Genetic analysis and meiotic role of the Saccharomyces cerevisiae RecQ helicase SGS1

SGS1, a Saccharomyces cerevisiae 3 '-5' DNA helicase, is a homologue of the Escherichia coli RecQ gene. It is essential for genomic stability of both during mitosis and meiosis. The purpose of this thesis is to provide a better understanding of the role of this helicase during meiotic recombination. In meiosis, SGS1 mutant cells display a decrease in sporulation efficiency and spore viability. In addition, the unusual spore viability pattern observed in SGS1 mutants cannot be explained solely by meiosis I or meiosis II missegregations. These problems could be partially explained by defects in mitotic chromosome segregation or problems with meiotic S-phase. Cytological experiments demonstrating an increase in synapsis initiation complexes and axial associations in sgs1Delta could be explained by an early function of Sgs1p in meiosis, such as the unwinding of inappropriate strand invasion events. Consistent with this, we observe increased gene conversion, increased homeologous recombination and increased interaction between sister choromatids.;Recent observations have suggested that, Sgs1p and Top3p in S. cerevisiae, and the human orthologue protein BLM, in conjunction with the Top3alpha protein, can dissolve double Holliday junctions. Physical analyses of double-strand break repair in meiosis, combined the genetic analysis of this work, indicate a late function of the Sgs1 protein in the dissolution of double Holliday junctions. We have shown an unusual class of tetrads in which non-sister spores and recombinant spores are dead. We interpret this as a consequence of the failure to untangle intertwined chromatids. This defect in SGS1 mutant strains could be explained by either the presence of pre-meiotic S-phase catenates, a defect in crossover resolution and/or a defect in the dissolution of closely spaced double Holliday junctions.