Fig. 1

Schematic representation of piRNA biogenesis and mechanisms of action. (1) In Drosophila, dual-strand piRNA clusters are marked by H3K9me3, which are targeted by Rhino. Subsequently, Rhino together with Deadlock and Cutoff recruits Moonshiner, leading to the activation of non-canonical transcription via RNA polymerase II. Canonical transcription at the dual-strand piRNA clusters is repressed by Maelstrom (Mael). UAP56 ensures the transport of piRNA precursors into the cytoplasm. (2) In mice, A-MYB recognizes piRNA clusters and ensures transcription catalyzed by RNA polymerase II. THOC1, and THOC2 proteins are responsible for the cytoplasmic transport of piRNA precursors. (3) In the cytoplasm, piRNA precursors undergo primary biogenetic pathway consisting of endonucleolytic cleavage by Zucchini/PLD6, PIWI loading, 3’ end trimming by PNLDC1 and 2’-O-methylation by HENMT1. (4) According to a unified model, the piRNA precursor is targeted by a piRISC with a complementary piRNA leading to the cleavage and generation of a new 5’ end. Subsequently, another PIWI protein is loaded and guides an endonuclease that cleaves the precursor. The resulting pre-piRNA is 3’ trimmed and 2’-O-methylated. The remaining piRNA precursor is repeatedly processed in the same manner, leading to the production of a variety of phased piRNAs. (5) Mature piRISCs target complementary piRNA precursors and transposon RNA transcripts, leading to their cleavage and further processing including amplification through the “ping-pong “ cycle. (6) piRISCs can also degrade protein-coding gene transcripts by targeting complementary sequences in the 3’ UTR of mRNAs in a miRNA-like manner. Alternatively, this interaction can (7) stabilize mRNAs or (8) activate translation. (9) In the nucleus, piRISCs target active transposons and repress their transcription by recruiting epigenetic machinery, which induces heterochromatinization. Created with BioRender.com