GENES INVOLVED IN SISTER-CHROMATID SEPARATION ARE NEEDED FOR B-TYPE CYCLIN PROTEOLYSIS IN BUDDING YEAST

B-type cyclin destruction is necessary for exit from mitosis and the i nitiation of a new cell cycle. Through the isolation of mutants, we ha ve identified three essential yeast genes, CDC16, CDC23, and CSE1, whi ch are required for proteolysis of the B-type cyclin CLBS but not of o ther unstable proteins. cdc23-1 mutants are defective in both entering and exiting anaphase. Their failure to exit anaphase can be explained by defective cyclin proteolysis. CDC23 is required at the metaphase/a naphase transition to separate sister chromatids, and we speculate tha t it might promote proteolysis of proteins that hold sister chromatids together. Proteolysis of CLB2 is initiated in early anaphase, but a f raction of CLB2 remains stable until anaphase is complete.