DNA instructed displacement of histones H2A and H2B at an inducible promoter

The ins and outs of ATP-dependent chromatin remodeling in budding yeast: biophysical and proteomic perspectives

ATP-dependent chromatin remodeling is performed by multi-subunit protein complexes. Over the last years, the identity of these factors has been unveiled in yeast and many parallels have been drawn with animal and plant systems, indicating that sophisticated chromatin transactions evolved prior to their divergence. Here we review current knowledge pertaining to the molecular mode of action of ATP-dependent chromatin remodeling, from single molecule studies to genome-wide genetic and proteomic studies. We focus on the budding yeast versions of SWI/SNF, RSC, DDM1, ISWI, CHD1, INO80 and SWR1.

Nucleolin: a multiFACeTed protein

Nucleolin is an abundant, ubiquitously expressed protein that is found in various cell compartments, especially in the nucleolus, of which it is a major component. This multifunctional protein has been described as being a part of many pathways, from interactions with viruses at the cellular membrane to essential processing of the ribosomal RNA in the nucleolus. However, most of the molecular details of these different functions are not understood. Here, we focus on the role of nucleolin in transcription, especially some recent findings describing the protein as a histone chaperone [with functional similarity to the facilitates chromatin transcription (FACT) complex] and a chromatin co-remodeler. These new properties could help reconcile discrepancies in the literature regarding the role of nucleolin in transcription.

Activation domains drive nucleosome eviction by SWI/SNF

ATP-dependent chromatin remodeling complexes play a critical role in chromatin dynamics. A large number of in vitro studies have pointed towards nucleosome sliding as the principal remodeling outcome of SWI/SNF action, whereas few have described histone octamer transfer as the principal outcome. In contrast, recent in vivo studies have linked the activity of SWI/SNF to histone eviction in trans from gene promoters. In this study, we have found that the chimeric transcription factor Gal4-VP16 can enhance SWI/SNF histone octamer transfer activity, resulting in targeted histone eviction from a nucleosome probe. This effect is dependent on the presence of the activation domain. We observed that under conditions mimicking the in vivo relative abundance of SWI/SNF with respect to the total number of nucleosomes in a cell nucleus, the accessibility of the transcription factor binding site is the first determinant in the sequence of events leading to nucleosome remodeling. We propose a model mechanism for this transcription factor-mediated enhancement of SWI/SNF octamer transfer activity.

Is histone loss a common feature of DNA metabolism regulation?

Chromatin modifications play a crucial role in regulating DNA metabolism. Chromatin structures can be remodeled by covalently modifying histones, by shifting nucleosomes along the DNA, and by changing the histone composition of nucleosomes. Lately, nucleosome displacement has been extensively described within transcribed genes and DNA breaks. This review focuses on recently published work that describes the relationships between histone modification/exchange and nucleosome displacement.

Mechanism of histone H1-stimulated glucocorticoid receptor DNA binding in vivo

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 H1o [corrected] (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.

Induction of progesterone target genes requires activation of Erk and Msk kinases and phosphorylation of histone H3

How genes are regulated in the context of chromatin is a central question of biology. Steroid hormones control gene expression via interaction of their receptors with target sequences on DNA but can also activate cytoplasmic signaling cascades. Here we report that rapid Erk activation by progestins participates in induction of target genes by preparing the chromatin for transcription. Five minutes after hormone treatment, Erk activation leads to phosphorylation of the progesterone receptor (PR), activation of Msk1, and recruitment of a complex of the three proteins to a nucleosome on the MMTV promoter. Msk1 phosphorylates histone H3, leading to displacement of HP1gamma and recruitment of Brg1 and RNA polymerase II. Cell-free experiments show a direct interaction between PR, Erk, and Msk1 and support the importance of H3 phosphorylation for nucleosome remodeling. Inhibition of Msk1 activation blocks recruitment of the kinase complex, H3 phosphorylation, and HP1gamma displacement, thus precluding remodeling and induction of the promoter.

