Tethering RITS to a nascent transcript initiates RNAi- and heterochromatin-dependent gene silencing

Cell. 2006 Jun 2;125(5):873-86. doi: 10.1016/j.cell.2006.04.025.


In the fission yeast Schizosaccharomyces pombe, the RNA-Induced Transcriptional Silencing (RITS) complex has been proposed to target the chromosome via siRNA-dependent base-pairing interactions to initiate heterochromatin formation. Here we show that tethering of the RITS subunit, Tas3, to the RNA transcript of the normally active ura4+ gene silences ura4+ expression. This silencing depends on a functional RNAi pathway, requires the heterochromatin proteins, Swi6/HP1, Clr4/Suv39h, and Sir2, and is accompanied by the generation of ura4+ siRNAs, histone H3-lysine 9 methylation, and Swi6 binding. Furthermore, the ability of the newly generated ura4+ siRNAs to silence a second ura4+ allele in trans is strongly inhibited by the conserved siRNA nuclease, Eri1. Surprisingly, silencing of tethered ura4+, or ura4+ inserted within centromeric heterochromatin, or some of the endogenous centromeric repeat promoters, is not associated with changes in RNA polymerase II occupancy. These findings support a model in which targeting of nascent transcripts by RITS mediates chromatin modifications and suggest that cotranscriptional processing events play a primary role in the silencing mechanism.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Carrier Proteins / genetics
  • Carrier Proteins / metabolism
  • Cell Cycle Proteins / genetics
  • Cell Cycle Proteins / metabolism
  • Chromosomal Proteins, Non-Histone / genetics
  • Chromosomal Proteins, Non-Histone / metabolism
  • DNA Methylation
  • Exoribonucleases / genetics
  • Exoribonucleases / metabolism
  • Gene Expression Regulation, Fungal / genetics
  • Heterochromatin / genetics*
  • Histone-Lysine N-Methyltransferase
  • Histones / metabolism
  • Methyltransferases / genetics
  • Methyltransferases / metabolism
  • RNA Interference / physiology*
  • RNA Precursors / genetics*
  • RNA, Small Interfering / genetics*
  • RNA-Binding Proteins / genetics
  • RNA-Induced Silencing Complex / genetics*
  • Regulatory Elements, Transcriptional / genetics
  • Schizosaccharomyces / genetics*
  • Schizosaccharomyces pombe Proteins / genetics
  • Schizosaccharomyces pombe Proteins / metabolism
  • Sirtuins / genetics
  • Sirtuins / metabolism


  • Carrier Proteins
  • Cell Cycle Proteins
  • Chromosomal Proteins, Non-Histone
  • Heterochromatin
  • Histones
  • RNA Precursors
  • RNA, Small Interfering
  • RNA-Binding Proteins
  • RNA-Induced Silencing Complex
  • Schizosaccharomyces pombe Proteins
  • Swi6 protein, S pombe
  • Tas3 protein, S pombe
  • ura4 protein, S pombe
  • Methyltransferases
  • Histone-Lysine N-Methyltransferase
  • clr4 protein, S pombe
  • Eri1 protein, S pombe
  • Exoribonucleases
  • Sirtuins