Mirror image stereoisomers of the major benzo[ a]pyrene N 2-dG adduct are bypassed by different lesion-bypass DNA polymerases in E. coli

Abstract

The potent mutagen/carcinogen benzo[ a]pyrene (B[ a]P) is metabolically activated to (+) -anti-B[ a]PDE, which induces a full spectrum of mutations (e.g., G-to-T, G-to-A, −1 frameshifts, etc.) via its major adduct [+ta]-B[ a]P-N 2-dG. We recently showed that the dominant G-to-T mutation depends on DNA polymerase V (DNAP V), but not DNAPs IV or II, when studied in a 5′-T G sequence in E. coli. Herein we investigate what DNAPs are responsible for non-mutagenic bypass with [+ta]-B[ a]P-N 2-dG, along with its mirror image adduct [−ta]-B[ a]P-N 2-dG. Each adduct is built into a 5′-T G sequence in a single stranded M13 phage vector, which is then transformed into eight different E. coli strains containing all combinations of proficiency and deficiency in the three lesion-bypass DNAPs II, IV and V. Based on M13 progeny output, non-mutagenic bypass with [−ta]-B[ a]P-N 2-dG depends on DNAP IV. In contrast, non-mutagenic bypass with [+ta]-B[ a]P-N 2-dG depends on both DNAPs IV and V, where arguments suggest that DNAP IV is involved in dCTP insertion, while DNAP V is involved in extension of the adduct-G:C base pair. Numerous findings indicate that DNAP II has a slight inhibitory effect on the bypass of [+ta]- and [−ta]-B[ a]P-N 2-dG in the case of both DNAPs IV and V. In conclusion, for efficient non-mutagenic bypass (dCTP insertion) in E. coli, [+ta]-B[ a]P-N 2-dG requires DNAPs IV and V, [−ta]-B[ a]P-N 2-dG requires only DNAP IV, while DNAP II is inhibitory to both, and experiments to investigate these differences should provide insights into the mechanism and purpose of these lesion-bypass DNAPs.