Progesterone signaling in breast and endometrium

In addition to transcriptional effects, steroid hormones rapidly activate cytoplasmic signaling cascades. The ultimate targets of these cascades are not well-defined and likely include transcription factors and coactivators. To better understand the role of the rapid "non-transcriptional" effects of progestins, we investigated the mechanisms leading to activation of these pathways and their relevance in the biological response, using two model systems: breast cancer and endometrial stromal cells. Our results demonstrated that progestins rapidly activate the Src/Erk1/2 and PI3K/Akt pathways in both cellular types via crosstalk between PR and ERalpha or ERbeta. This activation is essential for triggering proliferative response. However, even when the activation of kinase cascades is similar in both cellular types, the biological outcome of progestin treatment is different. A different ability of PR to mediate transcriptional effects might account for this discrepancy. Also differences in amount and subcellular location of PR, presence of ERalpha or ERbeta and alternative receptors could be also important for determining the cellular response. We also explored the connection between rapid activation of kinase cascades and transcriptional induction by progestins. Our results uncover a novel function of the rapid Erk activation by progestins, namely its direct involvement in transcriptional induction of MMTV promoter and other progesterone-target genes.

Epigenetic aberrations and cancer

The correlation between epigenetic aberrations and disease underscores the importance of epigenetic mechanisms. Here, we review recent findings regarding chromatin modifications and their relevance to cancer.

Chromatin Remodeling and Control of Cell Proliferation by Progestins via Cross Talk of Progesterone Receptor with the Estrogen Receptors and Kinase Signaling Pathways

Transcription from the mouse mammary tumor virus (MMTV) promoter can be induced by glucocorticoids or progestins. Progesterone treatment of cultured cells carrying an integrated single copy of an MMTV transgene leads to recruitment of progesterone receptor (PR), SWI/SNF, and SNF2h-related complexes to MMTV promoter. Recruitment is accompanied by selective displacement of histones H2A and H2B from the nucleosome B. In nucleosomes assembled on promoter sequences, SWI/SNF displaces histones H2A and H2B from MMTV nucleosome B, but not from other MMTV nucleosomes or from an rDNA promoter nucleosome. Thus, the outcome of nucleosome remodeling by purified SWI/SNF depends on the DNA sequence. On the other hand, 5 min after hormone treatment, the cytoplasmic signaling cascade Src/Ras/Erk is activated via an interaction of PR with the estrogen receptor, which activates Src. As a consequence of Erk activation PR is phosphorylated, Msk1 is activated, and a ternary complex PR-Erk-Msk1 is recruited to MMTV nucleosome B. Msk1 phosphorylates H3 at serine 10, which is followed by acetylation at lysine 14, displacement of HP1gamma, and recruitment of Brg1, PCAF, and RNA polymerase II. Blocking Erk activation or Msk1 activity prevents induction of the MMTV transgene. Thus, the rapid nongenomic effects of progestins are essential for their transcriptional effects on certain progestin target genes. In rat endometrial stromal cells, picomolar concentrations of progestins trigger the cross talk of PR with ERbeta that activates the Erk and Akt kinase pathways leading to cell proliferation in the absence of direct transcriptional effects of the ligand-activated PR. Thus, depending on the cellular context rapid kinase activation and transcriptional effect play different roles in the physiological response to progestins.

Epigenetic determination of a cell-specific gene expression program by ATF-2 and the histone variant macroH2A

Transcriptional activation of the interleukin-8 (IL-8) gene is restricted to distinct cell types, although the transcriptional regulatory proteins controlling IL-8 gene expression are ubiquitous. We show that cell-specific transcription of IL-8 is due to the distinct chromatin architecture on the enhancer/promoter before the arrival of the inducing signal. In expressing epithelial cells the enhancer/promoter is nucleosome-free, whereas in non-expressing B cells a nucleosome masks the entire regulatory region. The B-cell-specific nucleosome contains the histone variant macroH2A, which is responsible for preventing transcription factor binding. Recruitment of the repressive macroH2A nucleosome requires direct interactions between ATF-2 bound to the nearby AP1 site and macroH2A and it is regulated by DNA-induced protein allostery. siRNA against ATF-2 or macroH2A rescues IL-8 transcription in B cells. Thus, a transcription factor can work as a transcriptional repressor by orchestrating and maintaining the assembly of specialized local chromatin architectures.

