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RNA elements in precursors instruct directional and processive primary piRNA biogenesis

ABSTRACT: PIWI-interacting RNAs (piRNAs) are animal gonad-specific small RNAs that control the activity of transposable elements. Long single stranded RNAs from a variety of sources are substrates for the nebulous primary processing pathway that converts these into thousands of 24-30 nucleotide (nt) piRNAs. How these transcripts are selected as precursors is not known. Here we show that targeting a transcript with PIWI slicer activity of cysosolic Ago3 is sufficient to trigger ~30-nt waves of non-overlapping primary piRNAs in the fly ovarian germline. The generated primary piRNAs are almost exclusively loaded into the nuclear PIWI protein, Piwi. In the fly ovarian somatic environment we find that an RNA fragment from the 5′ end of a piRNA cluster is able to direct a heterologous sequence into primary processing. This piRNA trigger sequence (PTS) element drives generation of overlapping piRNAs from the transcript. Both mechanisms proceed with general 5′-3′ directionality. We propose that the former pathway serves to link cytoplasmic silencing of a target to nuclear transcriptional repression, while the latter extracts silencing information from a wide variety of genomic sources including piRNA clusters, select protein coding and transposon transcripts.

ORGANISM(S): Mus musculus Bombyx mori Drosophila melanogaster

PROVIDER: GSE69102 | GEO | 2015/07/09

SECONDARY ACCESSION(S): PRJNA284571

REPOSITORIES: GEO

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RNA elements in precursors instruct directional and processive primary piRNA biogenesis

Project description:PIWI-interacting RNAs (piRNAs) are animal gonad-specific small RNAs that control the activity of transposable elements. Long single stranded RNAs from a variety of sources are substrates for the nebulous primary processing pathway that converts these into thousands of 24-30 nucleotide (nt) piRNAs. How these transcripts are selected as precursors is not known. Here we show that targeting a transcript with PIWI slicer activity of cysosolic Ago3 is sufficient to trigger ~30-nt waves of non-overlapping primary piRNAs in the fly ovarian germline. The generated primary piRNAs are almost exclusively loaded into the nuclear PIWI protein, Piwi. In the fly ovarian somatic environment we find that an RNA fragment from the 5? end of a piRNA cluster is able to direct a heterologous sequence into primary processing. This piRNA trigger sequence (PTS) element drives generation of overlapping piRNAs from the transcript. Both mechanisms proceed with general 5?-3? directionality. We propose that the former pathway serves to link cytoplasmic silencing of a target to nuclear transcriptional repression, while the latter extracts silencing information from a wide variety of genomic sources including piRNA clusters, select protein coding and transposon transcripts. Total or immunoprecipitated small RNAs were purified from transfected BmN4 cells, Drosophila ovarian somatic cells (OSC) and from fly ovaries and high-throughput sequencing libraries were prepared. The mouse testicular RNAs were purified after ribozero treatment.

2015-07-09 | E-GEOD-69102 | biostudies-arrayexpress

Recruitment of Armitage and Yb to a transcript triggers its phased processing into primary piRNAs in Drosophila ovaries

Project description:Small RNAs called PIWI -interacting RNAs (piRNAs) are essential for transposon control and fertility in animals. Primary processing is the small RNA biogenesis pathway that uses long single-stranded RNA precursors to generate millions of individual piRNAs, but the molecular mechanisms that identify a transcript as a precursor are poorly understood. Here we demonstrate that artificial tethering of the piRNA biogenesis factor, Armi, to a transcript is sufficient to direct it into primary processing in Drosophila ovaries and in an ovarian cell culture model. In the fly ovarian somatic follicle cells, the transcript becomes cleaved in a stepwise manner, with a 5ʹ→3ʹ directionality, liberating U1-containing ~24 nt piRNAs that are loaded into Piwi. Although uridines are preferred for generation of piRNA 5ʹ ends, processing takes place even in their absence, albeit at a lower efficiency. We show that recombinant Armi has 5ʹ→3ʹ helicase activity, and mutations that abolish it reduce piRNA processing. Another somatic piRNA pathway factor Yb, and an interactor of Armi, is also able to trigger piRNA biogenesis when tethered to a transcript. Tethering-mediated primary piRNA biogenesis is also functional in the fly ovarian germline and loads all the three PIWI proteins present in this environment. Our study finds a broad correlation between piRNA processing and localization of the tethered factors to the cytoplasmic perinuclear ribonucleoprotein granules called germline nuage or somatic Yb bodies. We conclude that transcripts bound by Armi and Yb are identified as piRNA precursors, resulting in localization to cytoplasmic processing granules and their subsequent engagement by the resident piRNA biogenesis machinery.

