S-phase

Regulation

Accordingly, entry into S-phase is controlled by molecular pathways that facilitate a rapid, unidirectional shift in cell state. In yeast, for instance, cell growth induces accumulation of Cln3 cyclin, which complexes with the cyclin dependent kinase CDK2. The Cln3-CDK2 complex promotes transcription of S-phase genes by inactivating the transcriptional repressor Whi5. Since upregulation of S-phase genes drive further suppression of Whi5, this pathway creates a positive feedback loop that fully commits cells to S-phase gene expression.