Are base substitution and frameshift mutagenesis pathways interrelated? An analysis based upon studies of the frequencies and specificities of mutations induced by the (+)-anti diol epoxide of benzo[a]pyrene.

(+)-anti-B[a]PDE-induced mutagenesis is being investigated, including in a supF gene of the E. coli plasmid pUB3. Based upon various findings a working hypothesis was proposed that the major adduct of (+)-anti-B[a]PDE (formed at N2-Gua) is able to induce different base substitution mutations (e.g., GC-->TA vs. GC-->AT vs. GC-->CG) depending upon its conformation in DNA, which can be influenced by various factors, such as DNA sequence context. Frameshift mutations are also significant and are analyzed herein. In virtually all cases one of three possibilities is observed: (1) some treatments change frameshift and base substitution mutation frequency (MF) in a quantitatively parallel fashion; (2) other treatments, which change frameshift MF, can change base substitution MF in a quantitatively reciprocal fashion; finally, (3) there are treatments that do not change frameshift MF, and also do not change base substitution MF. (Changes can be brought about by SOS induction, differing DNA sequence context, or heating adducted pUB3 prior to transformation. Why different kinds of changes result in (1) vs. (2) vs. (3) is discussed.) Thus, base substitution and frameshift mutagenesis pathways appear to be coupled in some way, which is most easily rationalized if both pathways are interrelated. The simplest mechanism to rationalize this coupling is that a single (+)-anti-B[a]PDE adduct in a single conformation can be bypassed via either a frameshift or a base substitution pathway. The surprising implication is that--although different conformations are likely to be required to induce different base substitution mutations (e.g., GC-->TA vs. GC-->AT; see above)--a single conformation can give rise to either a base substitution or a frameshift mutation. Frameshift and base substitution pathways must eventually diverge, and it is proposed that this is controlled by factors such as DNA sequence context.