The Zn-finger of Saccharomyces cerevisiae Rad18 and its adjacent region mediate interaction with Rad5.

DNA damages that hinder the movement of the replication complex can ultimately lead to cell death. To avoid that, cells possess several DNA damage bypass mechanisms. The Rad18 ubiquitin ligase controls error-free and mutagenic pathways that help the replication complex to bypass DNA lesions by monoubiquitylating PCNA at stalled replication forks. In Saccharomyces cerevisiae, two of the Rad18 governed pathways are activated by monoubiquitylated PCNA and they involve translesion synthesis polymerases, whereas a third pathway needs subsequent polyubiquitylation of the same PCNA residue by another ubiquitin ligase the Rad5 protein, and it employs template switching. The goal of this study was to dissect the regulatory role of the multidomain Rad18 in DNA damage bypass using a structure-function based approach. Investigating deletion and point mutant RAD18 variants in yeast genetic and yeast two-hybrid assays we show that the Zn-finger of Rad18 mediates its interaction with Rad5, and the N-terminal adjacent region is also necessary for Rad5 binding. Moreover, results of the yeast two-hybrid and in vivo ubiquitylation experiments raise the possibility that direct interaction between Rad18 and Rad5 might not be necessary for the function of the Rad5 dependent pathway. The presented data also reveal that yeast Rad18 uses different domains to mediate its association with itself and with Rad5. Our results contribute to better understanding of the complex machinery of DNA damage bypass pathways.

Identification of Elg1 interaction partners and effects on post-replication chromatin re-formation.

Elg1, the major subunit of a Replication Factor C-like complex, is critical to ensure genomic stability during DNA replication, and is implicated in controlling chromatin structure. We investigated the consequences of Elg1 loss for the dynamics of chromatin re-formation following DNA replication. Measurement of Okazaki fragment length and the micrococcal nuclease sensitivity of newly replicated DNA revealed a defect in nucleosome organization in the absence of Elg1. Using a proteomic approach to identify Elg1 binding partners, we discovered that Elg1 interacts with Rtt106, a histone chaperone implicated in replication-coupled nucleosome assembly that also regulates transcription. A central role for Elg1 is the unloading of PCNA from chromatin following DNA replication, so we examined the relative importance of Rtt106 and PCNA unloading for chromatin reassembly following DNA replication. We find that the major cause of the chromatin organization defects of an ELG1 mutant is PCNA retention on DNA following replication, with Rtt106-Elg1 interaction potentially playing a contributory role.

Rif1 acts through Protein Phosphatase 1 but independent of replication timing to suppress telomere extension in budding yeast.

The Rif1 protein negatively regulates telomeric TG repeat length in the budding yeast Saccharomyces cerevisiae, but how it prevents telomere over-extension is unknown. Rif1 was recently shown to control DNA replication by acting as a Protein Phosphatase 1 (PP1)-targeting subunit. Therefore, we investigated whether Rif1 controls telomere length by targeting PP1 activity. We find that a Rif1 mutant defective for PP1 interaction causes a long-telomere phenotype, similar to that of rif1Δ cells. Tethering PP1 at a specific telomere partially substitutes for Rif1 in limiting TG repeat length, confirming the importance of PP1 in telomere length control. Ablating Rif1-PP1 interaction is known to cause precocious activation of telomere-proximal replication origins and aberrantly early telomere replication. However, we find that Rif1 still limits telomere length even if late replication is forced through deletion of nearby replication origins, indicating that Rif1 can control telomere length independent of replication timing. Moreover we find that, even at a de novo telomere created after DNA synthesis during a mitotic block, Rif1-PP1 interaction is required to suppress telomere lengthening and prevent inappropriate recruitment of Tel1 kinase. Overall, our results show that Rif1 controls telomere length by recruiting PP1 to directly suppress telomerase-mediated TG repeat lengthening.

Translesion synthesis DNA polymerase η exhibits a specific RNA extension activity and a transcription-associated function.

