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Conséquence physiologiques et mécanistiques de l'intéraction covalente du facteur Rm3 avec l'ARN polymérase I chez la levure Saccharomyces cerevisiae

Abstract : In Eukaryotes, the task of transcribing nuclear genes is shared between three DNA dependant RNA polymerases : RNA polymerase I II and III (Pol I, II and III), each of them transcribing a distinct set of genes. Pol I is dedicated to the synthesis of the large ribosomal RNA genes, and its activity monopolizes nearly 60% of the global transcriptional activity in rapidly growing yeast cells. Nuclear transcription is downregulated in many conditions and mostly during nutrient starvation which can be mimicked by rapamycin treatment. We constructed a yeast strain expressing a constituvely active form of Pol I (CARA strain). Using this strain, it has been demonstrated that Pol I activity contols the level of ribosome biogenesis. In this strain, the repression of Pol I activity is attenuated under rapamycin treatment comparing to the WT strain, but after 4 hours of treatment, the level of Pol I activity is nearly undectable in both strains. Surprisingly, in these conditions, the nucleolar structure in the CARA strain remained intact whereas the nucleolar structure falls apart in the WT strain. ChIP and Miller spread experiments revealed that even after 4 hours of rapamycin treatment, Pol I remained on the rDNA in CARA cells while we obsereved a drastic decrease of rDNA occupation by Pol I in WT cells. These results demonstrate for the first time that Pol I activity and nucleolar structure are uncoupled. We also investigated the mechanics of pol I transcription in vitro in order to determine the consequences of the non dissociability of the Pol I-Rrn3 complex. Our results suggest a model in which PolI-Rrn3 complex dissociates very rapidly shortly after or during initiation , this dissociation being a prerequisite for proper transcription elongation.
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Submitted on : Wednesday, January 13, 2010 - 3:11:45 PM
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Nayla Ayoub. Conséquence physiologiques et mécanistiques de l'intéraction covalente du facteur Rm3 avec l'ARN polymérase I chez la levure Saccharomyces cerevisiae. Biochimie [q-bio.BM]. Université Paris-Diderot - Paris VII, 2009. Français. ⟨tel-00446803⟩

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