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-ty pe cyclin, Cig2 from the fission yeast Schizosaccharomyces pombe. The cig2 gene encodes a 45-kDa protein that is most similar to a previousl y identified B-type cyclin in S. pombe, Cdc13. Deletion of cig2 had no observable effect on cell viability or progression through the cell c ycle. Strains carrying the cig2 null allele do, however, exhibit an en hanced ability to undergo conjugation relative to a wild-type strain. The cig2 transcript was found to undergo periodic oscillation during t he cell cycle, peaking at the G1/S-phase boundary. We have investigate d 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 sugges ts that there is a delay in septum formation, possibly as a result of defective nuclear separation.