MCB Accepts, published online ahead of print on 8 January 2007

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Mol. Cell. Biol. doi:10.1128/MCB.01509-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

On the mechanism of histone H1-stimulated glucocorticoid receptor DNA binding in vivo

Sergey Belikov, Carolina Åstrand, and Örjan Wrange^*

Dept. of Cell and Molecular Biology, Karolinska Institutet, SE-17177 Stockholm, Sweden

^* To whom correspondence should be addressed. Email: orjan.wrange{at}ki.se.

Xenopus oocytes lack somatic linker histone H1 but contain an oocyte specific variant, B4. The glucocorticoid receptor (GR) inducible mouse mammary tumor virus (MMTV) promoter was reconstituted in Xenopus oocytes to address the effects of histone H1. The expression of Xenopus H1A (H1) via cytoplasmic mRNA injection resulted in H1 incorporation into in vivo assembled chromatin based on: i) The appearance of a chromatosome stop. ii) The increased nucleosome repeat length (NRL). iii) H1-DNA binding assayed by chromatin immunopercipitation (ChIP). The H1-effect on the NRL was saturable and hence represents H1-binding to a specific site. A subsaturating level of H1 enhanced the hormone-dependent binding of GR to the glucocorticoid response elements (GREs) and the hormone-dependent MMTV transcription while it reduced the access to DNA as revealed by micrococcal nuclease (MNase) analysis. These H1-effects were lost at higher levels of H1. ChIP and MNase analysis revealed a hormone-dependent dissociation of H1 from the activated chromatin domain. The proposed mechanism of H1-induced GR binding is based on two effects: i) A GR-induced asymmetric distribution of H1 in favor of inactive chromatin. ii) An H1-induced reduction in DNA access. These effects results in increased concentration of free GR and hence in increased GR-GRE binding.

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