P34(CDC2) KINASE-ACTIVITY IS MAINTAINED UPON ACTIVATION OF THE REPLICATION CHECKPOINT IN SCHIZOSACCHAROMYCES-POMBE

All eukaryotes use feedback controls to order and coordinate cell cycl e events. In Schizosaccharomyces pombe, several classes of checkpoint genes serve to ensure that DNA replication is complete and free of err or before the onset of mitosis. Wild-type cells normally arrest upon i nhibition of DNA synthesis or in response to DNA damage, although the exact mechanisms controlling this arrest are unclear. Genetic evidence in fission yeast suggests that the dependence of mitosis upon complet ion of DNA replication is Linked to the regulation of the p34(cdc2) cy clin-dependent kinase. It has been hypothesized that inhibition of DNA synthesis triggers down-regulation of p34(cdc2) kinase activity, alth ough this has never been shown biochemically. We analyzed the activity of p34(cdc2) in mild-type and checkpoint-defective cells treated with a DNA synthesis inhibitor. Using standard in vitro assays we demonstr ate that p34(cdc2) kinase activity is maintained in wild-type cells ar rested at the replication checkpoint, We also used a novel in vivo ass ay for p34(cdc2) kinase activity, in which we expressed a fragment of the human retinoblastoma tumor suppressor protein in fission yeast. Ph osphorylation of this fragment of the human retinoblastoma tumor suppr essor protein is dependent on p34(cdc2) kinase activity, and this acti vity is also maintained in cells arrested at the replication checkpoin t. These data suggest that the mechanism for cell-cycle arrest in resp onse to incomplete DNA synthesis is not dependent an the attenuation o f p34(cdc2) activity.