The Major, N²-Gua Adduct of the (+)-anti-Benzo[a]pyrene Diol Epoxide Is Capable of Inducing G→A and G→C, in Addition to G→T, Mutations

Mutations induced by the (+)-anti-diol epoxide of benzo[a]pyrene [(+)-anh-B[a]PDE] were collected in the supF gene of the Escherichia coli plasmid pUB3. pUB3 was reacted with (+)-anti-B[a]-PDE and then either (1) transformed immediately into E. coli or (2) heated at 80 °C for 10 min and then cooled prior to transformation-the latter to probe mechanism [Rodriguez & Loechler (1993) Biochemistry 32, 1759], Qualitatively, heating did not affect the mutagenic pattern, except at the major base substitution hotspot in supF, G115, where principally G→T mutations were obtained prior to heating, while after heating, G→A and G→C mutations became statistically significantly more prevalent. Several studies have suggested that a heat-induced chemical transformation of a (+)-anti-B[a]PDE adduct at G115 (e.g., into an apurinic site) is not likely to explain the change in mutational pattern. The most likely model is that (+)-anti-B[a]P-N²-Gua is initially trapped in a metastable conformation giving principally G→T mutations, while heating induces a change to a stable conformation(s) resulting in G→T, A, and C mutations. This suggests that adduct conformational complexity is at the root of adduct mutational complexity. To investigate this model, a plasmid (B[a]P-G115-pREl) with (+)-anti-B[a]P-N²-Gua in the G115 sequence context is constructed using adduct site-specific techniques. Following transformation of B[a]P-G115- pREl into E. coli (ES87) cells, targeted G115→T (59%), A (22%), and C (19%) mutations are isolated from (+)-an?i-B[a]P-N²-Gua, which approximates the ratio obtained at G115 with (+)-anti-B[a]PDE itself. (+)-anti-B[a]P-N²-Gua principally induced G→T mutations in another sequence context [5'-TGC-3'; Mackay et al. (1992) Carcinogenesis 13, 1415]. Collectively, these findings demonstrate that (+)-anti-B[a]P-N²-Gua is able to induce all three base substitution mutations and that DNA sequence context can influence the qualitative pattern of mutations from (+)-anti-B[a]P-N²-Gua. Finally, it appears that (+)-anti-[a]P-N²-Gua at G115 may be able to induce semitargeted G116-A mutations as well, although this conclusion is more tentative.