DNA polymerase z has robust reverse transcriptase activity relative to other cellular DNA polymerases

Cell biology and genetic studies have demonstrated that DNA double-strand break (DSB) repair can be performed using an RNA transcript that spans the site of the DNA break as a template for repair. This type of DSB repair requires a reverse transcriptase to convert an RNA sequence into DNA to facilitate repair of the break, rather than copying from a DNA template as in canonical DSB repair. Translesion synthesis (TLS) DNA polymerases (Pol) are often more promiscuous than DNA Pols, raising the notion that reverse transcription could be performed by a TLS Pol. Indeed, several studies have demonstrated that human Pol h has reverse transcriptase activity, while others have suggested that the yeast TLS Polz is involved. Here, we purify all seven known nuclear DNA Pols of Saccharomyces cerevisiae and compare their reverse transcriptase activities. The comparison shows that Polz far surpasses Pol h and all other DNA Pols in reverse transcriptase activity. We fi nd that Polz reverse transcriptase activity is not affected by RPA or RFC/PCNA and acts distributively to make DNA complementary to an RNA template strand. Consistent with prior S. cerevisiae studies performed in vivo, we propose that Polz is the major DNA Pol that functions in the RNAtemplated DSB repair pathway.