Eukaryotic cells normally differentiate from G 1 ; here we investigate the mechanism preventing expression of differentiation-specific genes outside G 1 . In fission yeast, induction of the transcription factor Ste11 triggers sexual differentiation. We find that Ste11 is only active in G 1 when Cdk activity is low. In the remaining part of the cell cycle, Ste11 becomes Cdk-phosphorylated at Thr 82 (T82), which inhibits its DNA-binding activity. Since the ste11 gene is autoregulated and the Ste11 protein is highly unstable, this Cdk switch rapidly extinguishes Ste11 activity when cells enter S phase. When we mutated T82 to aspartic acid, mimicking constant phosphorylation, cells no longer underwent differentiation. Conversely, changing T82 to alanine rendered Ste11-controlled transcription constitutive through the cell cycle, and allowed mating from S phase with increased frequency. Thus, Cdk phosphorylation mediates periodic expression of Ste11 and its target genes, and we suggest this to be part of the mechanism restricting differentiation to G 1 .[Keywords: Cell cycle; differentiation; Cdk; Ste11; S. pombe] Supplemental material is available at http://www.genesdev.org.

Several genetic mutants of fission yeast (Schizosaccharomyces pombe) that form multiple septa and pseudohyphae (i.e., branching growth) have been isolated.(1-15) The current understanding of these mutants is that they lack the ability to separate the two sister cells after formation of the septum. Here it is shown that switching to multisepta and pseudohyphal growth can be induced in a reversible manner in wild-type S. pombe cells by changing the growth conditions, thus indicating an inherent cellular switch. Flow cytometry profiles of exponentially growing cultures of both wild-type and mutant cells further support that a bi-stable switch is controlling the morphological state of the cell in a stress-dependent manner.