Molecular Cell
Volume 63, Issue 3, 4 August 2016, Pages 408-419
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Article
Piwi Modulates Chromatin Accessibility by Regulating Multiple Factors Including Histone H1 to Repress Transposons

https://doi.org/10.1016/j.molcel.2016.06.008Get rights and content
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Highlights

  • Piwi forms a complex with H1 to regulate H1 association to target transposon loci

  • H1 regulates Piwi target transposons independently of H3K9me3 modification

  • Piwi transcriptionally silences its targets by modulating chromatin accessibility

  • H1 and HP1a interdependently regulate chromatin accessibility at Piwi target loci

Summary

PIWI-interacting RNAs (piRNAs) mediate transcriptional and post-transcriptional silencing of transposable element (TE) in animal gonads. In Drosophila ovaries, Piwi-piRNA complexes (Piwi-piRISCs) repress TE transcription by modifying the chromatin state, such as by H3K9 trimethylation. Here, we demonstrate that Piwi physically interacts with linker histone H1. Depletion of Piwi decreases H1 density at a subset of TEs, leading to their derepression. Silencing at these loci separately requires H1 and H3K9me3 and heterochromatin protein 1a (HP1a). Loss of H1 increases target loci chromatin accessibility without affecting H3K9me3 density at these loci, while loss of HP1a does not impact H1 density. Thus, Piwi-piRISCs require both H1 and HP1a to repress TEs, and the silencing is correlated with the chromatin state rather than H3K9me3 marks. These findings suggest that Piwi-piRISCs regulate the interaction of chromatin components with target loci to maintain silencing of TEs through the modulation of chromatin accessibility.

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