RESUMEN
Periodic transcriptional waves along the cell cycle ensure the accurate progression of the different cell cycle phases through the timely regulated expression of cell cycle proteins. The G1/S transition (Start) consists in the activation of a transcriptional program by G1 CDKs through the inactivation of Start transcriptional repressors, Whi5 and Whi7 in yeast or Rb in mammals. Here, we provide a comprehensive characterization of the transcriptional regulation of the Start repressor Whi7 in budding yeast. We found that WHI7 is a cell cycle regulated gene that shows periodic expression peaking in G1. Our results demonstrate that the three cell cycle transcriptional programs related to G1 and their corresponding transcription factors are involved in the transcriptional control of WHI7. Specifically, we identified that the transcriptional regulators Swi5 and Mcm1-Yox1, which are involved in late M and early G1 expression, and the transcription factors MBF and SBF, which are responsible for G1/S expression, are able to associate and regulate the WHI7 gene. In summary, in this work, we provide new mechanisms for the regulation of the Start repressor Whi7, which highlights the precise and complex control of the cell cycle machinery governing the G1/S transition.
RESUMEN
Pho85 is a multifunctional CDK that signals to the cell when environmental conditions are favorable. It has been connected to cell cycle control, mainly in Start where it promotes the G1/S transition. Here we describe that the Start repressor Whi7 is a key target of Pho85 in the regulation of cell cycle entry. The phosphorylation of Whi7 by Pho85 inhibits the repressor and explains most of the contribution of the CDK in the activation of Start. Mechanistically, Pho85 downregulates Whi7 protein levels through the control of Whi7 protein stability and WHI7 gene transcription. Whi7 phosphorylation by Pho85 also restrains the intrinsic ability of Whi7 to associate with promoters. Furthermore, although Whi5 is the main Start repressor in normal cycling cells, in the absence of Pho85, Whi7 becomes the major repressor leading to G1 arrest. Overall, our results reveal a novel mechanism by which Pho85 promotes Start through the regulation of the Whi7 repressor at multiple levels, which may confer to Whi7 a functional specialization to connect the response to adverse conditions with the cell cycle control.
Asunto(s)
Proteínas de Saccharomyces cerevisiae , Saccharomycetales , Ciclo Celular/genética , Quinasas Ciclina-Dependientes/genética , Quinasas Ciclina-Dependientes/metabolismo , Ciclinas/genética , Ciclinas/metabolismo , Regulación Fúngica de la Expresión Génica , Proteínas Represoras/genética , Proteínas Represoras/metabolismo , Saccharomyces cerevisiae/genética , Saccharomyces cerevisiae/metabolismo , Proteínas de Saccharomyces cerevisiae/genética , Proteínas de Saccharomyces cerevisiae/metabolismo , Saccharomycetales/metabolismoRESUMEN
Eukaryotic ribonucleotide reductases are iron-dependent enzymes that catalyze the rate-limiting step in the de novo synthesis of deoxyribonucleotides. Multiple mechanisms regulate the activity of ribonucleotide reductases in response to genotoxic stresses and iron deficiency. Upon iron starvation, the Saccharomyces cerevisiae Aft1 transcription factor specifically binds to iron-responsive cis elements within the promoter of a group of genes, known as the iron regulon, activating their transcription. Members of the iron regulon participate in iron acquisition, mobilization and recycling, and trigger a genome-wide metabolic remodeling of iron-dependent pathways. Here, we describe a mechanism that optimizes the activity of yeast ribonucleotide reductase when iron is scarce. We demonstrate that Aft1 and the DNA-binding protein Ixr1 enhance the expression of the gene encoding for its catalytic subunit, RNR1, in response to iron limitation, leading to an increase in both mRNA and protein levels. By mutagenesis of the Aft1-binding sites within RNR1 promoter, we conclude that RNR1 activation by iron depletion is important for Rnr1 protein and deoxyribonucleotide synthesis. Remarkably, Aft1 also activates the expression of IXR1 upon iron scarcity through an iron-responsive element located within its promoter. These results provide a novel mechanism for the direct activation of ribonucleotide reductase function by the iron-regulated Aft1 transcription factor.