New pathways regulating mbf-dependent transcription in fission yeast

Resumen

At the end of G1 phase, cells have to decide between continue proliferation or remain in a quiescent state (G0). This decision point, known as “START” in yeast and “Restriction Point” in metazoans, marks irreversibly the commitment to the completion of a new cell cycle, and is regulated mainly by the activity of the G1 CDK and the induction of the G1-to-S transcriptional program. The MBF transcription factor complex (functional homolog of pRB-E2F in metazoans) drives the G1-to-S transcriptional wave in the fission yeast Schizosaccharomyces pombe. We have previously described how the co-repressors Nrm1 and Yox1 bind to MBF complex at the end of S phase, inhibiting the MBF activity. However, the mechanisms involved in the activation of MBF at the onset of an unperturbed cell cycle have remained elusive. Here, we show that Nrm1 is the responsible for the activation of the MBF-dependent transcription through a two-step mechanism. Its phosphorylation by CDK1 and its posterior degradation by APCSte9 induce the irreversible MBF activation until the end of S phase. In addition, we have identified a role of TORC2-Gad8 in maintaining high levels of MBF-dependent transcription, mainly during G1-to-S transition, probably through regulation of the binding of the co-activator Rep2 to the MBF complex. We have also studied the role of chromatin remodelers in the control of the G1-to-S transcriptional program. In this sense, we have found that chromatin-remodeling complexes SWI/SNF and RSC are recruited to MBF-regulated genes, and RSC together with SAGA complex are important to activate the G1-to-S transcriptional wave. Furthermore, we have created a short-lived fluorescent reporter to measure small and transient changes in the MBF activity in vivo by flow cytometry, to further identify new MBF regulators.