TY - JOUR T1 - PIWI Proteins Act at Multiple Steps in the Production of Their Own Guides JF - bioRxiv DO - 10.1101/261545 SP - 261545 AU - Ildar Gainetdinov AU - Cansu Colpan AU - Katharine Cecchini AU - Phillip D. Zamore Y1 - 2018/01/01 UR - http://biorxiv.org/content/early/2018/02/07/261545.abstract N2 - In animals, PIWI-interacting RNAs (piRNAs) protect the germline and regulate gene expression. Two pathways make piRNAs. Endonucleolytic cleavage of long transcripts generates phased precursor piRNAs (pre-piRNAs) that mature into primary piRNAs, whereas PIWI-catalyzed slicing initiates production of secondary piRNAs. We report that, in mice and flies, PIWI proteins direct the stepwise fragmentation of long piRNA-producing transcripts: the sites of endonucleolytic cleavage along the transcript reflect both the identity of the PIWI protein bound to the precursor transcript’s 5′ end and the availability of uridines at the prospective pre-piRNA 3′ end. In mice, slicing of precursor transcripts by pachytene-piRNA guided PIWI proteins produces new pachytene piRNAs. Thus, PIWI proteins both initiate piRNA biogenesis and position the subsequent endonucleolytic cleavage that yield phased pre-piRNAs. Our data define a central role of mouse PIWI proteins in generating their own guides and suggest a unified model for insect and mammalian piRNA production. ER -