DNA lesions can often block DNA replication, so cells possess specialized low-fidelity, and often error-prone, DNA polymerases that can bypass such lesions and promote replication of damaged DNA. The Saccharomyces cerevisiae RAD30 and human hRAD30A encode Polη, which bypasses a cis-syn thymine-thymine dimer efficiently and accurately. Here we show that a related human gene, hRAD30B, encodes the DNA polymerase Polι, which misincorporates deoxynucleotides at a high rate. To bypass damage, Polι specifically incorporates deoxynucleotides opposite highly distorting or non-instructional DNA lesions. This action is combined with that of DNA polymerase Polζ, which is essential for damage-induced mutagenesis, to complete the lesion bypass. Polζ is very inefficient in inserting deoxynucleotides opposite DNA lesions, but readily extends from such deoxynucleotides once they have been inserted. Thus, in a new model for mutagenic bypass of DNA lesions in eukaryotes, the two DNA polymerases act sequentially: Polι incorporates deoxynucleotides opposite DNA lesions, and Polζ functions as a mispair extender.
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