Fig. 2

Left: piRNA cluster formation and germ line piRNA biogenesis. Rhino forms a terpolymer complex with Deadlock and Cutoff called RDC, and recognizes H3K9me3 on the piRNA cluster. First, RDC recruits Moonshiner and Trf2 to initiate transcription, and upon completion of transcription, piRNA cluster precursor is formed. Then RDC complex achieves the binding of NXF3-NXT1 complex by recruiting adapter protein Bootlegger (boot). Finally, Nxf3 binds Crm1 to mediate the transfer of piRNA precursor transcription out of the nucleus and localization to Nuage in the cytoplasm. In nuage, the 3 ‘end of the loaded piRNA precursor transcribed on Aub is trimmed by Nibbler, and then the end is subjected to 2’ -O-methylation by the methyltransferase Hen1 to become mature piRNAs. Finally, Krimper realized the transition of mature piRNA. In the mitochondria, Armi transfers the piRNA precursor transcript to the mitochondrial outer membrane, where it is cleaved by Zuc to produce the first piRNA. And then it is transported back to nuage. The rest of the piRNA precursor transcripts are repeatedly bound by PIWI and cleaved by Zuc, generating a string of PIWI-loaded piRNAs that are methylated by Hen1. Right: piRNA cluster formation and ovarian somatic cells piRNA biogenesis. The single-stranded piRNA cluster in ovarian cells contains a promoter itself, whose transcription initiation depends on transcription factor Pol II. The piRNA precursor transcripts form standard mRNA structures in the nucleus by standard splicing, 5 ‘-end closure, and 3’ -end polyadenylation. The Nxf1-Nxt1 complex and nuclear porins Nup54 and Nup58 then mediate the export of these transcripts to the Yb body. In Yb bodies, PIWI binds to the 5 ‘end of the flam precursor transcript and connects to Armi, then translocate to the mitochondrial outer membrane, where zuc cleavage produces a new 3’ end, which is finally subjected by Hen-1 to produce mature piRNA