A single fission yeast mitotic cyclin B p34cdc2 kinase promotes both S-phase and mitosis in the absence of G1 cyclins.

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Abstract
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‐replication. Activity of Start genes cdc2 and cdc10 is necessary and p34cdc2 kinase is active in re‐replicating cells. We tested to see whether other cyclins were required for re‐replication using cdc13delta. Further deletion of cig1 and puc1 had no effect, but deletion of cig2/cyc17 caused a severe delay in re‐replication. Deletion of cig1 and cig2/cyc17 together abolished re‐replication completely and cells arrested in G1. This, and analysis of the temperature sensitive cdc13–117 mutant, suggests that cdc13 can effectively substitute for the G1 cyclin activity of cig2/cyc17. We have characterized p56cdc13 activity and find evidence that in the absence of G1 cyclins, S‐phase is delayed until the mitotic p34cdc2‐p56cdc13 kinase is sufficiently active. These data suggest that a single oscillation of p34cdc2 kinase activity provided by a single B‐type cyclin can promote ordered progression into both DNA replication and mitosis, and that the level of cyclin‐dependent kinase activity may act as a master regulator dictating whether cells undergo S‐phase or mitosis.