Project description:Heterochromatic silencing is thought to occur through a combination of transcriptional silencing and RNA degradation, but the relative contribution of each pathway is not known. In this study we analyzed RNA Polymerase II (RNA Pol II) occupancy and levels of nascent and steady-state RNA in different strains of fission yeast, in order to quantify the contribution of each pathway to heterochromatic silencing. We found that transcriptional silencing consists of two components, reduced RNA Pol II accessibility and, unexpectedly, reduced transcriptional efficiency. Heterochromatic loci showed lower transcriptional output compared to euchromatic loci, despite the presence of comparable amounts of RNA Pol II in both types of regions. We determined that the Ccr4-Not complex and H3K9 methylation are required for reduced transcriptional efficiency in heterochromatin and that a subset of heterochromatic RNA is degraded more rapidly than euchromatic RNA. Finally, we quantified the contribution of different chromatin modifiers, RNAi and RNA degradation to each silencing pathway. Our data show that several pathways contribute to heterochromatic silencing in a locus-specific manner and reveal transcriptional efficiency as a new mechanism of silencing.
Project description:Heterochromatic silencing is thought to occur through a combination of transcriptional silencing and RNA degradation, but the relative contribution of each pathway is not known. In this study, we analyzed RNA Polymerase II (RNA Pol II) occupancy and levels of nascent and steady-state RNA in different mutants of Schizosaccharomyces pombe, in order to quantify the contribution of each pathway to heterochromatic silencing. We found that transcriptional silencing consists of two components, reduced RNA Pol II accessibility and, unexpectedly, reduced transcriptional efficiency. Heterochromatic loci showed lower transcriptional output compared to euchromatic loci, even when comparable amounts of RNA Pol II were present in both types of regions. We determined that the Ccr4-Not complex and H3K9 methylation are required for reduced transcriptional efficiency in heterochromatin and that a subset of heterochromatic RNA is degraded more rapidly than euchromatic RNA. Finally, we quantified the contribution of different chromatin modifiers, RNAi and RNA degradation to each silencing pathway. Our data show that several pathways contribute to heterochromatic silencing in a locus-specific manner and reveal transcriptional efficiency as a new mechanism of silencing.
Project description:The Ccr4-Not complex is an effector of multiple signaling pathways controlling gene transcription and mRNA turnover. Herein, we provide evidence that Ccr4-Not also activates nutrient signaling through the essential target of rapamycin complex 1 (TORC1) pathway. Mechanistically, Ccr4-Not loss decreases TORC1 signaling due to reduced stability of the vacuole V-ATPase that is required to activate TORC1 when associated with the Gtr1 GTPase containing EGO complex. Expressing a constitutively active Gtr1 in Ccr4-Not deficient cells bypasses the requirement for the V-ATPase and restores TORC1 signaling. Transcriptome analysis reveals that Ccr4-Not loss activates TORC1 repressed retrograde signaling to remodel metabolism and enhance mitochondrial function. Blocking retrograde signaling in a Ccr4-Not mutant further reduces TORC1 signaling, thus suggesting the enhanced mitochondrial metabolism due to Ccr4-Not loss is a metabolic adaptive response required to sustain TORC1 activity. Therefore, Ccr4-Not is a critical activator of TORC1 signaling that regulates cellular metabolism.