O(6)-Alkylguanine-DNA alkyltransferase (AGT) is a DNA repair protein that removes O(6)-alkylguanine adducts. The interaction of dibromomethane (CH(2)Br(2)) and bromomethyl acetate (BrCH(2)OAc) with AGT was studied in vitro, and the effect of AGT on their toxicity and mutagenicity was investigated using Escherichia coli strain TRG8 (lacking endogenous AGT) that expressed human AGT or its inactive C145A mutant. Both CH(2)Br(2) and BrCH(2)OAc reacted with AGT at its cysteine acceptor site, abolishing its DNA repair activity with the latter agent being much more potent. The formation of AGT-Cys(145)S-CH(2)OAc by BrCH(2)OAc was confirmed by mass spectral analysis, but the presumed AGT-Cys(145)S-CH(2)Br adduct from CH(2)Br(2) was too unstable for such characterization. In the presence of CH(2)Br(2), AGT was covalently cross-linked to an oligodeoxyribonucleotide, 5'-d(AG)(8)-3', but no cross-link was formed by BrCH(2)OAc. Survival of cells exposed to CH(2)Br(2) was reduced, and the number of mutants was greatly increased when wild-type AGT was present. The cytotoxicity of CH(2)Br(2) was similar to that of BrCH(2)CH(2)Br(2), but the mutagenicity was about four times less. Virtually all of the AGT-mediated mutants induced by CH(2)Br(2) in the rpoB gene were at G:C sites with equal numbers of transitions to A:T and transversions to T:A. In contrast, BrCH(2)OAc was more than 10-fold less genotoxic than CH(2)Br(2) and the survival of cells exposed to BrCH(2)OAc was not affected by AGT. The number of mutations (almost all G:C to A:T transitions) induced by BrCH(2)OAc was slightly reduced by the presence of wild-type AGT and substantially increased by the inactive C145A mutant. These results with CH(2)Br(2) are consistent with a mechanism in which reaction at the active site Cys145 residue followed by attack of AGT-Cys(145)S-CH(2)Br at guanine in DNA forms a covalent adduct, which leads to cytotoxicity and to mutagenicity. The results with BrCH(2)OAc suggest that it reacts directly with DNA to form O(6)-(CH(2)OAc)guanine, which, if unrepaired, causes G:C to A:T transitions. Our experiments reveal two novel pathways (direct inactivation of AGT and formation of AGT-Cys(145)S-CH(2)-DNA adducts) by which CH(2)Br(2) may cause damage to the genome in addition to the well-recognized pathway involving activation by GSTs.