Stb1 collaborates with other regulators to modulate the G1-specific transcriptional circuit.

G(1)-specific transcription in the budding yeast Saccharomyces cerevisiae depends upon SBF and MBF. Whereas inactivation of SBF-regulated genes during the G(1)/S transition depends upon mitotic B-type cyclins, inactivation of MBF has been reported to involve multiple regulators, Nrm1 and Stb1. Nrm1 is a transcriptional corepressor that inactivates MBF-regulated transcription via negative feedback as cells exit G(1) phase. Cln/cyclin-dependent kinase (CDK)-dependent inactivation of Stb1, identified via its interaction with the histone deacetylase (HDAC) component Sin3, has also been reported to inactivate MBF-regulated transcription. This report shows that Stb1 is a stable component of both SBF and MBF that binds G(1)-specific promoters via Swi6 during G(1) phase. It is important for the growth of cells in which SBF or MBF is inactive. Although dissociation of Stb1 from promoters as cells exit G(1) correlates with Stb1 phosphorylation, phosphorylation is only partially dependent upon Cln1/2 and is not involved in transcription inactivation. Inactivation depends upon Nrm1 and Clb/CDK activity. Stb1 inactivation dampens maximal transcriptional induction during late G(1) phase and also derepresses gene expression in G(1)-phase cells prior to Cln3-dependent transcriptional activation. The repression during G(1) also depends upon Sin3. We speculate that the interaction between Stb1 and Sin3 regulates the Sin3/HDAC complex at G(1)-specific promoters.