👤 Tatyana I Kalashnikova

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. Show less

In the budding yeast Saccharomyces cerevisiae, cell cycle initiation is prompted during G(1) phase by Cln3/cyclin-dependent protein kinase-mediated transcriptional activation of G(1)-specific genes. A recent screening performed to reveal novel interactors of SCB-binding factor (SBF) and MCB-binding factor (MBF) identified, in addition to the SBF-specific repressor Whi5 and the MBF-specific corepressor Nrm1, a pair of homologous proteins, Msa1 and Msa2 (encoded by YOR066w and YKR077w), as interactors of SBF and MBF, respectively. MSA1 is expressed periodically during the cell cycle with peak mRNA levels occurring at the late M/early G(1) phase and peak protein levels occurring in early G(1). Msa1 associates with SBF- and MBF-regulated target promoters consistent with a role in G(1)-specific transcriptional regulation. Msa1 affects cell cycle initiation by advancing the timing of transcription of G(1)-specific genes. Msa1 binds to SBF- and MBF-regulated promoters and binding is maximal during the G(1) phase. Binding depends upon the cognate transcription factor. Msa1 overexpression advances the timing of SBF-dependent transcription and budding, whereas depletion delays both indicators of cell cycle initiation. Similar effects on MBF-regulated transcription are observed. Based upon these results, we conclude that Msa1 acts to advance the timing of G(1)-specific transcription and cell cycle initiation. Show less

G1-specific transcriptional activation by Cln3/CDK initiates the budding yeast cell cycle. To identify targets of Cln3/CDK, we analyzed the SBF and MBF transcription factor complexes by multidimensional protein interaction technology (MudPIT). Whi5 was identified as a stably bound component of SBF but not MBF. Inactivation of Whi5 leads to premature expression of G1-specific genes and budding, whereas overexpression retards those processes. Whi5 inactivation bypasses the requirement for Cln3 both for transcriptional activation and cell cycle initiation. Whi5 associates with G1-specific promoters via SBF during early G1 phase, then dissociates coincident with transcriptional activation. Dissociation of Whi5 is promoted by Cln3 in vivo. Cln/CDK phosphorylation of Whi5 in vitro promotes its dissociation from SBF complexes. Mutation of putative CDK phosphorylation sites, at least five of which are phosphorylated in vivo, strongly reduces SBF-dependent transcription and delays cell cycle initiation. Like mammalian Rb, Whi5 is a G1-specific transcriptional repressor antagonized by CDK. Show less