Identification of pathways controlling DNA damage induced mutation in Saccharomyces cerevisiae.

Mutation in response to most types of DNA damage is thought to be mediated by the error-prone sub-branch of post-replication repair and the associated translesion synthesis polymerases. To further understand the mutagenic response to DNA damage, we screened a collection of 4848 haploid gene deletion strains of Saccharomyces cerevisiae for decreased damage-induced mutation of the CAN1 gene. Through extensive quantitative validation of the strains identified by the screen, we identified ten genes, which included error-prone post-replication repair genes known to be involved in induced mutation, as well as two additional genes, FYV6 and RNR4. We demonstrate that FYV6 and RNR4 are epistatic with respect to induced mutation, and that they function, at least partially, independently of post-replication repair. This pathway of induced mutation appears to be mediated by an increase in dNTP levels that facilitates lesion bypass by the replicative polymerase Pol delta, and it is as important as error-prone post-replication repair in the case of UV- and MMS-induced mutation, but solely responsible for EMS-induced mutation. We show that Rnr4/Pol delta-induced mutation is efficiently inhibited by hydroxyurea, a small molecule inhibitor of ribonucleotide reductase, suggesting that if similar pathways exist in human cells, intervention in some forms of mutation may be possible.

Phosphatases, DNA damage checkpoints and checkpoint deactivation.

Cells have evolved intricate and specialized responses to DNA damage, central to which are the DNA damage checkpoints that arrest cell cycle progression and facilitate the repair process. Activation of these damage checkpoints relies heavily on the activity of Ser/Thr kinases, such as Chk1 and Chk2 (Saccharomyces cerevisiae Rad53), which are themselves activated by phosphorylation. Only more recently have we begun to understand how cells disengage the checkpoints to reenter the cell cycle. Here, we review progress toward understanding the functions of phosphatases in checkpoint deactivation in S. cerevisiae, focusing on the non-redundant roles of the type 2A phosphatase Pph3 and the PP2C phosphatases Ptc2 and Ptc3 in the deactivation of Rad53. We discuss how these phosphatases may specifically recognize different phosphorylated forms of Rad53 and how each may independently regulate different facets of the checkpoint response. In conjunction with the independent dephosphorylation of other checkpoint proteins, such regulation may allow a more tailored response to DNA damage that is coordinated with the repair process, ultimately resulting in the resumption of growth.

Pph3-Psy2 is a phosphatase complex required for Rad53 dephosphorylation and replication fork restart during recovery from DNA damage.

Activation of the checkpoint kinase Rad53 is a critical response to DNA damage that results in stabilization of stalled replication forks, inhibition of late-origin initiation, up-regulation of dNTP levels, and delayed entry to mitosis. Activation of Rad53 is well understood and involves phosphorylation by the protein kinases Mec1 and Tel1 as well as in trans autophosphorylation by Rad53 itself. However, deactivation of Rad53, which must occur to allow the cell to recover from checkpoint arrest, is not well understood. Here, we present genetic and biochemical evidence that the type 2A-like protein phosphatase Pph3 forms a complex with Psy2 (Pph3-Psy2) that binds and dephosphorylates activated Rad53 during treatment with, and recovery from, methylmethane sulfonate-mediated DNA damage. In the absence of Pph3-Psy2, Rad53 dephosphorylation and the resumption of DNA synthesis are delayed during recovery from DNA damage. This delay in DNA synthesis reflects a failure to restart stalled replication forks, whereas, remarkably, genome replication is eventually completed by initiating late origins of replication despite the presence of hyperphosphorylated Rad53. These findings suggest that Rad53 regulates replication fork restart and initiation of late firing origins independently and that regulation of these processes is mediated by specific Rad53 phosphatases.

Role of Doa1 in the Saccharomyces cerevisiae DNA damage response.

The cellular response to DNA damage requires not only direct repair of the damage but also changes in the DNA replication machinery, chromatin, and transcription that facilitate survival. Here, we describe Saccharomyces cerevisiae Doa1, which helps to control the damage response by channeling ubiquitin from the proteosomal degradation pathway into pathways that mediate altered DNA replication and chromatin modification. DOA1 interacts with genes involved in PCNA ubiquitination, including RAD6, RAD18, RAD5, UBC13, and MMS2, as well as genes involved in histone H2B ubiquitination or deubiquitination, including RAD6, BRE1, LGE1, CDC73, UBP8, UBP10, and HTB2. In the absence of DOA1, damage-induced ubiquitination of PCNA does not occur. In addition, the level of ubiquitinated H2B is decreased under normal conditions and completely absent in the presence of DNA damage. In the case of PCNA, the defect associated with the doa1Delta mutant is alleviated by overexpression of ubiquitin, but in the case of H2B, it is not. The data suggest that Doa1 is the major source of ubiquitin for the DNA damage response and that Doa1 also plays an additional essential and more specific role in the monoubiquitination of histone H2B.