Nucleosome displacement in transcription

Recent reports reinforce the notion that nucleosomes are highly dynamic in response to the process of transcription. Nucleosomes are displaced at promoters during gene activation in a process that involves histone modification, ATP-dependent nucleosome remodeling complexes, histone chaperones and perhaps histone variants. During transcription elongation nucleosomes are acetylated and transferred behind RNA polymerase II where they are required to suppress spurious transcription initiation within the body of the gene. It is becoming increasingly clear that the eukaryotic transcriptional machinery is adapted to exploit the presence of nucleosomes in very sophisticated ways.

NXSensor web tool for evaluating DNA for nucleosome exclusion sequences and accessibility to binding factors

Nucleosomes, a basic structural unit of eukaryotic chromatin, play a significant role in regulating gene expression. We have developed a web tool based on DNA sequences known from empirical and theoretical studies to influence DNA bending and flexibility, and to exclude nucleosomes. NXSensor (available at ) finds nucleosome exclusion sequences, evaluates their length and spacing, and computes an ‘accessibility score’ giving the proportion of base pairs likely to be nucleosome-free. Application of NXSensor to the promoter regions of housekeeping (HK) genes and those of tissue-specific (TS) genes revealed a significant difference between the two classes of gene, the former being significantly more open, on average, particularly near transcription start sites (TSSs). NXSensor should be a useful tool in assessing the likelihood of nucleosome formation in regions involved in gene regulation and other aspects of chromatin function.

AFM imaging of protein movements: histone H2A-H2B release during nucleosome remodeling

Being able to follow assembly/disassembly reactions of biomolecular complexes directly at the single molecule level would be very useful. Here, we use an AFM technique that can simultaneously obtain topographic images and identify the locations of a specific type of protein within those images to monitor the histone H2A component of nucleosomes acted on by human Swi-Snf, an ATP-dependent nucleosome remodeling complex. Activation of remodeling results in significant H2A release from nucleosomes, based on recognition imaging and nucleosome height changes, and changes in the recognition patterns of H2A associated directly with hSwi-Snf complexes.

Probing SWI/SNF remodeling of the nucleosome by unzipping single DNA molecules

Chromatin-remodeling enzymes can overcome strong histone-DNA interactions within the nucleosome to regulate access of DNA-binding factors to the genetic code. By unzipping individual DNA duplexes, each containing a uniquely positioned nucleosome flanked by long segments of DNA, we directly probed histone-DNA interactions. The resulting disruption-force signatures were characteristic of the types and locations of interactions and allowed measurement of the positions of nucleosomes with 2.6-base-pair (bp) precision. Nucleosomes remodeled by yeast SWI/SNF were moved bidirectionally along the DNA, resulting in a continuous position distribution. The characteristic distance of motion was approximately 28 bp per remodeling event, and each event occurred with a catalytic efficiency of 0.4 min(-1) per nM SWI/SNF. Remodeled nucleosomes had essentially identical disruption signatures to those of unremodeled nucleosomes, indicating that their overall structure remained canonical. These results impose substantial constraints on the mechanism of SWI/SNF remodeling.

The human telomeric protein TRF1 specifically recognizes nucleosomal binding sites and alters nucleosome structure

Telomeres are dynamic nucleoprotein structures that cap the ends of eukaryotic chromosomes. In humans, the long (TTAGGG)(n) double-stranded telomeric DNA repeats are bound specifically by the two related proteins TRF1 and TRF2, and are organized in nucleosomes. Whereas the role of TRF1 and TRF2 in telomeric function has been studied extensively, little is known about the involvement of telomeric nucleosomes in telomere structures or how chromatin formation may affect binding of the TRFs. Here, we address the question of whether TRF1 is able to bind to telomeric binding sites in a nucleosomal context. We show that TRF1 is able to specifically recognize telomeric binding sites located within nucleosomes, forming a ternary complex. The formation of this complex is strongly dependent on the orientation of binding sites on the nucleosome surface, rather than on the location of the binding sites with respect to the nucleosome dyad. Strikingly, TRF1 binding causes alterations in nucleosome structure without dissociation of histone subunits. These results indicate that nucleosomes contribute to the establishment of a telomeric capping complex, whose structure and dynamics can be modulated by the binding of telomeric factors.

