PIWI Slicing and RNA Elements in Precursors Instruct Directional Primary piRNA Biogenesis.

PIWI proteins and PIWI-interacting RNAs (piRNAs) mediate repression of transposons in the animal gonads. Primary processing converts long single-stranded RNAs into ∼30-nt piRNAs, but their entry into the biogenesis pathway is unknown. Here, we demonstrate that an RNA element at the 5' end of a piRNA cluster­which we termed piRNA trigger sequence (PTS)­can induce primary processing of any downstream sequence. We propose that such signals are triggers for the generation of the original pool of piRNAs. We also demonstrate that endonucleolytic cleavage of a transcript by a cytosolic PIWI results in its entry into primary processing, which triggers the generation of non-overlapping, contiguous primary piRNAs in the 3' direction from the target transcript. These piRNAs are loaded into a nuclear PIWI, thereby linking cytoplasmic post-transcriptional silencing to nuclear transcriptional repression.

History of the discovery of a master locus producing piRNAs: the flamenco/COM locus in Drosophila melanogaster.

The discovery of transposable elements (TEs) in the 1950s by B. McClintock implied the existence of cellular regulatory systems controlling TE activity. The discovery of flamenco (flam) an heterochromatic locus from Drosophila melanogaster and its ability to survey several TEs such as gypsy, ZAM, and Idefix contributed to peer deeply into the mechanisms of the genetic and epigenetic regulation of TEs. flam was the first cluster producing small RNAs to be discovered long before RNAi pathways were identified in 1998. As a result of the detailed genetic analyses performed by certain laboratories and of the sophisticated genetic tools they developed, this locus has played a major role in our understanding of piRNA mediated TE repression in animals. Here we review the first discovery of this locus and retrace decades of studies that led to our current understanding of the relationship between genomes and their TE targets.

Transcriptional properties and splicing of the flamenco piRNA cluster.

In Drosophila, the piRNA cluster, flamenco, produces most of the piRNAs (PIWI-interacting RNAs) that silence transposable elements in the somatic follicle cells during oogenesis. These piRNAs are thought to be processed from a long single-stranded precursor transcript. Here, we demonstrate that flamenco transcription is initiated from an RNA polymerase II promoter containing an initiator motif (Inr) and downstream promoter element (DPE) and requires the transcription factor, Cubitus interruptus. We show that the flamenco precursor transcript undergoes differential alternative splicing to generate diverse RNA precursors that are processed to piRNAs. Our data reveal dynamic processing steps giving rise to piRNA cluster precursors.

Spatio-temporal requirements for transposable element piRNA-mediated silencing during Drosophila oogenesis.

During Drosophila oogenesis, transposable element (TE) repression involves the Piwi-interacting RNA (piRNA) pathway which ensures genome integrity for the next generation. We developed a transgenic model to study repression of the Idefix retrotransposon in the germline. Using a candidate gene KD-approach, we identified differences in the spatio-temporal requirements of the piRNA pathway components for piRNA-mediated silencing. Some of them (Aub, Vasa, Spn-E) are necessary in very early stages of oogenesis within the germarium and appear to be less important for efficient TE silencing thereafter. Others (Piwi, Ago3, Mael) are required at all stages of oogenesis. Moreover, during early oogenesis, in the dividing cysts within the germarium, Idefix anti-sense transgenes escape host control, and this is associated with very low piwi expression. Silencing of P-element-based transgenes is also strongly weakened in these cysts. This region, termed the 'Piwiless pocket' or Pilp, may ensure that new TE insertions occur and are transmitted to the next generation, thereby contributing to genome dynamics. In contrast, piRNA-mediated silencing is strong in germline stem cells in which TE mobilization is tightly repressed ensuring the continued production of viable germline cysts.