A bifunctional gene product involved in two phases of the yeast cell cycle.

The cell cycle in Saccharomyces cerevisiae is divided into two distinct phases. Unbudded, mononucleate cells in the G1 phase can react to relevant environmental changes by mating, sporulating, or by entering stationary phase. DNA synthesis and bud initiation occur almost simultaneously and mark 'commitment' to the completion of mitosis. Temperature-sensitive mutations at the cdc28 locus are known to cause arrest in the G1 phase of the cell cycle at the restrictive temperature. Here we show that the cdc28 gene product is also active in post-G1 cell cycle functions, and that a different property of the gene product may be required for each phase of the cycle in which it acts.