Schizosaccharomyces pombe Hsk1p is a potential Cds1p target required for genome integrity

The fission yeast Hsk1p kinase is an essential activator of DNA replication . Here we report the isolation and characterization of a novel mutant allel e of the gene. Consistent with its role in the initiation of DNA synthesis, hsk1(ts) genetically interacts with several S-phase mutants. At the restri ctive temperature, hsk1(ts) cells suffer abnormal S phase and loss of nucle ar integrity and are sensitive to both DNA-damaging agents and replication arrest. Interestingly, hsk1(ts) mutants released to the restrictive tempera ture after early S-phase arrest in hydroxyurea (HU) are able to complete bu lk DNA synthesis but they nevertheless undergo an abnormal mitosis. These f indings indicate a second role for hsk1 subsequent to RU arrest. Consistent with a later S-phase role, hsk1(ts) is synthetically lethal with Delta rqh 1 (RecQ helicase) or rad21ts (cohesin) mutants and suppressed by Delta cds1 (RAD53 kianse) mutants. We demonstrate that Hsk1p undergoes Cds1p-dependen t phosphorylation in response to HU and that it is a direct substrate of pu rified Cds1p kinase in vitro. These results indicate that the Hsk1p kinase is a potential target of Cds1p regulation and that its activity is required after replication initiation for normal mitosis.