Nuclear factor-I regulates glial fibrillary acidic protein gene expression in astrocytes differentiated from cortical precursor cells

The elucidation of the transcriptional mechanisms that regulate glial fibrillary acidic protein (GFAP) gene expression is important for the understanding of the molecular mechanisms that control astrocyte differentiation during brain development. We investigated regulatory elements located in a proximal region of the GFAP promoter, important for expression in cortical precursor cells differentiating into astrocytes. One of these elements recognizes transcription factors of the nuclear factor-I family (NFI). We found that, in primary cultures of cortical cells, NFI occupies the GFAP promoter prior to the induction of astrocyte differentiation. In the developing cerebral cortex, the onset of expression of NFI coincides chronologically with the beginning of astrocytogenesis. Mutational analysis of the GFAP gene and transfections in primary cortical precursors show that inhibition of binding of NFI to the GFAP promoter results in decreased levels of transcriptional activity and is required for the synergistic stimulation of the GFAP promoter by the astrogenic agents, pituitary adenylate cyclase-activating polypeptide and ciliary neurotrophic factor, which in combination enhance astrocyte differentiation to generate astrocytes with longer processes. Thus, NFI appears to be an important factor for the integration of astrogenic stimuli in the developing central nervous system.

Nucleolin is a histone chaperone with FACT-like activity and assists remodeling of nucleosomes

Remodeling machines play an essential role in the control of gene expression, but how their activity is regulated is not known. Here we report that the nuclear protein nucleolin possesses a histone chaperone activity and that this factor greatly enhances the activity of the chromatin remodeling machineries SWI/SNF and ACF. Interestingly, nucleolin is able to induce the remodeling by SWI/SNF of macroH2A, but not of H2ABbd nucleosomes, which are otherwise resistant to remodeling. This new histone chaperone promotes the destabilization of the histone octamer, helping the dissociation of a H2A-H2B dimer, and stimulates the SWI/SNF-mediated transfer of H2A-H2B dimers. Furthermore, nucleolin facilitates transcription through the nucleosome, which is reminiscent of the activity of the FACT complex. This work defines new functions for histone chaperones in chromatin remodeling and regulation of transcription and explains how nucleolin could act on transcription.

A high mobility group protein binds to long CAG repeat tracts and establishes their chromatin organization in Saccharomyces cerevisiae

Long CAG repeat tracts cause human hereditary neurodegenerative diseases and have a propensity to expand during parental passage. Unusual physical properties of CAG repeat tracts are thought to contribute to their instability. We investigated whether their unusual properties alter the organization of CAG repeat tract chromatin. We report that CAG repeat tracts, embedded in yeast chromosomes, have a noncanonical chromatin organization. Digestion of chromatin with the restriction enzyme Fnu4HI reveals hypersensitive sites occurring approximately 125 bp apart in the repeat tract. To determine whether a non-histone protein establishes this pattern, we performed a yeast one-hybrid screen using CAG repeat tracts embedded in front of two reporter genes. The screen identified the high mobility group box protein Hmo1. Chromatin immunoprecipitation of epitope-tagged Hmo1 selectively precipitates CAG repeat tracts DNAs that range from 26 to 126 repeat units. Moreover, deletion of HMO1 drastically alters the Fnu4HI digestion pattern of CAG repeat chromatin. These results show that Hmo1 binds to CAG repeat tracts in vivo and establish the basis of their novel chromatin organization.

Tax-dependent displacement of nucleosomes during transcriptional activation of human T-cell leukemia virus type 1

The human T-cell leukemia virus type 1 (HTLV-1) is integrated into the host cell DNA and assembled into nucleosomes. Within the repressive chromatin environment, the virally encoded Tax protein mediates the recruitment of the coactivators CREB-binding protein/p300 to the HTLV-1 promoter, located within the long terminal repeats (LTRs) of the provirus. These proteins carry acetyltransferase activity that is essential for strong transcriptional activation of the virus in the context of chromatin. Consistent with this, the amino-terminal tails of nucleosomal histones at the viral promoter are acetylated in Tax-expressing cells. We have developed a system in which we transfect Tax into cells carrying integrated copies of the HTLV-1 LTR driving the luciferase gene to analyze changes in "activating" histone modifications at the LTR. Unexpectedly, Tax transactivation led to an apparent reduction of these modifications at the HTLV-1 promoter and downstream region that correlates with a similar reduction in histone H3 and linker histone H1. Micrococcal nuclease protection analysis showed that less LTR-luciferase DNA is nucleosomal in Tax-expressing cells. Furthermore, nucleosome depletion correlated with RNA polymerase II recruitment and loss of SWI/SNF. The M47 Tax mutant, deficient in HTLV-1 transcriptional activation, was also defective for nucleosome depletion. Although this mutant formed complexes with CREB and p300 at the HTLV-1 promoter in vivo, it was unable to mediate RNA polymerase II recruitment or SWI/SNF displacement. These results support a model in which nucleosomes are depleted from the LTR and transcribed region during Tax-mediated transcriptional activation and correlate RNA polymerase II recruitment with nucleosome depletion.

