Repression of G₁/S Transcription Is Mediated via Interaction of the GTB Motifs of Nrm1 and Whi5 with Swi6

Abstract

bIn Saccharomyces cerevisiae, G 1 /S transcription factors MBF and SBF regulate a large family of genes important for entry to the cell cycle and DNA replication and repair. Their regulation is crucial for cell viability, and it is conserved throughout evolution. MBF and SBF consist of a common component, Swi6, and a DNA-specific binding protein, Mbp1 and Swi4, respectively. Transcriptional repressors bind to and regulate the activity of both transcription factors. Whi5 binds to SBF and represses its activity at the beginning of the G 1 phase to prevent early activation. Nrm1 binds to MBF to repress transcription as cells progress through S phase. Here, we describe a protein motif, the GTB motif (for G 1 /S transcription factor binding), in Nrm1 and Whi5 that is required to bind to the transcription factors. We also identify a region of the carboxy terminus of Swi6 that is required for Nrm1 and Whi5 binding to their target transcription factors and show that mutation of this region overrides the repression of MBF-and SBF-regulated genes by Nrm1 and Whi5. Finally, we show that the GTB motif is the core of a functional module that is necessary and sufficient for targeting of the transcription factors by their cognate repressors.

A common strategy for initiating the transition from proliferation to quiescence

“…Srl3 is induced by DNA damage and other forms of stress (Gasch, et al 2000, Lee, et al 2000, Ragni, et al 2011). It has been shown to bind SBF transiently in DNA damage (Travesa, et al 2013) and mediate the nuclear localization of Cln3 (Yahya, et al 2014). The whi5srl3 mutant is slow to arrest in G1 (Miles, et al 2016), but after seven days in culture, it achieves G1 arrest just like wild type (Fig 4A).…”

A systematic cell size screen uncovers coupling of growth to division by the p38/HOG network inCandida albicans

“…Size control in S. pombe is exerted through inhibition of G2/M phase CDK activity by the Wee1 kinase, which is encoded by the first size control gene discovered (9, 11). Size is also partially regulated at Start in S. pombe through an SBF/MBF- like G1/S transcription factor complex and the Nrm1 inhibitor (17). The CDK-dependent control of G1/S transcription in metazoans is analogously mediated by the cyclin D-Rb-E2F axis (15, 18, 19).…”

Topology and Control of the Cell-Cycle-Regulated Transcriptional Circuitry

“…Dissociation and nuclear export of Whi5, in turn, leads to the activation of SBF and the expression of its target genes. Although Cln3/CDK-dependent Whi5 phosphorylation appears to promote both dissociation from promoters and export from the nucleus, those two phenomena have not been separated in terms of their effect on the activation of G1/S transcription (Costanzo et al 2004;de Bruin et al 2004;Di Talia et al 2007;Skotheim et al 2008;Travesa et al 2013). A recent study has established that release of the Whi5 repressor is the critical determinant of the event historically referred to as START by Hartwell and colleagues (Doncic et al 2011;Eser et al 2011).…”

“…Unlike Whi5, which represses transcription from M phase through late G1 phase, Nrm1 acts only after G1/S transcription is strongly activated, leading to Nrm1 accumulation. Nrm1 binds to the carboxy terminus of Swi6 at MBF target promoters via interaction between its GTB-containing domain, a motif conserved with Whi5 and its fungal orthologs (Ofir et al 2012;Travesa et al 2013). This leads to the reestablishment of MBF as a transcriptional repressor such that expression of MBF target genes, including that of NRM1 itself, is terminated by negative feedback.…”

“…Cln3, once accumulated to a critical level in the nucleus, activates Cdk1, leading to the phosphorylation of the SBF-bound transcriptional repressor, Whi5 (Costanzo et al 2004;de Bruin et al 2004;Schaefer and Breeden 2004). Upon phosphorylation, Whi5, which binds to the carboxy terminus of Swi6 through its GTB-containing domain, dissociates from promoter-bound SBF and is relocalized to the cytoplasm (Figure 2) (Costanzo et al 2004;de Bruin et al 2004;Di Talia et al 2007;Skotheim et al 2008;Travesa et al 2013). Phosphorylation of Swi6 by Cln/CDK may also contribute to the regulation of Whi5 binding (Sidorova et al 1995;Costanzo et al 2004;Wagner et al 2009).…”