RNAi-mediated DNA silencing

After embracing RNAi's ability to silence and degrade transcripts, biologists have eyed reports that RNAi is required for heterochromatin assembly with skepticism. With new results showing that siRNAs are essential for targeting a heterochromatin-associated protein complex to DNA, André Verdel, Danesh Moazed (Harvard Medical School, Boston, Massachusetts), Shiv Grewal (National Cancer Institute, Bethesda, Maryland), and colleagues think that skepticism is now unfounded. “It leaves no doubt that RNAi is very directly involved in heterochromatin formation,” says Moazed.

Previous work showed that deletion of factors required for RNAi disrupted heterochromatin formation in yeast, implying that RNAi was involved in transcriptional gene silencing. Now, the authors have purified an RNAi effector complex (RITS) that contains Ago1 (the homologue of the siRNA-associated protein Argonaute), a heterochromatin associated protein Chp1, a novel protein Tas3, and small siRNAs complementary to centromeric DNA repeats.

Mutations of Tas3 disrupt heterochromatin formation and block association of methylated histone-3 with DNA, a phenotype that resembles the previously described mutants of Ago1 and Chp1. Targeting the RITS complex to DNA requires siRNAs—in yeast strains lacking Dicer, the enzyme that produces siRNAs, the proteins associate but fail to find chromatin. RITS may recruit histone modifying enzymes to chromatin and initiate heterochromatin assembly. Although Verdel et al. expect these associations to be transient, they are working to detect them directly. ▪

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