Zucchini consensus motifs determine the mechanism of pre-piRNA production

Abstract

PIWI-interacting RNAs (piRNAs) of between approximately 24 and 31 nucleotides in length guide PIWI proteins to silence transposons in animal gonads, thereby ensuring fertility1. In the biogenesis of piRNAs, PIWI proteins are first loaded with 5’-monophosphorylated RNA fragments called pre-pre-piRNAs, which then undergo endonucleolytic cleavage to produce pre-piRNAs1,2. Subsequently, the 3’-ends of pre-piRNAs are trimmed by the exonuclease Trimmer (PNLDC1 in mouse)3-6 and 2’-O-methylated by the methyltransferase Hen1 (HENMT1 in mouse)7-9, generating mature piRNAs. It is assumed that the endonuclease Zucchini (MitoPLD in mouse) is a major enzyme catalysing the cleavage of pre-pre-piRNAs into pre-piRNAs10-13. However, direct evidence for this model is lacking, and how pre-piRNAs are generated remains unclear. Here, to analyse pre-piRNA production, we established a Trimmer-knockout silkworm cell line and derived a cell-free system that faithfully recapitulates Zucchini-mediated cleavage of PIWI-loaded pre-pre-piRNAs. We found that pre-piRNAs are generated by parallel Zucchini-dependent and -independent mechanisms. Cleavage by Zucchini occurs at previously unrecognized consensus motifs on pre-pre-piRNAs, requires the RNA helicase Armitage, and is accompanied by 2’-O-methylation of pre-piRNAs. By contrast, slicing of pre-pre-piRNAs with weak Zucchini motifs is achieved by downstream complementary piRNAs, producing pre-piRNAs without 2’-O-methylation. Regardless of the endonucleolytic mechanism, pre-piRNAs are matured by Trimmer and Hen1. Our findings highlight multiplexed processing of piRNA precursors that supports robust and flexible piRNA biogenesis.