A Tudor-domain containing protein, SIMR-1, mediates silencing of piRNA target mRNAs by the Mutator Complex

piRNAs play a critical role in the regulation of transposons and other germline genes. In Caenorhabditis elegans, silencing of piRNA target genes is mediated by the Mutator complex, which synthesizes high levels of siRNAs through the activity of an RNA-dependent RNA polymerase. However, how mRNAs recognized by the piRNA pathway are handed off to the Mutator pathway is unclear. Here, we identify the Tudor domain-containing protein, SIMR-1, as a key mediator of this handoff. Interestingly, SIMR-1 also localizes to distinct subcellular foci adjacent to P granules and Mutator foci, two phase-separated condensates that are the sites of piRNA-dependent mRNA recognition and Mutator complex-dependent siRNA amplification, respectively. Thus, our data suggests a role for multiple perinuclear condensates in organizing the piRNA pathway and promoting silencing by the Mutator complex. 10 small-RNA seq libraries in duplicate, 14 single replicate small-RNA seq libraries, and 10 mRNA-seq libraries in triplicate were generated and analyzed.