Regulation of nucleosome stability as a mediator of chromatin function

Interactions among regulatory proteins, enzymes and DNA are required to use the information encoded in genomes. In eukaryotes, the location and timing of interactions between these proteins and DNA is determined in large part by whether DNA at a given locus is packaged into a nucleosome. Given the central role of nucleosome formation in regulating genome function, many mechanisms have evolved to control nucleosome stability at loci across the genome. New conclusions about the role of the DNA sequence itself in the regulation of nucleosome stability have come from two new types of experiment: high-resolution mapping of in vivo nucleosome occupancy on a genomic scale; and in vivo versus in vitro comparisons of nucleosome stability on natural DNA templates. These new types of data raise intriguing questions about the evolutionary constraints on DNA sequence with regard to nucleosome formation, and at long last might enable the derivation of DNA sequence-based rules that produce reliable predictions of nucleosome behavior.

Human SWI/SNF generates abundant, structurally altered dinucleosomes on polynucleosomal templates

Human SWI/SNF (hSWI/SNF) is an evolutionarily conserved ATP-dependent chromatin remodeling complex required for transcriptional regulation and cell cycle control. The regulatory functions of hSWI/SNF are correlated with its ability to create a stable, altered form of chromatin that constrains fewer negative supercoils than normal. Our current studies indicate that this change in supercoiling is due to the conversion of up to one-half of the nucleosomes on polynucleosomal arrays into asymmetric structures, termed "altosomes," each composed of two histone octamers and bearing an asymmetrically located region of nuclease-accessible DNA. Altosomes can be formed on chromatin containing the abundant mammalian linker histone H1 and have a unique micrococcal nuclease digestion footprint that allows their position and abundance on any DNA sequence to be measured. Over time, altosomes spontaneously revert to structurally normal but improperly positioned nucleosomes, suggesting a novel mechanism for transcriptional attenuation as well as transcriptional memory following hSWI/SNF action.

Nuclear receptor coactivators: the key to unlock chromatin

The biological effects of hormones, ranging from organogenesis, metabolism, and proliferation, are transduced through nuclear receptors (NRs). Over the last decade, NRs have been used as a model to study transcriptional control. The conformation of activated NRs is favorable for the recruitment of coactivators, which promote transcriptional activation by directly communicating with chromatin. This review will focus on the function of different classes of coactivators and associated complexes, and on progress in our understanding of gene activation by NRs through chromatin remodeling.

The dynamics of chromatin remodeling at promoters

The nucleosome, the structural unit of chromatin, is known to play a central role in regulating gene transcription from promoters. The last seven years have spawned a vast amount of data on the enzymes that remodel and modify nucleosomes and the rules governing how transcription factors interact with the epigenetic code on histones. Yet despite this effort, there has yet to emerge a unifying mechanism by which nucleosomes are remodeled during gene regulation. Recent advances have allowed nucleosome dynamics on promoters to be studied in real time, dramatically changing how we think about gene regulation on chromatin templates.

