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Abstract
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), and (iii) H1-DNA binding assayed by chromatin immunoprecipitation (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 and (ii) an H1-induced reduction in DNA access. These effects results in increased concentration of free GR and, hence, in increased GR-GRE binding.
SUPPLEMENTAL MATERIAL
This work was supported by grants to Ö.W. and S.B. from the Swedish Cancer Foundation (project 2222-B05-21XBB), to Ö.W. from the Medical Research Council (31BI-15338-01A), to Ö.W. from the Knut and Alice Wallenberg Foundation, and to S.B. from the Medical Research Council (project K2006-31X-20075-01-3). Ö.W. is an associate member of the EU NoE, The Epigenome.
We are grateful to Kiyoe Ura for a great deal of encouragement and support and for kindly providing cDNAs for xB4, and we are grateful to Ohsumi Keita for the generous gift of xB4 antiserum and the xH1A cDNA. We also thank Qiao Li and Ulla Björk for the construction of pGo2.5Go-39MTV:M13, and we are grateful to Per-Henrik Holmqvist for helpful discussions and support during this work.