Piwi proteins are important for germ cell development in most animals. These proteins are guided to specific targets by small guide RNAs, referred to as piRNAs or 21U RNAs in Caenorhabditis elegans. In this organism, even though genetic screens have uncovered 21U RNA biogenesis factors, little is known about how these factors interact or what they do. Based on the previously identified 21U biogenesis factor PID-1 (piRNA-induced silencing-defective 1), we here define a novel protein complex, PETISCO (PID-3, ERH-2, TOFU-6, and IFE-3 small RNA complex), that is required for 21U RNA biogenesis. PETISCO contains both potential 5 ′ cap and 5 ′ phosphate RNA-binding domains and interacts with capped 21U precursor RNA. We resolved the architecture of PETISCO and revealed a second function for PETISCO in embryonic development. This essential function of PETISCO is mediated not by PID-1 but by the novel protein TOST-1 (twenty-one U pathway antagonist). In contrast, TOST-1 is not essential for 21U RNA biogenesis. Both PID-1 and TOST-1 interact directly with ERH-2 using a conserved sequence motif. Finally, our data suggest a role for TOST-1:PETISCO in SL1 homeostasis in the early embryo. Our work describes a key complex for 21U RNA processing in C. elegans and strengthens the view that 21U RNA biogenesis is built on an snRNA-related pathway.

In Caenorhabditis elegans, the piRNA (21U RNA) pathway is required to establish proper gene regulation and an immortal germline. To achieve this, PRG‐1‐bound 21U RNAs trigger silencing mechanisms mediated by RNA‐dependent RNA polymerase (RdRP)‐synthetized 22G RNAs. This silencing can become PRG‐1‐independent and heritable over many generations, a state termed RNA‐induced epigenetic gene silencing (RNAe). How and when RNAe is established, and how it is maintained, is not known. We show that maternally provided 21U RNAs can be sufficient for triggering RNAe in embryos. Additionally, we identify PID‐2, a protein containing intrinsically disordered regions (IDRs), as a factor required for establishing and maintaining RNAe. PID‐2 interacts with two newly identified and partially redundant eTudor domain‐containing proteins, PID‐4 and PID‐5. PID‐5 has an additional domain related to the X‐prolyl aminopeptidase APP‐1, and binds APP‐1, implicating potential N‐terminal proteolysis in RNAe. All three proteins are required for germline immortality, localize to perinuclear foci, affect size and appearance of RNA inheritance‐linked Z granules, and are required for balancing of 22G RNA populations. Overall, our study identifies three new proteins with crucial functions in C. elegans small RNA silencing.

Transposable elements are genomic parasites that expand within and spread between genomes1. Piwi proteins control transposon activity, notably in the germline2,3. These proteins recognize their targets through small RNA co-factors named piRNAs, making piRNA biogenesis a key specificity-determining step in this crucial genome immunity system. While the processing of piRNA precursors is an essential step in this process, many molecular details of this process remain unknown. We identify a novel endoribonuclease, PUCH, that initiates piRNA processing in the nematode Caenorhabditis elegans. Genetic and biochemical studies show that PUCH, a trimer of Schlafen-like-domain proteins (SLFL proteins), executes 5-end piRNA precursor cleavage. PUCH-mediated processing strictly requires an m7G-Cap and a uracil at position three. We also demonstrate how PUCH interacts with PETISCO, a complex that binds piRNA precursors4, and that this interaction enhances piRNA production in vivo. The identification of PUCH completes the repertoire of C. elegans piRNA biogenesis factors and uncovers a novel type of RNA endonuclease formed by three SLFL proteins. Mammalian Schlafen (Slfn) genes have been associated with immunity responses5, exposing a thus far unknown molecular link between immune responses in mammals and deeply conserved RNA-based mechanisms that control transposable elements.

22Piwi proteins are important for germ cell development in almost all animals studied thus far. 23These proteins are guided to specific targets, such as transposable elements, by small guide 24RNAs, often referred to as piRNAs, or 21U RNAs in C. elegans. In this organism, even though 25 genetic screens have uncovered a number of potential 21U RNA biogenesis factors, little is 26 known about how these factors interact or what they do. Based on the previously identified 27 21U biogenesis factor PID-1, we here define a novel protein complex, PETISCO, that is 28 required for 21U RNA biogenesis. PETISCO contains both potential 5'-cap and 5'-phosphate 29 RNA binding domains, suggesting involvement in 5' end processing. We define the 30 interaction architecture of PETISCO and reveal a second function for PETISCO in embryonic 31 development. This essential function of PETISCO is not mediated by PID-1, but by Vice versa, TOST-1 is not involved in 21U RNA biogenesis. Both PID-1 and TOST-1 are small, 33intrinsically disordered proteins that interact directly with the PETISCO protein ERH-2 34 (enhancer of rudimentary homolog 2) using a conserved sequence motif. Finally, our data 35 suggest an important role for TOST-1:PETISCO in SL1 homeostasis in the early embryo. Our 36 work describes the first molecular platform for 21U RNA production in C. elegans, and 37 strengthens the view that 21U RNA biogenesis is built upon a much more widely used, 38 snRNA-related pathway. 39

In Caenorhabditis elegans, the piRNA (21U RNA) pathway is required to establish proper gene regulation and an immortal germline. To achieve this, PRG-1-bound 21U RNAs trigger silencing mechanisms mediated by RNA-dependent RNA polymerase (RdRP)-synthetized 22G RNAs. This 25 silencing can become PRG-1-independent, and heritable over many generations. This state is named RNAe. It is unknown how and when RNAe is established, and how it is maintained. We show that maternally provided 21U RNAs can be sufficient to trigger RNAe in embryos. Additionally, we identify the IDR-containing protein PID-2, as a factor required to establish and maintain RNAe. PID-2 interacts with two novel, partially redundant, eTudor domain proteins, PID-4 and PID-5. Additionally, PID-5 has 30 a domain related to the X-prolyl aminopeptidase protein APP-1, and binds APP-1, implicating Nterminal proteolysis in RNAe. All three proteins are required for germline immortality, localize to perinuclear foci, affect Z granules, and are required for balancing of 22G RNA populations. Overall, our study identifies three new proteins with crucial functions in the C. elegans small RNA silencing network. 35