The long-term goal of this work is to understand how the RNA interference (RNAi) pathway regulates heterochromatic gene silencing in fission yeast. RNAi is a widespread silencing mechanism that acts at both the posttranscriptional and transcriptional levels and is triggered by double stranded RNA molecules that are processed to small interfering RNAs (called siRNAs). siRNAs guide the inactivation of complementary target nucleic acids by effector complexes. Our laboratory has purified the RITS (RNA-Induced Transcriptional Gene Silencing) complex, containing the Ago1, Chp1, and Tas3 proteins, which physically links the RNAi pathway to heterochromatin. In addition, we have purified the RNAi complexes that synthesize dsRNA, process it into siRNAs, and help load siRNAs onto the RITS complex. We have provided evidence that these RNAi complexes localize to specific chromosome regions via base pairing interactions between siRNAs and nascent non-coding transcripts. This localization leads to recruitment of a histone H3 methyltransferase complex, called the CRC (Clr4-Rik1-Cul4) complex, which methylates histone H3 on lysine-9 and promotes heterochromatin formation. Our recent results suggest that cis-synthesis of dsRNA, at sites of heterochromatin assembly, is required for H3 lysine-9 methylation. The goals of this proposal are to understand how RITS and dsRNA synthesis recruit the CRC histone methyltransferase complex and how the nascent RNA transcript controls the spreading of heterochromatin. In addition, we will perform a biochemical dissection of steps that are involved in the formation of heterochromatic siRNAs.
This project addresses the role of RNA interference (RNAi), a conserved RNA silencing pathway, in gene silencing in the nucleus. Components of this pathway are involved in regulation of cell differentiation and cancer in humans. A basic understanding of the role of RNAi in gene silencing will not only provide a frame work for understanding how the process can fail, but also provides the substrate and knowledge to design therapeutic strategies based on intervention.
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