Arthur M. Sackler Colloqium on the Biology of RNAi

Rnai-Mediated Epigenetic Control of the Genome

Shiv Grewal
Laboratory of Molecular Cell Biology, National Cancer Institute, National Institutes of Health

In eukaryotes, the organization of chromatin into higher-order structures governs diverse chromosomal processes. Heterochromatin assembly at the centromeres facilitates proper segregation of chromosomes, and the formation of specialized chromatin structures at telomeres serves to maintain the length of telomeric repeats as well as suppress recombination. By using fission yeast as a model system, we previously defined precise sequence of events during heterochromatin assembly and showed that formation of constitutive heterochromatin involves an orchestrated event of histone modifications including histone deacetylation, histone methylation and recruitment of heterochromatin protein Swi6/HP1, which is conserved in higher eukaryotes. Moreover, we demonstrated that distinct site-specific histone H3 methylation patterns dictate the specific organization of chromosomes into discrete structural and functional domains.

The mechanisms that define a specific chromosomal domain as preferred site of heterochromatin assembly are not fully understood. Our recent studies have suggested the involvement of RNA interference (RNAi) machinery in nucleation of heterochromatin assembly. We find that centromeric repeat sequences and RNAi cooperate to nucleate heterochromatin, which then spreads to neighboring sequences in Swi6/HP1-dpendent manner, leading to silencing of nearby genes. Double stranded RNA generated from the centromeric repeats are processed by Dicer enzyme into ~22nt small interfering RNAs (siRNAs). These siRNAs join the RITS complex (which contains Argonaute protein, a heterochromatin-associated chromodomain protein Chp1 and a novel protein Tas3) and guide the complex to the homologous sequences, resulting in nucleation of heterochromatin assembly. Although formation of heterochromatin requires RNAi machinery, we find that other mechanisms are involved. We have discovered an RNAi independent pathway involving highly conserved proteins that cooperates with RNAi machinery to establish heterochromatin. Our recent progress on the mechanisms of heterochromatin assembly and its functions will be discussed.

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