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Somatic Primary piRNA Biogenesis Driven by Cis-Acting RNA Elements and Trans- Acting Yb

Project description: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. Thus, Yb determines primary piRNA sources by two modes of action; primary binding to cis-elements to specify substrates, and secondary binding to downstream regions to increase diversity in piRNA populations. HITS-CLIP of Yb in OSCs (Ovarian Somatic Cells) depleted for tj cis-element, and small RNA sequencing of Piwi-piRNAs in OSCs depleted for tj cis-element.

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Vreteno, a gonad-specific protein, is essential for germline development and primary piRNA biogenesis in Drosophila

Project description:In Drosophila, Piwi proteins associate with Piwi-interacting RNAs (piRNAs) and protect the germline genome by silencing mobile genetic elements. This defense system acts in germline and gonadal somatic tissue to preserve germline development. Genetic control for these silencing pathways varies greatly between tissues of the gonad. Here, we identified Vreteno (Vret), a novel gonad-specific protein essential for germline development. Vret is required for piRNA-based transposon regulation in both germline and somatic gonadal tissues. We show that Vret, which contains Tudor domains, associates physically with Piwi and Aubergine (Aub), stabilizing these proteins via a gonad-specific mechanism, absent in other fly tissues. In the absence of vret, Piwi-bound piRNAs are lost without changes in piRNA precursor transcript production, supporting a role for Vret in primary piRNA biogenesis. In the germline, piRNAs can engage in an Aub/Argonaute 3 (AGO3)-dependent amplification in the absence of Vret, suggesting that Vret function can distinguish between primary piRNAs loaded into Piwi/Aub complexes and piRNAs engaged in the amplification cycle. We propose that Vret acts at an early step in primary piRNA processing where it plays an essential role in transposon regulation. These studies show that vreteno (vret) has a role in germline development and primary piRNA regulation in Drosophila.

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Analysis of small RNAs in piRNA pathway mutant mouse testes

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PIWI slicing and EXD1 drive biogenesis of nuclear piRNAs from cytosolic targets of the mouse piRNA pathway

Project description:PIWI-interacting RNAs (piRNAs) guide PIWI proteins to suppress transposable elements in animal gonads. Here we demonstrate that in the mouse embryonic male germline, endonucleolytic cleavage (slicing) of a transcript by cytosolic MILI acts as a trigger to initiate its further 5??3? processing into non-overlapping fragments. These fragments accumulate as new piRNAs within the nuclear PIWI protein MIWI2. We identify Exonuclease domain-containing 1 (EXD1) as a partner of the established MIWI2 piRNA biogenesis factor TDRD12. Although EXD1 homodimers are inactive as a nuclease, it functions as an RNA adapter within a PET (PIWI-EXD1-Tdrd12) complex. Loss of Exd1 impacts biogenesis of MIWI2 piRNAs and displays a reduction in sequences generated by MILI slicing. This results in selective depletion of repeat piRNAs that target active retrotransposons like LINE1, which are de-repressed in the mutant. We propose that PIWI slicing and EXD1 promote coordination of nucleo-cytoplasmic silencing via piRNA biogenesis.

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