Centre National de la Recherche Scientifique, Unité de Recherche Associée 2578, Institut Pasteur, 25 rue du Dr Roux, F75015 Paris, France.
Proc Natl Acad Sci U S A. 2009 Dec 15;106(50):21258-63. doi: 10.1073/pnas.0809208105. Epub 2009 Nov 30.
A new class of small RNAs (endo-siRNAs) produced from endogenous double-stranded RNA (dsRNA) precursors was recently shown to mediate transposable element (TE) silencing in the Drosophila soma. These endo-siRNAs might play a role in heterochromatin formation, as has been shown in S. pombe for siRNAs derived from repetitive sequences in chromosome pericentromeres. To address this possibility, we used the viral suppressors of RNA silencing B2 and P19. These proteins normally counteract the RNAi host defense by blocking the biogenesis or activity of virus-derived siRNAs. We hypothesized that both proteins would similarly block endo-siRNA processing or function, thereby revealing the contribution of endo-siRNA to heterochromatin formation. Accordingly, P19 as well as a nuclear form of P19 expressed in Drosophila somatic cells were found to sequester TE-derived siRNAs whereas B2 predominantly bound their longer precursors. Strikingly, B2 or the nuclear form of P19, but not P19, suppressed silencing of heterochromatin gene markers in adult flies, and altered histone H3-K9 methylation as well as chromosomal distribution of histone methyl transferase Su(var)3-9 and Heterochromatin Protein 1 in larvae. Similar effects were observed in dcr2, r2d2, and ago2 mutants. Our findings provide evidence that a nuclear pool of TE-derived endo-siRNAs is involved in heterochromatin formation in somatic tissues in Drosophila.
一类新的小 RNA（endo-siRNAs）来源于内源性双链 RNA（dsRNA）前体，最近被证明可以在果蝇体中调节转座元件（TE）沉默。这些内源性 siRNAs 可能在异染色质形成中发挥作用，就像在 S. pombe 中那样，来自染色体着丝粒周围重复序列的 siRNAs 可以介导异染色质形成。为了研究这种可能性，我们使用了 RNA 沉默的病毒抑制剂 B2 和 P19。这些蛋白质通常通过阻止病毒衍生的 siRNA 的生物发生或活性来拮抗 RNAi 宿主防御。我们假设这两种蛋白质都会类似地阻断内源性 siRNA 的加工或功能，从而揭示内源性 siRNA 对异染色质形成的贡献。因此，P19 以及在果蝇体细胞中表达的核形式 P19 被发现可以将 TE 衍生的 siRNAs 隔离，而 B2 主要结合其更长的前体。引人注目的是，B2 或核形式的 P19，但不是 P19，抑制了成年果蝇中异染色质基因标记的沉默，并改变了幼虫中组蛋白 H3-K9 甲基化以及组蛋白甲基转移酶 Su(var)3-9 和异染色质蛋白 1 的染色体分布。在 dcr2、r2d2 和 ago2 突变体中也观察到了类似的效果。我们的研究结果为内源性 TE 衍生的 endo-siRNAs 参与果蝇体细胞中的异染色质形成提供了证据。
