Hyper-recombination and mutator effects of the mms9-1, mms13-1, and mms21-1 mutations in Saccharomyces cerevisiae

Requirement of Nse1, a subunit of the Smc5-Smc6 complex, for Rad52-dependent postreplication repair of UV-damaged DNA in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, postreplication repair (PRR) of UV-damaged DNA occurs by a Rad6-Rad18- and an Mms2-Ubc13-Rad5-dependent pathway or by a Rad52-dependent pathway. The Rad5 DNA helicase activity is specialized for promoting replication fork regression and template switching; previously, we suggested a role for the Rad5-dependent PRR pathway when the lesion is located on the leading strand and a role for the Rad52 pathway when the lesion is located on the lagging strand. In this study, we present evidence for the requirement of Nse1, a subunit of the Smc5-Smc6 complex, in Rad52-dependent PRR, and our genetic analyses suggest a role for the Nse1 and Mms21 E3 ligase activities associated with this complex in this repair mode. We discuss the possible ways by which the Smc5-Smc6 complex, including its associated ubiquitin ligase and SUMO ligase activities, might contribute to the Rad52-dependent nonrecombinational and recombinational modes of PRR.

A SUMO ligase is part of a nuclear multiprotein complex that affects DNA repair and chromosomal organization

Through a genetic screen using myosin-like protein strains mlp1Delta mlp2Delta and biochemical purification, we identified a complex of eight proteins, each required for growth and DNA repair in Saccharomyces cerevisiae. Among the subunits are Mms21 that contains a putative Siz/PIAS (protein inhibitor of activated signal transducer and activator of transcription) RING domain characteristic of small ubiquitin-like modifier (SUMO) ligases, two structural-maintenance-of-chromosome (Smc) proteins, Smc5 and Smc6, and a protein that contains an ubiquitin ligase signature domain. We show that these proteins colocalized to several distinct nuclear foci. Biochemical and genetic data demonstrated that Mms21 indeed functions as a SUMO ligase and that this activity requires the Siz/PIAS (protein inhibitor of activated signal transducer and activator of transcription) RING domain. The substrates for this SUMO ligase include a subunit of the octameric complex, Smc5, and the DNA repair protein Yku70. We further show that the abolition of the SUMO E3 activity of Mms21 leads to such disparate phenotypes as DNA damage sensitivity, defects in nucleolar integrity and telomere clustering, silencing, and length regulation. We propose that Mms21 sumoylates proteins involved in these diverse processes and that the other members of the complex, particularly Smc5/6, facilitate proper substrate sumoylation by localizing Mms21 to specific chromosomal regions.

Interactions among mutations affecting spontaneous mutation, mitotic recombination, and DNA repair in yeast

The mutant alleles mms9-1, mms13-1, or mms21-1 of Saccharomyces cerevisiae confer pleiotropic effects, including sensitivity to the alkylating agent methyl methanesulfonate, elevations in spontaneous mutation and mitotic recombination, defects in meiosis, and cross-sensitivity to radiation. We constructed double-mutant strains containing an mms mutation and a defect in either excision repair, mutagenic repair, or recombinational repair and measured the levels of spontaneous mutation and mitotic recombination. Double mutants lacking excision repair show elevations in spontaneous mutation but with predominantly unchanged levels of mitotic recombination. RAD52 function was required for the expression of the hyper-recombination phenotype of the mms9-1, mms13-1, and mms21-1 alleles; double mutants displayed the very low recombination levels characteristic of rad52 mutants. Phenotypes of double mutants containing one of the mms alleles and either of the hyper-recombination/mutator rad6-1 or rad3-102 alleles suggest that the mutagenic lesions in mms strains may not be identical to the recombinogenic lesions.