Fig. 3.

The ping-pong amplification pathway of piRNA biogenesis in Drosophila. In the primary pathway, primary piRNA/Piwi complexes translocate to the nucleus where they induce heterochromatin formation of target loci. In the ping-pong cycle, primary piRNAs in complex with Aub triggers amplification. Antisense piRNAs guide Aub to sense transposon transcripts to induce cleavage. Cleaved sense transcripts are then loaded onto Ago3 and trimmed by 3′-5′ exonuclease to generate secondary sense piRNAs. These in turn guide Ago3 to antisense piRNA precursor transcripts to generate more antisense piRNAs. In this way, the ping-pong cycle incorporates secondary piRNA biogenesis and posttranscriptional transcript silencing by endonucleolytic cleavage.