A SINGLE FISSION YEAST MITOTIC CYCLIN-B P34(CDC2) KINASE PROMOTES BOTH S-PHASE AND MITOSIS IN THE ABSENCE OF G(1) CYCLINS

Deletion of the fission yeast mitotic B-type cyclin gene cdc13 causes cells to undergo successive rounds of DNA replication. We have used a strain which expresses cdc13 conditionally to investigate re-replicati on. Activity of Start genes cdc2 and cdc10 is necessary and p34(cdc2) kinase is active in re-replicating cells. We tested to see whether oth er cyclins were required for rereplication using cdc13 Delta. Further deletion of cig1 and puc1 had no effect, but deletion of cig2/cyc17 ca used a severe delay in re-replication. Deletion of cig1 and cig2/ cyc1 7 together abolished re-replication completely and cells arrested in G (1). This, and analysis of the temperature sensitive cdc13-117 mutant, suggests that cdc13 can effectively substitute for the G(1) cyclin ac tivity of cig2/cyc17. We have characterized p56(cdc13) activity and fi nd evidence that in the absence of G(1) cyclins, S-phase is delayed un til the mitotic p34(cdc2)-p56(cdc13) kinase is sufficiently active. Th ese data suggest that a single oscillation of p34(cdc2) kinase activit y provided by a single B-type cyclin can promote ordered progression i nto both DNA replication and mitosis, and that the level of cyclin-dep endent kinase activity may act as a master regulator dictating whether cells undergo S-phase or mitosis.