G1/S transcription and the DNA synthesis checkpoint: common regulatory mechanisms.

When DNA replication is challenged, cells activate a DNA-synthesis checkpoint blocking cell cycle progression until they are able to overcome the replication defects. In fission yeast, Cds1 is the effector kinase of this checkpoint, inhibiting M phase entry through inactivation of the phosphatase Cdc25, stabilizing stalled replication forks to prevent deleterious DNA structures and triggering transcriptional activation of S-phase genes. The MBF complex controls the transcription of genes required for the S phase and Yox1, a homeodomain-containing protein, binds and represses MBF-dependent transcription at the end of S phase in a cell cycle-regulated manner. Interestingly, when the DNA synthesis checkpoint is activated, Yox1 is phosphorylated by Cds1 resulting in the abrogation of its binding to MBF. As a consequence, MBF-dependent transcription is maintained active until cells are able to overcome the replication challenge. Thus, Yox1 couples normal cell cycle regulation and the DNA synthesis checkpoint in a single transcriptional complex.