Polymerase eta (Polη) is a low fidelity translesion synthesis DNA polymerase that rescues damage-stalled replication by inserting deoxy-ribonucleotides opposite DNA damage sites resulting in error-free or mutagenic damage bypass. In this study we identify a new specific RNA extension activity of Polη of Saccharomyces cerevisiae. We show that Polη is able to extend RNA primers in the presence of ribonucleotides (rNTPs), and that these reactions are an order of magnitude more efficient than the misinsertion of rNTPs into DNA. Moreover, during RNA extension Polη performs error-free bypass of the 8-oxoguanine and thymine dimer DNA lesions, though with a 103 and 102-fold lower efficiency, respectively, than it synthesizes opposite undamaged nucleotides. Furthermore, in vivo experiments demonstrate that the transcription of several genes is affected by the lack of Polη, and that Polη is enriched over actively transcribed regions. Moreover, inactivation of its polymerase activity causes similar transcription inhibition as the absence of Polη. In summary, these results suggest that the new RNA synthetic activity of Polη can have in vivo relevance.

Mutations at the Subunit Interface of Yeast Proliferating Cell Nuclear Antigen Reveal a Versatile Regulatory Domain.

Proliferating cell nuclear antigen (PCNA) plays a key role in many cellular processes and due to that it interacts with a plethora of proteins. The main interacting surfaces of Saccharomyces cerevisiae PCNA have been mapped to the interdomain connecting loop and to the carboxy-terminal domain. Here we report that the subunit interface of yeast PCNA also has regulatory roles in the function of several DNA damage response pathways. Using site-directed mutagenesis we engineered mutations at both sides of the interface and investigated the effect of these alleles on DNA damage response. Genetic experiments with strains bearing the mutant alleles revealed that mutagenic translesion synthesis, nucleotide excision repair, and homologous recombination are all regulated through residues at the subunit interface. Moreover, genetic characterization of one of our mutants identifies a new sub-branch of nucleotide excision repair. Based on these results we conclude that residues at the subunit boundary of PCNA are not only important for the formation of the trimer structure of PCNA, but they constitute a regulatory protein domain that mediates different DNA damage response pathways, as well.

PCNA Retention on DNA into G2/M Phase Causes Genome Instability in Cells Lacking Elg1.

Loss of the genome maintenance factor Elg1 causes serious genome instability that leads to cancer, but the underlying mechanism is unknown. Elg1 forms the major subunit of a replication factor C-like complex, Elg1-RLC, which unloads the ring-shaped polymerase clamp PCNA from DNA during replication. Here, we show that prolonged retention of PCNA on DNA into G2/M phase is the major cause of genome instability in elg1Δ yeast. Overexpression-induced accumulation of PCNA on DNA causes genome instability. Conversely, disassembly-prone PCNA mutants that relieve PCNA accumulation rescue the genome instability of elg1Δ cells. Covalent modifications to the retained PCNA make only a minor contribution to elg1Δ genome instability. By engineering cell-cycle-regulated ELG1 alleles, we show that abnormal accumulation of PCNA on DNA during S phase causes moderate genome instability and its retention through G2/M phase exacerbates genome instability. Our results reveal that PCNA unloading by Elg1-RLC is critical for genome maintenance.

Def1 promotes the degradation of Pol3 for polymerase exchange to occur during DNA-damage--induced mutagenesis in Saccharomyces cerevisiae.

DNA damages hinder the advance of replication forks because of the inability of the replicative polymerases to synthesize across most DNA lesions. Because stalled replication forks are prone to undergo DNA breakage and recombination that can lead to chromosomal rearrangements and cell death, cells possess different mechanisms to ensure the continuity of replication on damaged templates. Specialized, translesion synthesis (TLS) polymerases can take over synthesis at DNA damage sites. TLS polymerases synthesize DNA with a high error rate and are responsible for damage-induced mutagenesis, so their activity must be strictly regulated. However, the mechanism that allows their replacement of the replicative polymerase is unknown. Here, using protein complex purification and yeast genetic tools, we identify Def1 as a key factor for damage-induced mutagenesis in yeast. In in vivo experiments we demonstrate that upon DNA damage, Def1 promotes the ubiquitylation and subsequent proteasomal degradation of Pol3, the catalytic subunit of the replicative polymerase δ, whereas Pol31 and Pol32, the other two subunits of polymerase δ, are not affected. We also show that purified Pol31 and Pol32 can form a complex with the TLS polymerase Rev1. Our results imply that TLS polymerases carry out DNA lesion bypass only after the Def1-assisted removal of Pol3 from the stalled replication fork.