Translesion synthesis past guanine(C8)-thymine(N3) intrastrand cross-links catalyzed by selected A- and Y-family polymerases

Oxidatively generated guanine radicals in DNA can undergo various nucleophilic reactions including the formation of C8-guanine cross-links with adjacent or nearby N3-thymines in DNA in the presence of O₂. These G[8-3]T lesions have been identified in the DNA of human cells exposed to oxidative stress, and are most likely genotoxic if not removed by cellular defence mechanisms. The abilities of several representative polymerases to bypass the G[8-3]T lesions in two different sequence contexts, G∗T∗ and G∗CT∗, were assessed in vitro. The polymerase BF (bacillus fragment) from Bacillus stearothermophilus, the Y-family archaeal polymerases Dpo4 from Sulfolobus sulfataricus P2, and human DNA pol κ and pol η were selected for the study. The A-family polymerase BF was strongly blocked, while relatively weak translesion synthesis was observed in the case of Y-family polymerases Dpo4 and pol κ. Primer extension catalyzed by pol η was also partially stalled at various positions at or near the G[8-3]T cross-linked bases, but a significant and distributive primer extension was observed beyond the sites of the lesions with the efficiency being consistently greater in the case of G∗CT∗ than in the case of G∗T∗ lesions. The results obtained with pol η are compared with translesion synthesis past other intrastrand cross-linked lesions with previously published results of others that include the isomeric G[8-5m]T lesions generated by ionizing radiation, the cis-syn cyclobutane pyrimidine dimer and the 6-4 photoproduct generated by UV irradiation, and the Pt-G∗G∗ lesions derived from the reactions of the chemotherapeutic agent cisplatin with DNA.