Somatic Primary piRNA Biogenesis Driven by cis-Acting RNA Elements and trans-Acting Yb

Primary piRNAs in Drosophila ovarian somatic cells arise from piRNA cluster transcripts and the 3′ UTRs of a subset of mRNAs, including Traffic jam (Tj) mRNA. However, it is unclear how these RNAs are determined as primary piRNA sources. Here, we identify a cis-acting 100-nt fragment in the Tj 3′ UTR that is sufficient for producing artificial piRNAs from unintegrated DNA. These artificial piRNAs were effective in endogenous gene transcriptional silencing. Yb, a core component of primary piRNA biogenesis center Yb bodies, directly bound the Tj-cis element. Disruption of this interaction markedly reduced piRNA production. Thus, Yb is the trans-acting partner of the Tj-cis element. Yb-CLIP revealed that Yb binding correlated with somatic piRNA production but Tj-cis element downstream sequences produced few artificial piRNAs. We thus propose that Yb determines primary piRNA sources through two modes of action: primary binding to cis elements to specify substrates and secondary binding to downstream regions to increase diversity in piRNA populations. [Display omitted] •cis elements for producing somatic primary piRNAs were identified•Yb primary binding to cis elements in piRNA precursors initiates piRNA production•Yb secondary binding downstream of cis elements does not initiate piRNA production•Artificial piRNAs from the Yb-cis-element system elicit transcriptional silencing piRNAs protect the germline genome from transposons, although the underlying mechanism remains elusive. Ishizu et al. focused on understanding how piRNA sources are selectively determined from all cellular RNAs. They successfully identified the particular RNA elements necessary and sufficient for producing piRNAs and determined Yb protein as the trans-acting partner.