Genetic and epigenetic mechanisms regulating the g1-s transcriptional wave in fission yeast

Resum

When cells start their life cycle have to face the decisive decision whether to remain in quiescence state (G0) or to pass a point of no return which leads to cellular proliferation. This point, which is known as “Start” in yeast and “Restriction Point” in mammalians, takes place at the end of G1 phase of cell cycle and depends on the activation of specific transcription factors. These transcription factors regulate a wide variety of genes involved in the DNA replication, DNA repair and the progression of the cell cycle. The transcription of genes required for S phase has to be finely regulated. Alteration in some components of this pathway can trigger uncontrolled cell cycle progression and tumorigenic processes in higher eukaryotes. In this work, we use the fission yeast Schizosaccharomyces pombe to study the regulation of the transcription factor MBF (functional homolog of E2F-RB in metazoans). MBF regulates the expression of genes required for the G1-S transition and its alteration leads to genetic instability in yeast. In previous work, we have described how the co-repressors Nrm1 and Yox1 bind to MBF at the end of S phase, inhibiting the MBF-dependent transcription. But the molecular mechanisms that regulate the activation of MBF at the onset of each cell cycle it is still an unresolved question. We have developed a reporter system to measure the changes in the activity of MBF in live cells using flow cytometry. Thanks to this reporter system, we have been able to find new regulators of MBF such as tRNA methyltransferases and COP9/Signalosome which can alter positively and negatively the MBF-dependent expression respectively. Furthermore, we have also studied which proteins are necessary to modify the chromatin structure when the MBF-dependent genes are active. In this sense, we have found that the remodeler complex INO80 (which participates in the exchange of histone variant H2A.Z with H2A) and the histone acetyltransferase Gcn5 (through the acetylation of histone H3 at lysine 9 and lysine 18) help to active the MBF-regulated genes.