Benzo[a]pyrene, a potent human carcinogen, is metabolized in vivo to a diol epoxide that reacts with the N2-position of guanine to produce N2-BP-dG adducts. These adducts are mutagenic causing G to T transversions. These adducts block replicative polymerases but can be bypassed by the Y-family translesion synthesis polymerases. The mechanisms by which mutagenic bypass occurs is not well-known. We have evaluated base pairing structures using atomic substitution of the dNTP with two stereoisomers, 2′-deoxy-N-[(7R,8S,9R,10S)-7,8,9,10-tetrahydro-7,8,9-trihydroxybenzo[a]pyren-10-yl]guanosine and 2′-deoxy-N-[(7S,8R,9S,10R)-7,8,9,10-tetrahydro-7,8,9-trihydroxybenzo[a]pyren-10-yl]guanosine. We have examined the kinetics of incorporation of 1-deaza-dATP, 7-deaza-dATP, 2′-deoxyinosine triphosphate, and 7-deaza-dGTP, analogues of dATP and dGTP in which single atoms are changed. Changes in rate will occur if that atom provided a critical interaction in the transition state of the reaction. We examined two polymerases, Escherichia coli DNA polymerase I (Kf) and Sulfolobus solfataricus DNA polymerase IV (Dpo4), as models of a high fidelity and TLS polymerase, respectively. We found that with Kf, substitution of the nitrogens on the Watson-Crick face of the dNTPs resulted in decreased rate of reactions. This result is consistent with a Hoogsteen base pair in which the template N2-BP-dG flipped from the anti to syn conformation. With Dpo4, while the substitution did not affect the rate of reaction, the amplitude of the reaction decreased with all substitutions. This result suggests that Dpo4 bypasses N2-BP-dG via Hoogsteen base pairs but that the flipped nucleotide can be either the dNTP or the template.
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