Primer extension by various polymerases using oligonucleotide templates containing stereoisomeric benzo[a]pyrene-deoxyadenosine adducts.

Abstract

Four isomeric benzo[a]pyrene-deoxyadenosine adducts, corresponding to the products of trans opening of the epoxide ring in the four configurationally isomeric benzo[a]pyrene dihydrodiol epoxides by the amino group of deoxyadenosine, were separately introduced into each of two 16-mer sequence contexts. The sequences were from the supF gene, and the site of the adducted adenine was known, for some hydrocarbon dihydrodiol epoxides, to be a hotspot for mutation in Context I and a coldspot for mutation in Context II. Using primers complementary to the 3' ends of these oligonucleotides, the abilities of several polymerases to replicate these templates in vitro were investigated. Each adduct proved to be an effective block to primer extension such that only with high concentrations of exo- Klenow fragment was any bypass of adducts seen. DNA polymerase alpha and HIV-1 reverse transcriptase were blocked 3' to the adduct when the configuration at C10 of the hdyrocarbon was S, and some introduction of thymine opposite the adenine adduct was seen with the R configuration. Incorporation of a nucleotide opposite the adduct occurred more readily with Sequenase and the Klenow fragment, and the mutagenic introduction of adenine was apparent in most cases. This corresponded to the A-->T transversions frequently seen in mutation studies with hydrocarbon dihydrodiol epoxides that react extensively with adenine in DNA. Overall, it was clear that sequence context, adduct stereochemistry, and the choice of polymerase all influenced the polymerization reaction. With these in vitro systems, no major differences correlating with the differing tumorigenicities of the isomeric dihydrodiol epoxides or with the hotspot or coldspot nature of the sequences were detected.