The Y-family DNA polymerase Dpo4, from the thermophilic crenarchaeon S. solfataricus P2, offers a valuable opportunity to investigate the effect of conformational flexibility on the bypass of bulky lesions because of its ability to function efficiently at a wide range of temperatures. Combined molecular modeling and experimental kinetic studies have been carried out for (+)-trans-[BP]-N²-dG, a lesion derived from the covalent reaction of a benzo[a]pyrene metabolite with guanine in DNA, at 55 °C and results compared with an earlier study at 37 °C (Perlow-Poehnelt et al. (2004) J. Biol. Chem. 279, 36951-36961). The experimental results show that there is more overall nucleotide insertion opposite (+)-trans-[BP]-N²-dG due to particularly enhanced mismatch incorporation at 55 °C compared to 37 °C. The molecular dynamics simulations suggest that mismatched nucleotide insertion opposite (+)-trans-[BP]-N²-dG is increased at 55 °C compared to 37 °C because the higher temperature shifts the preference of the damaged base from the anti to the syn conformation, with the carcinogen on the more open major groove side. The mismatched dNTP structures are less distorted when the damaged base is syn than when it is anti, at the higher temperature. However, with the normal partner dCTP, the anti conformation with close-to Watson-Crick alignment remains more favorable. The molecular dynamics simulations are consistent with the k_cat values for nucleotide incorporation opposite the lesion studied, providing structural interpretation of the experimental observations. The observed temperature effect suggests that conformational flexibility plays a role in nucleotide incorporation and bypass fidelity opposite (+)-trans-[BP]-N²-dG by Dpo4.