question: What is the role of the Ada O6-alkylguanine alkyltransferase in bacteria?
answer: The Ada O6-methylguanine-DNA methyltransferase is a multifunctional protein, product of the ada gene. Ada functions in DNA repair by direct dealkylation of alkylated DNA lesions, such as the toxic, mutagenic and carcinogenic O6-alkylguanine (O6-AlkG) and O4-alkylthymine (O4-AlkT) which are restored to guanine and thymine. Ada accepts stoichiometrically the alkyl group from O6-alkylguanine in DNA at the Cys-321 residue and from alkyl phosphotriester at the Cys-69 residue. When methylated at Cys-69, Ada becomes a transcriptional activator of the genes in the ada regulon, including its own. The ada gene controls the inducible resistance to alkylation mutagenesis and killing (the adaptive response). Ada alkyltransferase (ATase) is induced by exposure to low doses of methylating agents. During exponential growth, Ada removes lesions responsible for G:C to A:T transitions and G:C to C:G transversions, while in stationary populations it removes lesions causing G:C to A:T and A:T to G:C transitions, and G:C to C:G, A:T to C:G, and A:T to T:A transversions. Thus, Ada protein acts both as a positive regulator of the ada response and as a DNA repair enzyme.