Effects of Accessory Proteins on the Bypass of a cis-syn Thymine−Thymine Dimer by Saccharomyces cerevisiae DNA Polymerase η

Among several hypotheses to explain how translesion synthesis (TLS) by DNA polymerase η (pol η) suppresses ultraviolet light-induced mutagenesis in vivo despite the fact that pol η copies DNA with low fidelity, here we test whether replication accessory proteins enhance the fidelity of TLS by pol η. We first show that the single-stranded DNA binding protein RPA, the sliding clamp PCNA, and the clamp loader RFC slightly increase the processivity of yeast pol η and its ability to recycle to new template primers. However, these increases are small, and they are similar when copying an undamaged template and a template containing a cis-syn TT dimer. Consequently, the accessory proteins do not strongly stimulate the already robust TT dimer bypass efficiency of pol η. We then perform a comprehensive analysis of yeast pol η fidelity. We show that it is much less accurate than other yeast DNA polymerases and that the accessory proteins have little effect on fidelity when copying undamaged templates or when bypassing a TT dimer. Thus, although accessory proteins clearly participate in pol η functions in vivo, they do not appear to help suppress UV mutagenesis by improving pol η bypass fidelity per se.