Figure 1: Yeast G1/S Transcription and Replication Stress Response.
A) Transcription by SBF and MBF is inhibited prior to S-phase. SBF is inhibited by Whi5; MBF acts as a transcriptional repressor by an unknown mechanism. Upon entrance to S-phase, G1 cyclin/CDK complex promotes transcription by SBF and MBF. The cyclin/CDK complex promotes Whi5 dissociation from SBF by phosphorylation. At the end of S-phase, SBF and MBF transcription are downregulated by negative feedback loops. The Cyclin B/Cdc28 complex promotes SBF dissociation from chromatin, and Nrm1 acts as a transcriptional co-repressor for MBF. B) Replication stress results in activated Rad53, which phosphorylates Nrm1, interfering with its interaction with MBF. This results in continued MBF transcription as repair occurs.