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.