Polymerase eta is a short-lived, proteasomally degraded protein that is temporarily stabilized following UV irradiation in Saccharomyces cerevisiae.

Saccharomyces cerevisiae Rad30 is the homolog of human DNA polymerase eta whose inactivation leads to the cancer-prone syndrome xeroderma pigmentosum variant. Both human and yeast polymerase eta are responsible for error-free bypass of UV-induced cis-syn pyrimidine dimers and several other DNA lesions. Here we show, using yeast strains expressing TAP-tagged Rad30, that the level of this protein is post-translationally regulated via ubiquitination and proteasome-mediated degradation. The half-life of Rad30 is 20 min and it increases due to proteasomal defects. Mutations inactivating components of the Skp1/cullin/ F-box (SCF) ubiquitin ligase complex: Skp1 and the F-box protein Ufo1 stabilize Rad30. Our results indicate also that ultraviolet irradiation causes transient stabilization of Rad30, which leads, in turn, to temporary accumulation of this polymerase in the cell. We conclude that proteolysis plays an important role in regulating the cellular abundance of Rad30. These results are the first indication of a role for controlled proteasomal degradation in modulating cellular level of translesion DNA polymerase in eukaryotes.

The influence of the mismatch-repair system on stationary-phase mutagenesis in the yeast Saccharomyces cerevisiae.

Stationary-phase (also called adaptive) mutation occurs in non-dividing cells during prolonged non-lethal selective pressure, e.g. starvation for an essential amino acid. Because in such conditions no DNA replication is observed, mutations probably arise as a result of inefficient DNA repair. In order to understand the role of the yeast mismatch-repair (MMR) system in the mutagenesis of stationary-phase cells, we studied the effects of deletions in genes encoding MutS- and MutL-related proteins on the reversion frequency of the lys2 Delta Bgl frameshift mutation. We found that the level of Lys(+) reversion was increased in all MMR mutants, with the strongest effect observed in a MSH2 (MUTS homologue)-deprived strain. Disruption of the MSH3 or MSH6 genes (also MUTS homologues) resulted in elevation of the mutation frequency and rate, but to a lesser degree than that caused by the inactivation of MSH2. MutL-related proteins were also required for mutation avoidance in stationary-phase cells, but to a lesser extent than MutS homologues. Among MutL homologues, Mlh1 seems to play the major role in this process, while Pms1 and Mlh3 are partially redundant and appear to substitute for each other. These data suggest that MMR proteins, particularly MutS homologues, are involved in the control of mutability in stationary-phase yeast cells.