Identification of a strand-related bias in the PCNA-mediated bypass of spontaneous lesions by yeast Polη

Translesion synthesis (TLS) DNA polymerases are specialized to bypass lesions that block replicative polymerases and prevent complete genome duplication. Current TLS models hypothesize that PCNA, the polymerase processivity clamp, is important for regulating the access and loading of the low fidelity TLS polymerases onto DNA in response to replication-blocking lesions. PCNA binds to the C-terminus of yeast Polη, for example, and this interaction is required for cell survival after UV irradiation. Previously, we identified two spontaneous, Polζ-dependent “complex” mutation hotspots using the lys2ΔA746 frameshift reversion assay in repair-compromised cells. In the current study we observed an accumulation of Polζ-dependent complex frameshifts at a third hotspot in Polη-deficient cells. Interestingly, the sequence of this third hotspot is the reverse complement of the two hotspots previously identified, suggesting that the utilization of Polζ and Polη may be related to the position of the relevant lesion on either the leading- or lagging-strand template. Using the lys2ΔA746 assay system, we investigated changes in the accumulation of complex events at hotspots when the direction of replication was reversed in repair-compromised cells with either wildtype Polη, a deletion of Polη, or a mutant of Polη that cannot interact with PCNA. Our results suggest that there is a polymerase hierarchy between Polη and Polζ in the bypass of certain lesions and that the interaction of Polη with PCNA is needed for some, but not all, spontaneous lesion bypass.