Requirement for three novel protein complexes in the absence of the sgs1 DNA helicase in Saccharomyces cerevisiae

The Saccharomyces cerevisiae Sgs1 is a member of the RccQ family of DNA hel icases and is required for genome stability, but not cell viability. To ide ntify proteins that function in the absence of Sgs1, a synthetic-lethal scr een was performed. We obtained mutations in six complementation groups that we refer to as SLX genes. Most of the SLX genes encode uncharacterized ope n reading frames that are conserved in other species. None of these genes i s required for viability and all SLX null mutations are synthetically letha l with mutations in TOP3,encoding the SGS1-interacting DNA topoisomerase. A nalysis of the null mutants identified a pair of genes in each of three phe notypic classes. Mutations in MMS4 (SLX2) and SLX3 generate identical pheno types, including weak UV and strong MMS hypersensitivity complete loss of s porulation, and synthetic growth defects with mutations in TOP1. Mms4 and S lx3 proteins coimmunoprecipitate from cell extracts, suggesting that they f unction in a complex. Mutations in SLX5 and SLX8 generate hydroxyurea sensi tivity, reduced sporulation efficiency, and a slow-growth phenotype charact erized by heterogeneous colony morphology. The SLX1 and SLX4 proteins conta in RING finger domains coimmunoprecipitate from cell extracts. The SLX1 and SLX4 genes are required for viability in the presence of an sgs1 temperatu re-sensitive allele at the restrictive temperature and Slx1 and Slx4 protei ns are similarly associated in cell extracts. We propose that the MMS4/SLX3 , SLX5/8, and SLX1/4 gene pairs encode heterodimeric complexes and speculat e that these complexes are required to resolve recombination intermediates that arise in response to DNA damage, during meiosis, and in the absence of SGS1/TOP3.