Fig. 1. Ping-pong model for piRNA production. (A) The Piwi-class Argonaute protein Argonaute 3 (Ago3) binds to sense-strand piRNAs (blue) and directs the cleavage of target antisense-strand transcripts (red), producing the 5' end of antisense-strand piRNAs. (B) Aubergine (Aub) and Piwi (not shown) bind to the resulting piRNA precursor, which is trimmed to its final length. This might be catalyzed by the putative nucleases Squash and Zucchini. (C) Drosophila Hen1 methylates the 3' ends of piRNAs (3'OMe). (D) Aub-antisense-strand piRNA complexes catalyze the cleavage of sense transcripts (blue), producing the 5' end of sense piRNAs. (E) Ago3 binds the resulting sense piRNA precursors, which are trimmed and (F) methylated, as described for antisense-strand piRNAs. Based on the model proposed by Brennecke et al. (Brennecke et al., 2007).