Fig. 1

The basics of piRNA biogenesis and function in flies and mice. PiRNA precursors are transcribed form genomic regions called piRNA clusters. Long single-stranded RNA precursors are exported into the cytoplasm and transported to piRNA processing sites on the surface of mitochondria and in germ granules (also called ‘intermitochondrial cement’ or ‘nuage’). Primary piRNA biogenesis is initiated by the endonuclease D.m. Zucchini (ZUC)/ M.m. PLD6/MitoPLD on the mitochondrial surface (Biogenesis module 1). An individual long precursor gives rise to multiple piRNAs. PLD6/ZUC generates 5’phosphorylated RNA fragments that are loaded into PIWI proteins to form pre-piRNA complexes. 3’ end processing involves another endonucleolytic cleavage by PLD6/ZUC followed by exonucleolytic trimming in some organisms. During the effector phase of piRNA-guided silencing, mature PIWI-piRNA silencing complexes (piRISC) target RNAs with base-pairing complementarity. Nuclear PIWI-piRNA complexes recruit histone and DNA methyltransferases, depending on the organism, to establish transcriptional gene silencing (TGS) at target loci (silencing module 2). Cytoplasmic PIWI-piRNA complexes induce post-transcriptional gene silencing (PTGS) using PIWI’s intrinsic nuclease activity (slicer) (silencing module 1). Slicing of target RNAs produces 5’ monophosphorylated fragments that can either be further degraded by exonucleases or loaded onto a PIWI protein to generate secondary piRNAs (biogenesis module 2). Secondary piRNAs can induce further amplification of a piRNA-pair via coordinated slicing during ping-pong. (D.m … Drosophila melanogaster; M.m … Mus musculus)