Tobacco-Specific Nitrosamine-Derived O2-Alkylthymidines Are Potent Mutagenic Lesions in SOS-Induced Escherichia coli

To investigate the biological effects of the O2-alkylthymidines induced by the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), we have replicated a plasmid containing O2-methylthymidine (O2-Me-dT) or O2-[4-(3-pyridyl-4-oxobut-1-yl]thymidine (O2-POB-dT) in Escherichia coli with specific DNA polymerase knockouts. High genotoxicity of the adducts was manifested in the low yield of transformants from the constructs, which was 2–5% in most strains but increased 2–4-fold with SOS. In the SOS-induced wild type E. coli, O2-Me-dT and O2-POB-dT induced 21% and 56% mutations, respectively. For O2-POB-dT, the major type of mutation was T → G followed by T → A, whereas for O2-Me-dT, T → G and T → A occurred in equal frequency. For both lesions, T → C also was detected in low frequency. The T → G mutation was reduced in strains with deficiency in any of the three SOS polymerases. By contrast, T → A was abolished in the pol V– strain, while its frequency in other strains remained unalt...