Genetic dissection the Sgs1-Top3 pathway in budding yeast

In budding yeast Saccharomyces cerevisiae, mutation of the TOP3 gene encoding topoisomerase III results in slow growth and genome maintenance defects, including hyper-recombination, increased chromosome missegregation, and hyper-sensitivity to DNA damaging agents. Mutation of the organism's sole RecQ family helicase, Sgs1, largely suppresses top3 defects. Top3 and Sgs1 also interact with each other physically. The physical and genetic associations between RecQ helicases and topoisomerase III are evolutionarily conserved, suggesting that these two classes of proteins function in concert with one another. An current view regarding the roles of these proteins suggests that they control genetic recombination, particularly during the process of DNA replication. In this dissertation, we describe several genetic approaches directed to further our understanding of the roles of these proteins in maintenance of genome stability and to identify other cellular factors that exhibit functional interactions with the Sgs1-Top3 pathway. We find that mutations in genes encoding homologous recombination (HR) factors partially suppress top3 defects, supporting the view that HR is detrimental in the absence of a functional Sgs1-Top3 complex. We also identify several other mutational suppressors of top3 slow growth, such as SHU1, SHU2, PSY3, and CSM2, whose cellular functions are only beginning to be elucidated. Our results suggest that these four genes have roles in promoting efficient and rearrangement-free recombinational repair and in protecting the genome from mutagenesis.