In mammals and plants, cytosine DNA methylation is essential for the epigenetic repression of transposable elements and foreign DNA. In plants, DNA methylation is guided by small interfering RNAs (siRNAs) in a self-reinforcing cycle termed RNA-directed DNA methylation (RdDM). RdDM requires the specialized RNA polymerase V (Pol V), and the key unanswered question is how Pol V is first recruited to new target sites without pre-existing DNA methylation. We find that Pol V follows and is dependent on the recruitment of an AGO4-clade ARGONAUTE protein, and any siRNA can guide the ARGONAUTE protein to the new target locus independent of pre-existing DNA methylation. These findings reject long-standing models of RdDM initiation and instead demonstrate that siRNA-guided ARGONAUTE targeting is necessary, sufficient and first to target Pol V recruitment and trigger the cycle of RdDM at a transcribed target locus, thereby establishing epigenetic silencing.

在哺乳动物和植物中，胞嘧啶 DNA 甲基化对于转座因子和外源 DNA 的表观遗传抑制是必不可少的。在植物中，DNA 甲基化由小干扰 RNA (siRNA) 在称为 RNA 指导的 DNA 甲基化 (RdDM) 的自我强化循环中引导。RdDM 需要专门的 RNA 聚合酶 V (Pol V)，而未解决的关键问题是 Pol V 如何在没有预先存在的 DNA 甲基化的情况下首先被招募到新的靶位点。我们发现 Pol V 跟随并依赖于 AGO4 进化枝 ARGONAUTE 蛋白的募集，并且任何 siRNA 都可以将 ARGONAUTE 蛋白引导至新的靶位点，而与预先存在的 DNA 甲基化无关。这些发现拒绝了长期存在的 RdDM 启动模型，而是证明 siRNA 引导的 ARGONAUTE 靶向是必要的，