CDK actively contributes to establishment of the stationary phase state in fission yeast

Upon exhaustion of essential environmental nutrients, unicellular organisms cease cell division and enter stationary phase, a metabolically repressed state essential for cell survival in stressful environments. In the fission yeast Schizosaccharomyces pombe, cell size is reduced by cell division before entry into stationary phase; thus cyclin-dependent kinase (CDK) must actively contribute to stationary phase establishment. However, the contribution of CDK to stationary phase remains largely uncharacterized. Here, we examine the role of the sole S. pombe CDK, Cdc2, in the establishment of stationary phase. We show that in stationary phase, nuclear and chromosomal volumes and the nucleus-to-cell volume ratio are reduced, and sister chromatid separation and chromosome fluctuation are repressed. Furthermore, Cdc2 accumulates in the nucleolus. Most of these changes are induced by glucose depletion. Reduction in Cdc2 activity before and upon stationary phase entry alleviates the changes and shortens the survival time of stationary phase cells, whereas Cdc2 inhibition represses nucleolar Cdc2 accumulation and glucose depletion-induced nuclear volume reduction. These results demonstrate that CDK actively regulates stationary phase, both before and upon stationary phase entry.