The translesion DNA polymerases Pol ζ and Rev1 are activated independently of PCNA ubiquitination upon UV radiation in mutants of DNA polymerase δ

Replicative DNA polymerases cannot insert efficiently nucleotides at sites of base lesions. This function is taken over by specialized translesion DNA synthesis (TLS) polymerases to allow DNA replication completion in the presence of DNA damage. In eukaryotes, Rad6and Rad18-mediated PCNA ubiquitination at lysine 164 promotes recruitment of TLS polymerases, allowing cells to efficiently cope with DNA damage. However, several studies showed that TLS polymerases can be recruited also in the absence of PCNA ubiquitination. We hypothesized that the stability of the interactions between DNA polymerase delta (Pol delta) subunits and/or between Pol delta and PCNA at the primer/template junction is a crucial factor to determine the requirement of PCNA ubiquitination. To test this hypothesis, we used a structural mutant of Pol d in which the interaction between Pol3 and Pol31 is inhibited. We found that in yeast, rad18 Delta-associated UV hypersensitivity is suppressed by pol3-ct, a mutant allele of the POL3 gene that encodes the catalytic subunit of replicative Pol delta pol3-ct suppressor effect was specifically dependent on the Rev1 and Pol zeta TLS polymerases. This result strongly suggests that TLS polymerases could rely much less on PCNA ubiquitination when Pol delta interaction with PCNA is partially compromised by mutations. In agreement with this model, we found that the pol3-FI allele suppressed rad18 Delta-associated UV sensitivity as observed for pol3-ct. This POL3 allele carries mutations within a putative PCNA Interacting Peptide (PIP) motif. We then provided molecular and genetic evidence that this motif could contribute to Pol delta-PCNA interaction indirectly, although it is not a bona fide PIP. Overall, our results suggest that the primary role of PCNA ubiquitination is to allow TLS polymerases to outcompete Pol delta for PCNA access upon DNA damage.