RNA interference (RNAi) is a conserved pathway that utilizes Argonaute proteins and their associated small RNAs to exert gene regulatory function on complementary transcripts. While the majority of germline-expressed RNAi proteins reside in perinuclear germ granules, it is unknown whether and how RNAi pathways are spatially organized in other cell types. Here, we find that the small RNA biogenesis machinery is spatially and temporally organized during Caenorhabditis elegans embryogenesis. Specifically, the RNAi factor, SIMR-1, forms visible concentrates during mid-embryogenesis that contain an RNA-dependent RNA polymerase, a poly-UG polymerase, and the unloaded nuclear Argonaute protein, NRDE-3. Curiously, coincident with the appearance of the SIMR granules, the small RNAs bound to NRDE-3 switch from predominantly CSR-class 22G-RNAs to ERGO-dependent 22G-RNAs. NRDE-3 binds ERGO-dependent 22G-RNAs in the somatic cells of larvae and adults to silence ERGO-target genes; here we further demonstrate that NRDE-3-bound, CSR-class 22G-RNAs repress transcription in oocytes. Thus, our study defines two separable roles for NRDE-3, targeting germline-expressed genes during oogenesis to promote global transcriptional repression, and switching during embryogenesis to repress recently duplicated genes and retrotransposons in somatic cells, highlighting the plasticity of Argonaute proteins and the need for more precise temporal characterization of Argonaute-small RNA interactions.

A new method for mapping torsion provides insights into the ways that the genome responds to the torsion generated by RNA polymerase II.