The dependence of cell-cycle progression on the integrity of the genome has been described as checkpoint control1,2. A number of mutants of the fission yeast Schizosaccharomyces pombe, selected for their sensitivity to DNA damage caused by radiation (rad mutants) or to the DNA synthesis inhibitor hydroxyurea (hus mutants) have been classified as checkpoint mutants because they fail to arrest the cell cycle in response to DNA damage or incompletely replicated DNA3-6. Coupling of the checkpoint pathways that monitor DNA repair a'nd replication to control of the cell cycle is essential. In a search for components that interact with the cell-cycle regulatory kinase p34cdc2, we have identified a novel fission yeast protein kinase homologue which is involved in cell-cycle arrest when DNA damage has occurred or when unligated DNA is present. We have called the gene encoding this protein chkl for checkpoint kinase. Multiple copies of chkl partially rescue the ultraviolet sensitivity of rad 1-1, a mutant deficient in checkpoint control3-5. Identification of a gene involved in check-point control as a rescue of a cdc2 mutant links the rad1-dependent DNA-damage-sensing pathway and p34cdc2 activity.
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