Repair of O6-alkylguanine lesions in DNA by chromatin enzymes.

Disappearance of 7-ethylguanine and O6-ethylguanine lesions from nuclear DNA is observed on incubation of isolated rat liver nuclei previously treated with ethylnitrosourea. Free 7-ethylguanine but no free O6-ethylguanine is released in the medium. Incubation of methylated or ethylated DNA with chromatin proteins leads to the disappearance of O6-methylguanine or O6-ethylguanine from DNA; the methyl group or the ethyl group is transferred to proteins where it is accepted by cystein residues. Repair of O6-methylguanine and repair of O6-ethylguanine lesions in DNA consume a common reactant which is likely the acceptor protein. The repair does not seem however to be a simple bimolecular reaction between the O6-alkylguanine and the cystein of the acceptor protein.