Enzymatic repair of O-alkylated thymidine residues in DNA: involvement of a O4-methylthymine-DNA methyltransferase and a O2-methylthymine DNA glycosylase.

Alkylation of poly(dT) by N-[methyl-3H] (N-nitrosomethylurea) and subsequent annealing with poly(dA) yield a substrate containing O2 and O4-methylthymidine, 3-methylthymidine and phosphotriesters. In an in vitro assay using this substrate, cell extracts from Escherichia coli catalyse i) the transfer of the O4-methyl present in O4 methylthymidine to a protein which becomes alkylated; ii) the release of O2-methylthymine by a glycosylase activity. The two DNA repair activities described above appear to be involved in the adaptive response.