Getting a grip on piRNA cluster transcription

Abstract

The generation of piRNAs from long primary transcripts requires specialized factors that distinguish these precursors from canonical RNA polymerase II transcripts. Mohn et al. and Zhang et al. provide evidence that in Drosophila melanogaster noncanonical transcription coupled with splicing inhibition differentiates piRNA precursors from mRNAs and ensures their correct processing.

Figure 1

Model for Dual-Strand piRNA Cluster Expression in Drosophila melanogaster (A) Dual-strand cluster (blue box) transcription is achieved by read-through transcription from convergent neighboring genes (green boxes) or by noncanonical transcription inititation by RNA polymerase II (Pol II). piRNA-mediated recruitment of Piwi to some source loci leads to H3K9 trimethylation (me3) by Eggless (Egg) and subsequent Rhino (Rhi) recruitment in complex with Deadlock (Del) and Cuttoff (Cuff). This licenses dual-strand cluster expression. (B) Rhi binding to chromatin brings Cuff into close proximity to the newly formed 5′ end of a nascent piRNA precursor transcript after the upstream transcript has undergone 3′ end processing. Cuff binding prevents degradation of the transcript and probably inhibits splicing together with UAP56, which marks the precursor for export and processing in perinuclear bodies.