A yeast mutant conditionally defective only for reentry into the mitotic cell cycle from stationary phase.

We report the isolation of a cold-sensitive mutant of the yeast Saccharomyces cerevisiae that is conditionally defective only for reentry into the mitotic cell cycle from stationary phase. Although actively dividing mutant cells shifted to the restrictive temperature continued to divide, stationary-phase mutant cells placed in fresh medium at the restrictive temperature failed to divide or even perform the cell cycle regulatory step "start" but did lose the characteristic stationary-phase properties of thermotolerance, accumulation of storage carbohydrates, and resistance to cell-wall-lytic enzymes. Order-of-function analysis indicated that the cold-sensitive defect blocked cells during reentry before start of the first mitotic cell cycle. Genetic analysis showed that the mutant phenotype is due to the interaction between two mutations, a cold-sensitive mutation gcs1 and a suppressor mutation sed1. These mutations thus provide the genetic basis for further analysis of stationary phase and the G0 state.