Solution AFM studies of human Swi-Snf and its interactions with MMTV DNA and chromatin

ATP-dependent nucleosome remodeling complexes are crucial for relieving nucleosome repression during transcription, DNA replication, recombination, and repair. Remodeling complexes can carry out a variety of reactions on chromatin substrates but precisely how they do so remains a topic of active inquiry. Here, a novel recognition atomic force microscopy (AFM) approach is used to characterize human Swi-Snf (hSwi-Snf) nucleosome remodeling complexes in solution. This information is then used to locate hSwi-Snf complexes bound to mouse mammary tumor virus promoter nucleosomal arrays, a natural target of hSwi-Snf action, in solution topographic AFM images of surface-tethered arrays. By comparing the same individual chromatin arrays before and after hSwi-Snf activation, remodeling events on these arrays can be monitored in relation to the complexes bound to them. Remodeling is observed to be: inherently heterogeneous; nonprocessive; able to occur near and far from bound complexes; often associated with nucleosome height decreases. These height decreases frequently occur near sites of DNA release from chromatin. hSwi-Snf is usually incorporated into nucleosomal arrays, with multiple DNA strands entering into it from various directions, + or - ATP; these DNA paths can change after hSwi-Snf activation. hSwi-Snf appears to interact with naked mouse mammary tumor virus DNA somewhat differently than with chromatin and ATP activation of surface-bound DNA/hSwi-Snf produces no changes detectable by AFM.

Changes in attitude, changes in latitude: nuclear receptors remodeling chromatin to regulate transcription

Nuclear receptors (NRs) are a large family of ligand-dependent transcription factors that regulate important physiological processes. To activate or repress genes assembled naturally as chromatin, NRs recruit two distinct enzymatic activities, namely histone-modifying enzymes and ATP-dependent chromatin remodeling complexes, to alter local chromatin structure at target gene promoters. In this review, we examine the functional relationship between ATP-dependent chromatin remodeling complexes and NRs in the context of transcriptional regulation. Using the steroid-responsive mouse mammary tumor virus promoter as a model system, we discuss in detail the molecular mechanisms underlying the recruitment of these complexes and subsequent chromatin structure changes catalyzed by this group of enzymes. In addition, we extend the discussion to other NR-regulated promoters including the pS2 promoter. Finally, we summarize specific principles governing this critical relationship, identify unanswered questions and discuss the potential application of these principles in rational drug design.

Differentiation-dependent alterations in histone methylation and chromatin architecture at the inducible chicken lysozyme gene

It is now well established that locus-wide chromatin remodeling and dynamic alterations of histone modifications are required for the developmentally regulated activation of tissue-specific genes. However, little is known about the dynamics of these events during cell differentiation and how chromatin of an entire gene locus responds to signal transduction processes. To address this issue we investigated chromatin accessibility, linker histone distribution, and the histone methylation status at the macrophage-specific chicken lysozyme locus and the ubiquitously expressed gas41 locus in multipotent precursor cell lines and BM2 monoblast cells. The latter can be induced to go through macrophage maturation by treatment with phorbol-12-myristate acetate and can be further stimulated with bacterial lipopolysaccharide. We show that expression of the lysozyme gene in undifferentiated monoblasts is low and that a high level of gene expression requires both cell differentiation and lipopolysaccharide stimulation. However, depletion of the linker histone H1 is observed already in lysozyme non-expressing multipotent precursor cells. In undifferentiated monoblasts, the lysozyme regulatory regions are marked by the presence of monomethylated histone H3 lysine 4, which becomes increasingly converted into trimethylated H3 lysine K4 during cell differentiation. We also present evidence for extensive, differentiation-dependent alterations in nuclease accessibility at the lysozyme promoter without alterations of nucleosome and transcription factor occupancy.

Histone dynamics on the interleukin-2 gene in response to T-cell activation

Several models have been proposed for the mechanism of chromatin remodelling across the promoters of inducible genes in mammalian cells. The most commonly held model is one of cooccupation where histone proteins are modified by acetylation or phosphorylation and nucleosomes are remodelled, allowing the assembly of transcription factor complexes. Using chromatin immunoprecipitation, we observed an apparent decrease of histone acetylation and phosphorylation signals at the proximal promoter region of the inducible interleukin-2 and granulocyte-macrophage colony-stimulating factor genes in response to T-cell activation. We showed that this apparent decrease was due to a loss of histone H3 and H4 proteins corresponding to a decrease in nucleosome occupation of the promoter. This histone loss is reversible; it is dependent on the continual presence of appropriate activating signals and transcription factors and is not dependent on the acetylation status of the histone proteins. These data show for the first time that histone proteins are lost from a mammalian promoter upon activation of transcription and support a model of activation-dependent disassembly and reassembly of nucleosomes.