MUTATIONAL AND DNA-BINDING SPECIFICITY OF THE CARCINOGEN 2-AMINO-3,8-DIMETHYLIMIDAZO[4,5-F]QUINOXALINE

The mutagenic specificity of 2-amino3,8-dimethylimidazo[4,5-f]quinoxal ine (MeIQx), a food-borne mutagen and carcinogen, was studied, Plasmid pK19 was modified by photolysis with the 2-azido form of the carcinog en, High pressure liquid chromatography confirmed that the photoactiva ted azide formed primarily C8 and N-2 guanyl adducts. Transformation o f modified pK19 into excision repair competent Escherichia coli result ed in dose-dependent increases in genotoxicity and in mu tagenesis wit hin the lacZ alpha target sequence. Upon induction of the SOS response , a 20-fold increase in mutation frequency over background was observe d, A mutational spectrum for MeIQx, generated by sequencing 125 indepe ndent mutants, revealed base substitutions (41%), frameshifts (54%), a nd complex mutations (5.6%); >90% of the mutations occurred at G-C bas e pairs, Two hotspots were evident at runs of three or five G-C base p airs; similar to 60% of the mutations occurred at the hotspot sites, T he hotspot at position 2532 produced mainly base substitutions, while that at position 2576 gave exclusively frameshift mutations, A polymer ase inhibition assay mapped the sites of MeIQx adducts, Arrest sites w ere primarily at or one base 3' to a guanine residue, which correlated well with the distribution of mutations. No direct correlation was se en, however, between intensity of modification and hotspots for mutati on.