Figure 2.

Role of Rad18 and Rad6 in the prevention of spontaneous mutations by 8-oxoG in S.cerevisiae. Endogenous oxidative stress generates 8-oxoG in DNA yielding (G⁰/C). 8-OxoG mispaired with cytosine (G⁰/C) is removed by the Ogg1 glycosylase in the course of a BER process restoring G/C (WT). If Ogg1 is absent or not available, Polδ replicates past 8-oxoG by inserting an adenine across from the lesion. The resulting 8-oxoG/A (G⁰/A) is recognized and a DNA fragment containing adenine in G⁰/A is excised by MMR. In the absence of MMR, cells accumulate GC to TA transversions (MUTANT). In MMR-proficient cells, the removal of adenine opposite 8-oxoG generates single-stranded DNA gaps that contain the lesions in the template strand nearby the 3′-OH end for DNA repair synthesis. Such DNA structures could be recognized by PCNA (red buoy). Due to its affinity for single-stranded DNA, Rad18 (black rectangle) binds the gap, interacts with PCNA and recruits Rad6 (violet oval). The Rad18–Rad6 complex mono-ubiquinates PCNA (small yellow buoy), allowing the recruitment of Polη (green cube). Then, Polη undergoes error-free TLS across from 8-oxoG, restoring G⁰/C, i.e. the substrate of Ogg1. In the absence of Rad18 or Rad6, unmodified PCNA preferentially recruits Polδ at excision gaps and promotes the mutagenic incorporation of adenine. This model does not exclude minor pathway(s), with recruitment of Polη at unmodified PCNA or that of another DNA polymerase (Polx) at modified PCNA yielding MUTANT and WT.