Adaptive response to alkylating agents involves alteration in situ of O⁶-methylguanine residues in DNA

Abstract

THE mutagenic lesion O⁶-methylguanine (O⁶-MeG) is slowly but actively removed from DNA in Escherichia coli cells treated with alkylating agents¹. In bacteria exposed to sublethal concentrations of chemical mutagens such as N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) or N- methyl -N-nitrosourea (MNUA), an apparently error-free repair system is induced that confers increased resistance to the mutagenic and lethal effects of challenge doses of the same or related compounds^2–4. This adaptive response is associated with an increased ability of the induced E. coli to remove 0⁶-MeG residues from their DNA (see ref. 5 and preceding paper⁶). We have investigated the action of cell-free extracts from adapted ^{E. coli} on DNA containing O⁶-MeG residues. Here we report that O⁶-MeG disappears from alkylated DNA after incubation with a crude enzyme fraction from adapted cells, although no concomitant release of the methyl group or the alkylated base or nucleotide occurs. Instead, this process is due to a previously unrecognised DNA repair mechanism involving enzyme-catalysed structural alteration of the alkylated residue.