Replication past O⁶-Methylguanine by yeast and human DNA polymerase η

O⁶-Methylguanine (m6G) is formed by the action of alkylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on DNA. m6G is a highly mutagenic and carcinogenic lesion, and it presents a block to synthesis by DNA polymerases. Here, we provide genetic and biochemical evidence for the involvement of yeast and human DNA polymerase η (Polη) in the replicative bypass of m6G lesions in DNA. The formation of MNNG-induced mutations is almost abolished in the rad30Δ pol32Δ double mutant of yeast, which lacks the RAD30 gene that encodes Polη and the Pol32 subunit of DNA polymerase δ (Polδ). Although Polδ can function in the mutagenic bypass of m6G lesions, our biochemical studies indicate that Polη is much more efficient in replicating through m6G than Polδ. Both Polη and Polδ insert a C or a T residue opposite from m6G; Polη, however, is more accurate, as it inserts a C about twice as frequently as Polδ. Alkylating agents are used in the treatment of malignant tumors, including lymphomas, brain tumors, melanomas, and gastrointestinal carcinomas, and the clinical effectiveness of these agents derives at least in part from their ability to form m6G in DNA. Inactivation of Polη could afford a useful strategy for enhancing the effectiveness of these agents in cancer chemotherapy.