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Drosophila H1 Regulates the Genetic Activity of Heterochromatin by Recruitment of Su(var)3-9

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Science  05 Apr 2013:
Vol. 340, Issue 6128, pp. 78-81
DOI: 10.1126/science.1234654

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Silencing Transposons

Eukaryotic DNA is packaged onto nucleosomes, which are composed of four core histones (H2A, H2B, H3, and H4). Chromatin also contains a fifth histone, H1, which binds to both the core particles and the “linker” DNA that joins adjacent nucleosomes, where it helps chromatin to fold into higher-order structures and generally silences gene expression. In Drosophila germline and somatic cells and tissues, Lu et al. (p. 78) found that the repressive function of H1 in vivo was directed toward transposable elements and independent of small RNA silencing pathways. Instead, H1 acted through direct recruitment of the histone methyltransferase Su(var)3–9, which methylates histone H3 lysine 9, a repressive histone mark.

Abstract

Eukaryotic genomes harbor transposable elements and other repetitive sequences that must be silenced. Small RNA interference pathways play a major role in their repression. Here, we reveal another mechanism for silencing these sequences in Drosophila. Depleting the linker histone H1 in vivo leads to strong activation of these elements. H1-mediated silencing occurs in combination with the heterochromatin-specific histone H3 lysine 9 methyltransferase Su(var)3-9. H1 physically interacts with Su(var)3-9 and recruits it to chromatin in vitro, which promotes H3 methylation. We propose that H1 plays a key role in silencing by tethering Su(var)3-9 to heterochromatin. The tethering function of H1 adds to its established role as a regulator of chromatin compaction and accessibility.

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