63"/>). The major mutagenic and carcinogenic effect of methylating agents in DNA is the formation of O6-alkylguanine. The alkyl group at the O-6 position is transferred to a cysteine residue in the enzyme [<cite idref="PUB00000053"/>]. This is a suicide reaction since the enzyme is irreversibly inactivated and the methylated protein accumulates as a dead-end product. Most, but not all of the methyltransferases are also able to repair O-4-methylthymine. DNA-[protein]-cysteine S-methyltransferases are widely distributed and are found in various prokaryotic and eukaryotic sources [<cite idref="PUB00004404"/>].</p>