CDC18P CAN BLOCK MITOSIS BY 2 INDEPENDENT MECHANISMS

The DNA replication checkpoint is required to maintain the integrity o f the genome, inhibiting mitosis until S phase has been successfully c ompleted. The checkpoint preventing premature mitosis in Schizosacchar omyces pombe relies on phosphorylation of the tyrosine-15 residue on c dc2p to prevent its activation and hence mitosis, The cdc18 gene is es sential for both generating the DNA replication checkpoint and the ini tiation of S phase, thus providing a key role for the overall control and coordination of the cell cycle. We show that the C terminus of the protein is capable of both initiating DNA replication and the checkpo int function of cdc18p. The C terminus of cdc18p acts upstream of the DNA replication checkpoint genes rad1, rad3, rad9, rad17, hus1 and cut 5 and requires the wee1p/mik1p tyrosine kinases to block mitosis, The N terminus of cdc18p can also block mitosis but does so in the absence of the DNA replication checkpoint genes and the wee1p/mik1p kinases t herefore acting downstream of these genes. Because the N terminus of c dc18p associates with cdc2p in vivo, we suggest that by binding the cd c2p/cdc13p mitotic kinase directly, it exerts an effect independently of the normal checkpoint control, probably in an unphysiological manne r.