Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA.
Cell. 2010 Feb 19;140(4):504-16. doi: 10.1016/j.cell.2010.01.019.
Assembly of fission yeast pericentromeric heterochromatin and generation of small interfering RNAs (siRNAs) from noncoding centromeric transcripts are mutually dependent processes. How this interdependent positive feedback loop is first triggered is a fundamental unanswered question. Here, we show that two distinct Argonaute (Ago1)-dependent pathways mediate small RNA generation. RNA-dependent RNA polymerase complex (RDRC) and Dicer act on specific noncoding RNAs to generate siRNAs by a mechanism that requires the slicer activity of Ago1 but is independent of pre-existing heterochromatin. In the absence of RDRC or Dicer, a distinct class of small RNAs, called primal small RNAs (priRNAs), associates with Ago1. priRNAs are degradation products of abundant transcripts, which bind to Ago1 and target antisense transcripts that result from bidirectional transcription of DNA repeats. Our results suggest that a transcriptome surveillance mechanism based on random association of RNA degradation products with Argonaute triggers siRNA amplification and heterochromatin assembly within DNA repeats.
裂殖酵母着丝粒异染色质的组装和非编码着丝粒转录本产生小干扰 RNA（siRNA）是相互依赖的过程。这个相互依赖的正反馈环是如何首先被触发的，这是一个基本的未解决的问题。在这里，我们表明两种不同的 Argonaute（Ago1）依赖性途径介导小 RNA 的产生。RNA 依赖性 RNA 聚合酶复合物（RDRC）和 Dicer 作用于特定的非编码 RNA，通过一种需要 Ago1 的切割活性但不依赖于预先存在的异染色质的机制产生 siRNA。在缺乏 RDRC 或 Dicer 的情况下，一类称为原始小 RNA（primal small RNAs，priRNAs）的小 RNA 与 Ago1 结合。priRNAs 是大量转录本的降解产物，与 Ago1 结合，并靶向来自 DNA 重复双向转录的反义转录本。我们的结果表明，基于 RNA 降解产物与 Argonaute 随机结合的转录组监测机制触发了 siRNA 的扩增和 DNA 重复内异染色质的组装。
