CIG2, A B-TYPE CYCLIN, PROMOTES THE ONSET OF S IN SCHIZOSACCHAROMYCES-POMBE

Cdc2, a catalytic subunit of cyclin-dependent kinases, is required for both the G(1)-to-S and G(2)-to-M transitions in the fission yeast Sch izosaccharomyces pombe, Cdc13, a B-type cyclin, is required for the M- phase induction function of Cdc2. Two additional B-type cyclins, Cig1 and Cig2, have been identified in S. pombe, but none of the B-type cyc lins are individually required for the onset of S. We report that Cdc1 3 is important for DNA replication in a strain lacking Cig2, Unlike De lta cdc13 cells, double-mutant Delta cdc13 Delta cig2 cells are defect ive in undergoing multiple rounds of DNA replication, The conclusion t hat Cig2 promotes S is further supported by the finding that Cig2 prot ein and Cig2-associated kinase activity appear soon after the completi on of M and peak during S, as well as the observation that S is delaye d in Delta cig2 cells as they recover from a G(1) arrest induced by ni trogen starvation, These studies indicate that Cig2 is the primary S-p hase-promoting cyclin in S. pombe but that Cdc13 can effectively subst itute for Cig2 in Delta cig2 cells, These observations also suggest th at the gradual increase in the activity of Cdc2-Cdc13 kinase can be su fficient for the correct temporal ordering of S and M phases in Delta cig2 cells.