PRR includes translesion DNA synthesis that is error prone and a second activity that is largely error free. In selleck chem inhibitor budding yeast, the UBC13 gene codes for an Ub conjugating enzyme involved in the error free DNA PRR pathway. After DNA damage, Ubc13p interacts with Mms2p to assemble Ub chains at the Ub Lys63 residue of PCNA, instead of the conventional Lys48 residue that is the main signal to target a substrate for proteolysis by 26S proteasome. The involvement of UBC13 in cellular tol erance to DNA damage is further supported by its indu cibility in response to treatment with DNA damaging agents such as MMS and UV radiation. The human homolog of S. pombe Ubc13, is UBE2N UBC13, a Ub conjugating enzyme requiring the presence of a Ubc variant for poly ubiquitination.

In particular, divergent activities of mammalian Ubc13 rely on its pairing with either of two Uevs, Uev1A or Mms2. Pmt3 gene product is SUMO, one of a number of Ub like protein that are post translationally covalently attached to one or more Lys residues on target proteins. Although it has only 18% sequence identity with Ub, its structure resembles that of Ub. However, unlike Ub, mammalian SUMO and its budding yeast homologue SMT3 have been shown to be more important for post translational protein modification than for protein degradation. Indeed, SUMO modification has a variety of cellular functions, including roles in transcrip tion, DNA damage response, cell cycle and nuclear transport. Recently, Pmt3 has been shown to be required for SUMO targeted Ub ligase dependent ubi quitination of target proteins.

As an example, S. pombe PCNA is sumoylated in S phase following DNA damage. The process of sumoylation resem bles that of ubiquitination. SUMO is produced as a pre cursor protein that needs to be cleaved to the mature form by one or more specific SUMO proteases. Genetic analyses showed that the pmt3 gene is not essential for viability, but it may be essential for the checkpoint coupling mitosis to the completion of DNA replication and the DNA damage response. Dele tion mutants for pmt3 were strikingly sensitive to the DNA synthesis inhibitor hydroxyurea, MMS and UV radiation, and the microtubule destabilizing agent thiabendazole. However, it has been proposed that pmt3 is involved in the DNA damage tolerance process rather than in the checkpoint itself, similarly to rad31 and hus5.

In fission yeast, sumoylation is involved also in chromo some segregation and telomere length maintenance. Loss of pmt3 function caused a striking increase in telo mere length. More recently, a role for SUMO chain formation in Entinostat response to replication arrest in S. pombe has been established. In addition, a variable pattern of response to DNA damaging agents has been reported in the budding yeast SIZ1 gene mutant, which is charac terized by resistance to anthracyclines and sensitivity to cisplatin and camp tothecin. Since SIZ1 is an E3 ligase of the SUMO pathway, sumoylation defects may impair drug response.