piRNA processing by a trimeric Schlafen-domain nuclease

Transposable elements are genomic parasites that expand within and spread between genomes. Piwi proteins control transposon activity, notably in the germline. 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.