Authors: Michael R. Harris, Dave Lee, Sarah Farmer, Noel F. Lowndes, Robertus A. M. de Bruin.
Model of SBF transcriptional regulation. During G1, SBF (Swi4-Swi6) is bound to target promoters in complex with the transcriptional inhibitor Whi5, which represses transcription. Activation of Mcm1-dependent transcription results in the initial accumulation of Cln3 and Swi4. Cln3/CDK-dependent phosphorylation removes Whi5 from SBF at promoters, activating transcription. Initial transcriptional activation results in SBF-dependent accumulation of Cln1-2 and MBF-dependent further accumulation of Swi4. Cln1-2 in complex with CDK is involved in a positive feedback loop to further phosphorylate Whi5, which leads to robust activation of G1/S transcription. Accumulation of Swi4 during G1 coincides with enhanced detection of Swi4 at the CLN2 promoter, possibly representing an additional positive feedback loop to ensure timely activation of G1/S transcription.