Project description:In the fission yeast Schizosaccharomyces pombe, the RNA interference (RNAi) pathway is required to generate small interfering RNAs (siRNAs) that mediate heterochromatic silencing of centromeric repeats. Here we demonstrate that RNAi also functions to repress genomic elements other than constitutive heterochromatin. Using DamID (DNA adenine methyltransferase identification) we show that Dcr1 and Rdp1 physically associate with some euchromatic genes, non-coding RNA (ncRNA) genes, and retrotransposon long terminal repeats (LTRs), and that this association is independent of the Clr4 histone methyltransferase. Physical association of RNAi with chromatin is sufficient to trigger a silencing response but not to assemble heterochromatin. The mode of silencing at the newly identified RNAi targets is consistent with a co-transcriptional gene silencing model as proposed earlier and functions with trace amounts of siRNAs. We anticipate that similar mechanisms could also be operational in other eukaryotes.
Project description:Expression profiling, ChiP-CHIP and phenotypic analysis were used to investigate the functional relationships of class III NAD+-dependent HDACs (Sirtuins) in fission yeast. We detected significant histone acetylation increases in Sirtuin mutants at their specific genomic binding targets and were thus able to identify an in vivo substrate preference for each Sirtuin. At heterochromatic loci, we demonstrate that although Hst2 is mainly cytoplasmic, a nuclear pool of Hst2 co-localises with the other Sirtuins at silent regions (cen, mat, tel, rDNA) and that like the other Sirtuins, Hst2 is required for rDNA and centromeric silencing. Interestingly we found specific functions for the fission yeast Sirtuins Hst2 and Hst4 in gene regulation. Hst2 directly represses genes involved in transport and membrane function whereas Hst4 represses amino acid biosynthesis genes and Tf2 retrotransposons. A specific role for Hst4 in Tf2 5’ mRNA processing was revealed. Thus, Sirtuins share functions at many genomic targets, but Hst2 and Hst4 have also evolved unique functions in gene regulation. Keywords: chromatin; fission yeast; gene silencing; retrotransposon; Sirtuin; HDAC
