Project description:Epigenetic gene silencing plays a critical role in regulating gene expression and contributes to organismal development and cell fate acquisition in eukaryotes. In fission yeast, Schizosaccharomyces pombe, heterochromatin-associated gene silencing is known to be mediated by RNA processing pathways including RNA interference (RNAi) and a 3’-5’ exoribonuclease complex exosome. Here, we report a new RNA-processing pathway that contributes to epigenetic gene silencing and assembly of heterochromatin mediated by 5’-3’ exoribonuclease Dhp1/Xrn2. Dhp1 mutation causes defective gene silencing both at peri-centromeric regions and at the silent mating type locus. Intriguingly, mutation of either one of two well-characterized Dhp1-interacting proteins, the Din1 pyrophosphohydrolase or the Rhn1 transcription termination factor, does not show silencing defects at the main heterochromatic regions. Dhp1 is essential in the sequential steps of establishing silencing in a manner independent of both RNAi and the exosome. Genomic and genetic analysis suggest that Dhp1 is involved in post-transcriptional silencing of repetitive regions through its catalytic activity. Our study is the first investigation into an unexpected role of Dhp1/Rat1/Xrn2 in chromatin-based silencing. These results elucidate how various RNA-processing pathways, acting together or independently, contribute to epigenetic regulation of the eukaryotic genome.
Project description:Epigenetic gene silencing plays a critical role in regulating gene expression and contributes to organismal development and cell fate acquisition in eukaryotes. In fission yeast, Schizosaccharomyces pombe, heterochromatin-associated gene silencing is known to be mediated by RNA processing pathways including RNA interference (RNAi) and a 3’-5’ exoribonuclease complex exosome. Here, we report a new RNA-processing pathway that contributes to epigenetic gene silencing and assembly of heterochromatin mediated by 5’-3’ exoribonuclease Dhp1/Xrn2. Dhp1 mutation causes defective gene silencing both at peri-centromeric regions and at the silent mating type locus. Intriguingly, mutation of either one of two well-characterized Dhp1-interacting proteins, the Din1 pyrophosphohydrolase or the Rhn1 transcription termination factor, does not show silencing defects at the main heterochromatic regions. Dhp1 is essential in the sequential steps of establishing silencing in a manner independent of both RNAi and the exosome. Genomic and genetic analysis suggest that Dhp1 is involved in post-transcriptional silencing of repetitive regions through its catalytic activity. Our study is the first investigation into an unexpected role of Dhp1/Rat1/Xrn2 in chromatin-based silencing. These results elucidate how various RNA-processing pathways, acting together or independently, contribute to epigenetic regulation of the eukaryotic genome.
Project description:Co-transcriptional RNA processing and surveillance factors mediate heterochromatin formation in fission yeast. In addition to RNAi, RNA elimination machinery including MTREC (Mtl1-Red1 core) and the exosome are involved in facultative heterochromatin assembly, however, the exact mechanisms remain unclear. Here we show that RNA elimination factors cooperate with the conserved exoribonuclease Dhp1/Rat1/Xrn2, which couples pre-mRNA 3â-end processing to transcription termination, to promote premature termination and facultative heterochromatin formation at meiotic genes. Dhp1 also affects termination of transcripts at genes that are targets of RNAi-mediated heterochromatin assembly. Moreover, Dhp1 facilitates constitutive heterochromatin formation and silencing at centromeric and mating-type loci. Remarkably, we find that Dhp1 interacts with the Clr4/Suv39h methyltransferase complex and acts directly to nucleate heterochromatin. Our results uncover a novel role for 3â-end processing and termination machinery in gene silencing through premature termination and suggest that non-canonical termination by Dhp1 and RNA elimination factors is linked to heterochromatin assembly. These findings have important implications for understanding mechanisms of gene silencing in higher eukaryotes. Sequencing and analysis of small RNA in two S. pombe mutants
