Down-regulation of Cdk1 activity in G1 coordinates the G1/S gene expression programme with genome replication.

Cell division is regulated by cyclin-dependent kinases (Cdks) and requires the periodic activation and inactivation of transcription factors that generate waves of gene expression in different cell-cycle phases. In fission yeast, the MCB-binding transcription factor (MBF) is activated at the end of G1 and regulates the expression of a set of genes that encode for proteins involved in the G1/S transition and DNA replication. Here, we review the importance of controlling MBF by Cdk activity at the onset of S phase. Furthermore, we emphasize that MBF regulation by Cdk is particularly critical under conditions in which G1 is extended, such as in nitrogen-poor environments, where down-regulation of Cdk activity in G1 is crucial to generate a proper wave of MBF-dependent transcription at the end of G1, which is critical to promote a successful S phase.