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An epigenetic activation role of Piwi and a Piwi-associated piRNA in Drosophila melanogaster

Nature volume 450, pages 304–308 (2007)Cite this article

Abstract

Heterochromatin, representing the silenced state of transcription, consists largely of transposon-enriched and highly repetitive sequences. Implicated in heterochromatin formation and transcriptional silencing in Drosophila are Piwi (P-element induced wimpy testis)1,2 and repeat-associated small interfering RNAs (rasiRNAs)3,4,5. Despite this, the role of Piwi in rasiRNA expression and heterochromatic silencing remains unknown. Here we report the identification and characterization of 12,903 Piwi-interacting RNAs (piRNAs) in Drosophila, showing that rasiRNAs represent a subset of piRNAs. We also show that Piwi promotes euchromatic histone modifications and piRNA transcription in subtelomeric heterochromatin (also known as telomere-associated sequence, or TAS), on the right arm of chromosome 3 (3R-TAS). Piwi binds to 3R-TAS and a piRNA uniquely mapped to 3R-TAS (3R-TAS1 piRNA). In piwi mutants, 3R-TAS loses euchromatic histone modifications yet accumulates heterochromatic histone modifications and Heterochromatin Protein 1a (HP1a). Furthermore, the expression of both the 3R-TAS1 piRNA and a white reporter gene in 3R-TAS becomes suppressed. A P element inserted 128 base pairs downstream of the 3R-TAS1 piRNA coding sequence restores the euchromatic histone modifications of 3R-TAS and the expression of 3R-TAS1 piRNA in piwi mutants, as well as partly rescuing their defects in germline stem-cell maintenance. These observations suggest that Piwi promotes the euchromatic character of 3R-TAS heterochromatin and its transcriptional activity, opposite to the known roles of Piwi and the RNA-mediated interference pathway in epigenetic silencing. This activating function is probably achieved through interaction with at least 3R-TAS1 piRNA and is essential for germline stem-cell maintenance.

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Figure 1: Identification and characterization of Piwi-associated piRNAs.
Figure 2: The expression of 3R-TAS1 piRNA is Piwi-dependent and correlates with germline stem-cell maintenance.
Figure 3: Piwi protein is associated with 3R-TAS and is required for its euchromatic state.
Figure 4: piwi is an enhancer of telomere position effect.

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Acknowledgements

We thank L. Rusche for discussions and help with quantitative PCR; S. Sweeney for RNase protection assays for TAS2 piRNA, HeT-A1 piRNA and 2R-42AB-B1 piRNA; R. Levis for w1118;P{w+,ry+}A4-4 stocks; and S. Elgin, T. Hsieh, E. Beyret, H. Megosh, S. Findley, M. Nolde and V. Ganguraju for comments on the manuscript. This study was supported by the NIH and the Mathers Foundation.

Author Contributions H.Y. contributed to project design, all experimental work, data analysis and manuscript preparation. H.L. contributed to project design, data analysis and manuscript preparation.

The Piwi-associated piRNA sequences have been deposited in the NCBI Gene Expression Omnibus with the accession number GSE9138.

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Authors and Affiliations

  1. Department of Cell Biology, Duke University Medical School, Durham, North Carolina 27710, USA,

    Hang Yin & Haifan Lin

  2. Yale Stem Cell Center and Department of Cell Biology, Yale University School of Medicine, New Haven, Connecticut 06509, USA,

    Hang Yin & Haifan Lin

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Correspondence to Haifan Lin.

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The file contains Supplementary Results, Supplementary Discussion, Supplementary Figures S1-S11 and Supplementary Tables 1-2 and additional references. (PDF 3088 kb)

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Yin, H., Lin, H. An epigenetic activation role of Piwi and a Piwi-associated piRNA in Drosophila melanogaster. Nature 450, 304–308 (2007). https://doi.org/10.1038/nature06263

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