G1-SPECIFIC CYCLINS - IN SEARCH OF AN S-PHASE-PROMOTING FACTOR

In budding yeast, Saccharomyces cerevisiae, the two principal cell cycle transitions, from G1 to S phase and from G2 to M phase, are controlled by the same protein kinase, CDC28, a homology of the cdc2 protein kinase in fission yeast and other organisms. The G1 to S phase activity of the kinase is associated with accumulation of a novel family of G1 cyclins, distinct from cyclins that are required to activate the kinase for G2 to M phase functions. It remains to be determined whether G1 cyclins with similar functions exist in higher cells.