Position of a single acetylaminofluorene adduct within a mutational hot spot is critical for the related mutagenic event.

2-Acetylaminofluorene, a potent rat liver carcinogen, which binds primarily to C8 of guanines, has been shown to induce mainly frameshift mutations in the bacteria Escherichia coli. Mutations occur at specific sequences, known as mutation hot spots, of which two types may be considered. First, repetitive sequences, where deletions of a single unit occur (GGGGG----GGGG). Second, the so-called NarI site, 5'GGCGCC3', where only -2-bp deletions are observed (G1G2CG3CC----GGCC). Mutagenesis within repetitive sequences is dependent on the UmuCD+ gene functions, whereas mutagenesis in the NarI site is not. These differences in the genetic requirements of mutagenesis at these hot spots suggest that two different pathways operate. In order to precisely determine the actual involvement of each of the three premutagenic lesions that may form in the NarI site in the course of the mutational process, we designed a single adduct mutagenesis experiment, and found that AAF binding to the G3 induced only a -2 frameshift mutation event. This result will be discussed in terms of local DNA conformation.