Single adduct mutagenesis: strong effect of the position of a single acetylaminofluorene adduct within a mutation hot spot.

2-Acetylaminofluorene (AAF), a potent rat liver carcinogen that binds covalently to the C-8 position of guanine residues in DNA, is an effective frameshift mutagen. The mutations are distributed nonrandomly, in that most are located at a few specific DNA sequences (i.e., mutation hot spots). Among these hot spots, the Nar I sequence (GGCGCC) is especially susceptible to the induction of -2 frameshift mutations (GGCGCC----GGCC). Due to the nature of the Nar I sequence, G1G2CG3CC, three different molecular events, each involving the deletion of two contiguous base pairs (i.e., G2C, CG3, G3C), can give rise to the observed end point (GGCC). To compare the potential role of each of the three possible guanine-AAF adducts in the Nar I site to induce the -2 frameshift mutation, we constructed double-stranded plasmid molecules containing a single-AAF adduct bound to one of the three guanine positions. Using these plasmids, we found that only the adduct in the G3 position induces the -2 frameshift mutation. This strong effect of the position of the -AAF adduct within the Nar I site is discussed in relation to the possible involvement of an unusual DNA conformation in the mutagenic processing.