FISSION YEAST CUT5 LINKS NUCLEAR CHROMATIN AND M-PHASE REGULATOR IN THE REPLICATION CHECKPOINT CONTROL

Fission yeast temperature-sensitive cut5 (cell untimely torn) mutants are defective in initiation and/or elongation of DNA replication but a llow mitosis and cell division at a restrictive temperature. We show t hat the cut5 protein (identical to rad4) (i) is an essential component of the replication checkpoint system but not the DNA damage checkpoin t, and (ii) negatively regulates the activation of M phase kinase at m itotic entry. Even if the replication checkpoint has been activated pr eviously, cut5 mutations allow mitosis and cell division after shift t o 36 degrees C. Transcription of cut5(+) is not under the control of t he START gene cdc10(+) The cut5 protein is enriched in the nucleus, co nsisting of repeating domains. An essential domain which resembles the proto-oncoprotein Ect2 has a strong negative effect on the entry into mitosis when overexpressed. Expression of the cut5 mutant phenotype r equires the function of the M phase regulator genes cdc2(+), cdc25(+) and cdc13(+). The cut5 protein forms a novel, essential link between D NA synthesis and M phase activation in the replication checkpoint cont rol pathway.