Structure and dynamic properties of a glucocorticoid receptor-induced chromatin transition

Activation of the mouse mammary tumor virus (MMTV) promoter by the glucocor ticoid receptor (GR) is associated with a chromatin structural transition i n the B nucleosome region of the viral long terminal repeat (LTR), Recent e vidence indicates that this transition extends upstream of the B nucleosome , encompassing a region larger than a single nucleosome (G. Fragoso, W, D, Pennie, S, John, and G, L, Eager, Mol. Cell. Biol, 18:3633-3644). We have r econstituted MMTV LTR DNA into a polynucleosome array using Drosophila embr yo extracts. We show binding of purified GR to specific GR elements within a large, multinucleosome array and describe a GR-induced nucleoprotein tran sition that is dependent on ATP and a HeLa nuclear extract. Previously unch aracterized GR binding sites in the upstream C nucleosome region are involv ed in the extended region of chromatin remodeling. We also show that GR-dep endent chromatin remodeling is a multistep process; in the absence of ATP, GR binds to multiple sites on the chromatin array end prevents restriction enzyme access to recognition sites. Upon addition of ATP, GR induces remode ling and a large increase in access to enzymes sites within the transition region. These findings suggest a dynamic model in which CR first binds to c hromatin after ligand activation, recruits a remodeling activity, and is th en lost from the template. This model is consistent with the recent descrip tion of a "hit-and-run" mechanism for GR action in living cells (J, G, McNa lly, W, G. Muller, D, Walker, and G, L, Hager, Science 287:1262-1264, 2000) .