FULL ACTIVATION OF P34(CDC28) HISTONE H1 KINASE-ACTIVITY IS UNABLE TOPROMOTE ENTRY INTO MITOSIS IN CHECKPOINT-ARRESTED CELLS OF THE YEAST SACCHAROMYCES-CEREVISIAE

In most cells, mitosis is dependent upon completion of DNA replication . The feedback mechanisms that prevent entry into mitosis by cells wit h damaged or incompletely replicated DNA have been termed checkpoint c ontrols. Studies with the fission yeast Schizosaccharomyces pombe and Xenopus egg extracts have shown that checkpoint controls prevent activ ation of the master regulatory protein kinase, p34cdc2, that normally triggers entry into mitosis. This is achieved through inhibitory phosp horylation of the Tyr-15 residue of p34cdc2. However, studies with the budding yeast Saccharomyces cerevisiae have shown that phosphorylatio n of this residue is not essential for checkpoint controls to prevent mitosis. We have investigated the basis for checkpoint controls in thi s organism and show that these controls can prevent entry into mitosis even in cells which have fully activated the cyclin B (Clb)-associate d forms of the budding yeast homolog of p34cdc2, p34CDC28, as assayed by histone H1 kinase activity. However, the active complexes in checkp oint-arrested cells are smaller than those in cycling cells, suggestin g that assembly of mitosis-inducing complexes requires additional step s following histone H1 kinase activation.