Although mammals encode multiple family X DNA polymerases implicated in DNA repair, Saccharomyces cerevisiae has only one, DNA polymerase IV (pol IV). To better understand the repair functions of pol IV, here we characterize its biochemical properties. Like mammalian pol ␤ and pol , but not pol , pol IV has intrinsic 5-2-deoxyribose-5-phosphate lyase activity. Pol IV has low processivity and can fill short gaps in DNA. Unlike the case with pol ␤ and pol , the gap-filling activity of pol IV is

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... t enhanced by a 5-phosphate on the downstream primer but is stimulated by a 5-terminal synthetic abasic site. Pol IV incorporates rNTPs into DNA with an unusually high efficiency relative to dNTPs, a property in common with pol but not pol ␤ or pol . Finally, pol IV is highly inaccurate, with an unusual error specificity indicating the ability to extend primer termini with limited homology. These properties are consistent with a possible role for pol IV in base excision repair and with its known role in non-homologous end joining of double strand breaks, perhaps including those with damaged ends.