Characterization of the DNA repair role of the Saccharomyces cerevisiae NSE1 gene

In eukaryotes, the structural maintenance of chromosomes (SMC) complexes play important roles in chromosome dynamics. The Saccharomyces cerevisiae Smc5-Smc6 complex is composed of eight essential subunits and is particularly required for a normal response to DNA damage. Hypomorphic mutations in these subunits render cells sensitive to DNA damaging agents. Of the non-SMC subunits, Nse1 and Mms21 are of particular interest, because Nse1 contains a RING finger motif characteristic of ubiquitin ligases and Mms21 is a SUMO ligase that sumoylates different subunits of the complex and also other DNA repair proteins.\r\nIn this study, we have isolated a mutation, nse1-101, which results in sensitivity to UV irradiation and MMS. We used this mutant for genetic and postreplication repair (PRR) experiments to examine the role of Nse1 in promoting the bypass of DNA lesions. From epistasis analyses, we inferred a role for Nse1 in the Rad52-dependent repair pathway. Our PRR experiments further support our genetic results, as the newly synthesized DNA does not attain the normal size in UV irradiated nse1-101 cells. On the basis of these and other studies we have provided evidence for the role of Nse1 in the PRR of UV-damaged DNA in a Rad5-independent but Rad52-dependent manner.\r\nWe also constructed mutations in the RING domains of Nse1 and Mms21 to investigate the role of their ligase functions in DNA repair. As was observed with nse1-101, epistasis analyses with the nse1 and mms21 RING mutants also suggest a role for their ligase activities in Rad52-dependent PRR. As this pathway has been suggested to promote PRR when the lesion is on the lagging strand, our studies have indicated a role for Nse1 and Mms21 in promoting PRR on the lagging strand together with the proteins of the Rad52 group. We propose that the Smc5-Smc6 complex functions in this pathway by holding the DNA duplexes in close proximity using the ring structure formed by the Smc5-Smc6 dimer. Further, the ubiquitin ligase and SUMO ligase functions of Nse1 and Mms21, respectively, can contribute to this process by mediating physical interactions between the Smc5-Smc6 complex and the Rad52 group of recombinational proteins.