RITS acts in cis to promote RNA interference-mediated transcriptional and post-transcriptional silencing

RNA interference is a conserved mechanism by which double-stranded RNA is processed into short interfering RNAs (siRNAs) that can trigger both post-transcriptional and transcriptional gene silencing. In fission yeast, the RNA-induced initiation of transcriptional gene silencing (RITS) complex contains Dicer-generated siRNAs and is required for heterochromatic silencing. Here we show that BITS components, including Argonaute protein, bind to all known heterochromatic loci. At the mating-type region, BITS is recruited to the centromere-homologous repeat cenH in a Dicer-dependent manner, whereas the spreading of BITS across the entire 20-kb silenced domain, as well as its subsequent maintenance, requires heterochromatin machinery including Swi6 and occurs even in the absence of Dicer. Furthermore, our analyses suggest that RNA interference machinery operates in cis as a stable component of heterochromatic domains with BITS tethered to silenced loci by methylation of histone H3 at Lys9. This tethering promotes the processing of transcripts and generation of additional siRNAs for heterochromatin maintenance.