MicroRNA (miRNA) production entails the step-wise processing of primary miRNAs (pri-miRNAs) into precursor miRNAs (pre-miRNAs) and miRNA/* duplexes by Dicing complexes containing DCL1, HYL1 and SE, which are localized in nuclear dicing bodies (D-bodies)^1,2. Here, we show that D-bodies are phase-separated condensates. SE forms droplets and drives DCL1, HYL1 and pri/pre-miRNAs into the droplets in vitro, and mutation of SE abrogates the formation of D-bodies in vivo, which indicates that D-bodies arise through SE-mediated phase separation. Disruption of SE phase separation greatly reduces its activity in promoting miRNA processing both in vitro and in vivo. We further show that pre-miRNAs are processed into miRNA/* duplexes in the droplets and, after processing, miRNA/* duplexes are bound by HYL1 and released from the droplets. Our findings provide evidence that efficient miRNA processing depends on the SE-phase-separation-mediated formation of D-bodies and suggest a paradigm that the products made in phase-separated condensates can be shipped out for subsequent processes.

微小 RNA (miRNA) 的产生需要通过包含 DCL1、HYL1 和 SE 的切割复合物将初级 miRNA (pri-miRNA) 逐步加工成前体 miRNA (pre-miRNA) 和 miRNA/* 双链体，这些复合物位于核切割体中（ D 体）^1,2. 在这里，我们表明 D 体是相分离的凝聚态。SE 在体外形成液滴并驱动 DCL1、HYL1 和 pri/pre-miRNA 进入液滴，SE 的突变消除了体内 D 体的形成，这表明 D 体是通过 SE 介导的相分离产生的。SE 相分离的破坏大大降低了其在体外和体内促进 miRNA 加工的活性。我们进一步表明，pre-miRNA 在液滴中被加工成 miRNA/* 双链体，并且在加工后，miRNA/* 双链体被 HYL1 结合并从液滴中释放出来。我们的研究结果证明，有效的 miRNA 加工取决于 SE 相分离介导的 D 体形成，并提出了一种范例，即在相分离的凝聚物中制成的产品可以运出用于后续过程。