Interaction between the Cig1 and Cig2 B-type cyclins in the fission yeast cell cycle.

In this report, we describe the cloning and characterization of a B-type cyclin, Cig2 from the fission yeast Schizosaccharomyces pombe. The cig2 gene encodes a 45-kDa protein that is most similar to a previously identified B-type cyclin in S. pombe, Cdc13. Deletion of cig2 had no observable effect on cell viability or progression through the cell cycle. Strains carrying the cig2 null allele do, however, exhibit an enhanced ability to undergo conjugation relative to a wild-type strain. The cig2 transcript was found to undergo periodic oscillation during the cell cycle, peaking at the G1/S-phase boundary. We have investigated the relationship between Cig2 and the other B-type cyclins, Cig1 and Cdc13, in the fission yeast. We found that cells carrying disruptions of both the cig1 and cig2 genes contain multiple nuclei with a 1C DNA content, suggesting that they are delayed in progression through the G1 phase of the cell cycle. The phenotype of this double mutant suggests that there is a delay in septum formation, possibly as a result of defective nuclear separation.