Rapid purification of intact minichromosomes over a glycerol cushion

Chromatin remodeling and transcriptional activation: the cast (in order of appearance)

The number of chromatin modifying and remodeling complexes implicated in genome control is growing faster than our understanding of the functional roles they play. We discuss recent in vitro experiments with biochemically defined chromatin templates that illuminate new aspects of action by histone acetyltransferases and ATP-dependent chromatin remodeling engines in facilitating transcription. We review a number of studies that present an 'ordered recruitment' view of transcriptional activation, according to which various complexes enter and exit their target promoter in a set sequence, and at specific times, such that action by one complex sets the stage for the arrival of the next one. A consensus emerging from all these experiments is that the joint action by several types of chromatin remodeling machines can lead to a more profound alteration of the infrastructure of chromatin over a target promoter than could be obtained by these enzymes acting independently. In addition, it appears that in specific cases one type of chromatin structure alteration (e.g., histone hyperacetylation) is contingent upon prior alterations of a different sort (i.e., ATP-dependent remodeling of histone-DNA contacts). The striking differences between the precise sequence of action by various cofactors observed in these studies may be - at least in part - due to differences between the specific promoters studied, and distinct requirements exhibited by specific loci for chromatin remodeling based on their pre-existing nucleoprotein architecture.

ATP-driven chromatin remodeling activity and histone acetyltransferases act sequentially during transactivation by RAR/RXR In vitro

Using a "crude" chromatin-based transcription system that mimics transactivation by RAR/RXR heterodimers in vivo, we could not demonstrate that chromatin remodeling was required to relieve nucleosomal repression. Using "purified" chromatin templates, we show here that, irrespective of the presence of histone H1, both ATP-driven chromatin remodeling activities and histone acetyltransferase (HAT) activities of coactivators recruited by liganded receptors are required to achieve transactivation. DNA footprinting, ChIP analysis, and order of addition experiments indicate that coactivator HAT activities and two ATP-driven remodeling activities are sequentially involved at distinct steps preceding initiation of transcription. Thus, both ATP-driven chromatin remodeling and HAT activities act in a temporally ordered and interdependent manner to alleviate the repressive effects of nucleosomal histones on transcription by RARalpha/RXRalpha heterodimers.

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

Activation of the mouse mammary tumor virus (MMTV) promoter by the glucocorticoid receptor (GR) is associated with a chromatin structural transition in the B nucleosome region of the viral long terminal repeat (LTR). Recent evidence 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. Hager, Mol. Cell. Biol. 18:3633-3644). We have reconstituted MMTV LTR DNA into a polynucleosome array using Drosophila embryo extracts. We show binding of purified GR to specific GR elements within a large, multinucleosome array and describe a GR-induced nucleoprotein transition that is dependent on ATP and a HeLa nuclear extract. Previously uncharacterized GR binding sites in the upstream C nucleosome region are involved in the extended region of chromatin remodeling. We also show that GR-dependent chromatin remodeling is a multistep process; in the absence of ATP, GR binds to multiple sites on the chromatin array and prevents restriction enzyme access to recognition sites. Upon addition of ATP, GR induces remodeling and a large increase in access to enzymes sites within the transition region. These findings suggest a dynamic model in which GR first binds to chromatin after ligand activation, recruits a remodeling activity, and is then lost from the template. This model is consistent with the recent description of a "hit-and-run" mechanism for GR action in living cells (J. G. McNally, W. G. Müller, D. Walker, and G. L. Hager, Science 287:1262-1264, 2000).