The major, N²-dG adduct of (+)-anti-B[a]PDE induces G→A mutations in a 5'-AGA-3' sequence context

Previously, in a random mutagenesis study, the (+)-anti diol epoxide of benzo[a]pyrene [(+)-anti-B[a]PDE] was shown to induce a complex mutational spectrum in the supF gene of an Escherichia coli plasmid, which included insertions, deletions and base substitution mutations, notably a significant fraction of GC→TA, GC→AT and GC→CG mutations. At some sites, a single type of mutation dominated and to understand individual mutagenic pathways these sites were chosen for study by site-specific means to determine whether the major adduct, [+ta]-B[a]P-N²-dG, was responsible, [+ta]-B[a]P-N²-dG was shown to induce ~ 95% G→T mutations in a 5'-TGC-3' sequence context and ~ 80% G→A mutations in a 5'-CGT-3' sequence context. (+)-anti-B[a]PDE induced principally GC→CG mutations in the G133 sequence context (5'-AGA-3') in studies using both SOS-uninduced or SOS-induced E. coli. Herein, [+ta]-B[a]P-N²-dG is shown to induce principally G→A mutations (> 90%) either without or with SOS induction in a closely related 5'-AGA-3' sequence context (identical over 7 bp). This is the first time that there has been a discrepancy between the mutagenic specificity of (+)-anti-B[a]PDE versus [+ta]-B[a]P-N²-dG. Eight explanations for this discordance are considered. Four are ruled out; e.g. the second most prevalent adduct [+ca]-B[a]P-N²-dG also induces a preponderance of G→A mutations (> 90%), so it also is not responsible for (+)anti-B[a]PDE-induced G133→C mutations. The four explanations not ruled out are discussed and include that another minor adduct might be responsible and that the 5'-AGA-3' sequence context differed slightly in the studies with [+ta]-B[a]P-N²-dG versus (+)-anti-B[a]PDE. In spite of the discordance, [+ta]-B[a]P-N²-dG induces G→A mutations in the context studied herein and this result has proven useful in generating a hypothesis for what conformations of [+ta]-B[a]P-N²-dG are responsible for G→T versus G→A mutations.