Asymmetric Linker Histone Association Directs the Asymmetric Rearrangement of Core Histone Interactions in a Positioned Nucleosome Containing a Thyroid Hormone Response Element

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Abstract
We describe histone−DNA cross-linking in a positioned nucleosome containing a thyroid hormone response element (TRE) from the Xenopuslaevis thyroid hormone receptor βA gene (TRβA). Histones H3 and H4 are cross-linked to DNA in the nucleosome core within 30 base pairs to either side of the dyad axis. Histone H2A cross-links to DNA in the core at the dyad axis, and histones H2A and H2B have extensive interactions with DNA 40−80 bp away from the dyad axis. Linker histone H5 and the globular domain of Xenopus H1° associate asymmetrically with DNA at one edge of the TRβA nucleosome. Nevertheless, the asymmetric association of H5 leads to a significant rearrangement of core histone−DNA contacts at the dyad axis of the nucleosome. In the presence of linker histone, cross-linkings of H4 within 15 bp to one side of the dyad axis, of histone H2A at the dyad axis, and of H2A and H2B 40−80 bp to one side of the dyad axis are all reduced. This reduction in cross-linking occurs preferentially on the side of the nucleosome to which H5 is bound. Our results indicate that core histone contacts within mononucleosomes are conformationally dynamic and that linker histone incorporation at the edge of the nucleosome can influence core histone−DNA interactions in an asymmetric way including contacts at the dyad axis.