HHMI and Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, MD, USA.
Department of Molecular Genetics, University of Toronto, Toronto, ON, Canada.
Dev Cell. 2019 Sep 23;50(6):716-728.e6. doi: 10.1016/j.devcel.2019.07.026. Epub 2019 Aug 8.
P granules are perinuclear condensates in C. elegans germ cells proposed to serve as hubs for self/non-self RNA discrimination by Argonautes. We report that a mutant (meg-3 meg-4) that does not assemble P granules in primordial germ cells loses competence for RNA-interference over several generations and accumulates silencing small RNAs against hundreds of endogenous genes, including the RNA-interference genes rde-11 and sid-1. In wild type, rde-11 and sid-1 transcripts are heavily targeted by piRNAs and accumulate in P granules but maintain expression. In the primordial germ cells of meg-3 meg-4 mutants, rde-11 and sid-1 transcripts disperse in the cytoplasm with the small RNA biogenesis machinery, become hyper-targeted by secondary sRNAs, and are eventually silenced. Silencing requires the PIWI-class Argonaute PRG-1 and the nuclear Argonaute HRDE-1 that maintains trans-generational silencing of piRNA targets. These observations support a "safe harbor" model for P granules in protecting germline transcripts from piRNA-initiated silencing.
P 颗粒是线虫生殖细胞中的核周凝聚物，被认为是 Argonautes 进行自我/非自我 RNA 区分的中心。我们报告说，一个不能在原始生殖细胞中组装 P 颗粒的突变体（meg-3 meg-4）在几代内失去了 RNA 干扰的能力，并积累了针对数百个内源性基因的沉默小 RNA，包括 RNA 干扰基因 rde-11 和 sid-1。在野生型中，rde-11 和 sid-1 转录本被 piRNAs 大量靶向，并在 P 颗粒中积累，但仍保持表达。在 meg-3 meg-4 突变体的原始生殖细胞中，rde-11 和 sid-1 转录本与小 RNA 生物发生机制一起在细胞质中分散，被二级 sRNA 过度靶向，并最终被沉默。沉默需要 PIWI 类 Argonaute PRG-1 和核 Argonaute HRDE-1，它们维持 piRNA 靶标的跨代沉默。这些观察结果支持 P 颗粒作为“安全港”模型，保护生殖细胞转录本免受 piRNA 起始的沉默。
