Mutational specificity of the (+)-anti-diol epoxide of benzo[a]pyrene in a supF gene of an Escherichia coli plasmid: DNA sequence context influences hotspots, mutagenic specificity and the extent of SOS enhancement of mutagenesis.

Abstract

Mutagenesis by the suspected major mutagenic metabolite of activated benzo[a]pyrene, which is (+)-anti-BPDE, was analyzed with a new system, which permits the selection of supF- mutations in an Escherichia coli plasmid using lactose minimal plates. (+)-anti-BPDE enhances base pairing mutations--principally at G:C base pairs, frameshift mutations and large deletions. Frameshift mutagenesis principally involves deletions and insertions of a single G:C base pair in runs of G:C base pairs. Base pairing mutations are significantly enhanced by SOS induction, especially GC-->TA mutations. Nearest neighbor analysis was performed assuming that a guanine (underlined) is being mutated, and (+)-anti-BPDE base pairing mutagenesis is enhanced by SOS induction in 5'-(A/T)G-3' sequences approximately 4-fold more than in 5'-(G/C)G-3' sequences, and in 5'-G(C/G)-3' sequences approximately 4-fold more than in 5'-G(A/T)-3' sequences; this is discussed. The influence of sequence context on quantitative aspects of (+)-anti-BPDE mutagenesis is considered, and hotspots are found at most, but not all, 5'-GG-3' sequences. The influence of sequence context on qualitative aspects of (+)-anti-BPDE mutagenesis (i.e. mutagenic specificity) is also considered. For example, the sequences, 5'-AG-3', 5'-CG-3' and 5'-GG-3', all have examples of G-->T, G-->A and G-->C mutations, while in the sequence, 5'-TG-3', only G-->T mutations have been detected. (The latter finding correlates with a recent site-specific study on the major adduct of (+)-anti-BPDE formed at N2-Gua in a 5'-TG-3' context, where G-->T mutations predominated [Carcinogenesis (1992) 13, 1415-1425].) These results suggest that sequence context plays a role in defining the kind of mutation (i.e. GC-->TA versus GC-->AT versus GC-->CG) induced by (+)-anti-BPDE, where the base on the 5' side of the guanine undergoing mutation seems to be influential. The most likely model for this is that sequence context influences adduct conformation, which controls mutagenic specificity.