STOCKPILING OF CDC25 DURING A DNA-REPLICATION CHECKPOINT ARREST IN SCHIZOSACCHAROMYCES-POMBE

The DNA replication checkpoint couples the onset of mitosis with the c ompletion of S phase. It is clear that in the fission yeast Schizosacc haromyces pombe, operation of this checkpoint requires maintenance of the inhibitory tyrosyl phosphorylation of Cdc2. Cdc25 phosphatase indu ces mitosis by dephosphorylating tyrosine 15 of Cdc2. In this report, Cdc25 is shown to accumulate to a very high level in cells arrested in S. This shows that mechanisms which modulate the abundance of Cdc25 a re unconnected to the DNA replication checkpoint. Using a Cdc2/cyclin B activation assay, we found that Cdc25 activity increased similar to 10-fold during transit through M phase. Cdc25 was activated by phospho rylations that were dependent on Cdc2 activity in vivo. Cdc25 activati on was suppressed in cells arrested in G(1) and S. However, Cdc25 was more highly modified and appeared to be somewhat more active in S than in G(1). This finding might be connected to the fact that progression from G(1) to S increases the likelihood that constitutive Cdc25 overp roduction will cause inappropriate mitosis.