I propose to investigate the role of S. pombe Rqh1p in the process of recovery from S-phase arrest. In response to incompletely replicated DNA, S-phase checkpoint activation causes cells to transiently arrest cell-cycle progression. While activation of checkpoint arrest is essential for cell viability, cells also must be able to maintain genomic stability throughout arrest so that recovery from arrest occurs successfully. Although much is known about how checkpoint arrest is activated, little is known about the molecular basis of the recovery process. It has been discovered that rqh1+ encodes a DNA helicase that is essential for the process of recovery from S-phase arrest. Interestingly, rqh1+ is homologous to the human BLM (Bloom's syndrome) and WRN (Werner's Syndrome) genes which when mutated cause genomic instability and increased incidences of cancer in affected individuals. To examine the role of eukaryotic DNA helicases in maintaining genomic stability, I propose to identify additional cellular components of this Rqh1p-dependent arrest-recovery pathway by screening for suppressors of rqh1, and investigating possible roles for Rqh1p in this pathway. The numerous tractable phenotypes in rqh1 mutants makes fission yeast an ideal model system for understanding of BLM and WRN in human disease.
