(1/3798) Gadd45, a p53-responsive stress protein, modifies DNA accessibility on damaged chromatin.
This report demonstrates that Gadd45, a p53-responsive stress protein, can facilitate topoisomerase relaxing and cleavage activity in the presence of core histones. A correlation between reduced expression of Gadd45 and increased resistance to topoisomerase I and topoisomerase II inhibitors in a variety of human cell lines was also found. Gadd45 could potentially mediate this effect by destabilizing histone-DNA interactions since it was found to interact directly with the four core histones. To evaluate this possibility, we investigated the effect of Gadd45 on preassembled mononucleosomes. Our data indicate that Gadd45 directly associates with mononucleosomes that have been altered by histone acetylation or UV radiation. This interaction resulted in increased DNase I accessibility on hyperacetylated mononucleosomes and substantial reduction of T4 endonuclease V accessibility to cyclobutane pyrimidine dimers on UV-irradiated mononucleosomes but not on naked DNA. Both histone acetylation and UV radiation are thought to destabilize the nucleosomal structure. Hence, these results imply that Gadd45 can recognize an altered chromatin state and modulate DNA accessibility to cellular proteins. (+info)
(2/3798) A novel H2A/H4 nucleosomal histone acetyltransferase in Tetrahymena thermophila.
Recently, we reported the identification of a 55-kDa polypeptide (p55) from Tetrahymena macronuclei as a catalytic subunit of a transcription-associated histone acetyltransferase (HAT A). Extensive homology between p55 and Gcn5p, a component of the SAGA and ADA transcriptional coactivator complexes in budding yeast, suggests an immediate link between the regulation of chromatin structure and transcriptional output. Here we report the characterization of a second transcription-associated HAT activity from Tetrahymena macronuclei. This novel activity is distinct from complexes containing p55 and putative ciliate SAGA and ADA components and shares several characteristics with NuA4 (for nucleosomal H2A/H4), a 1.8-MDa, Gcn5p-independent HAT complex recently described in yeast. A key feature of both the NuA4 and Tetrahymena activities is their acetylation site specificity for lysines 5, 8, 12, and 16 of H4 and lysines 5 and 9 of H2A in nucleosomal substrates, patterns that are distinct from those of known Gcn5p family members. Moreover, like NuA4, the Tetrahymena activity is capable of activating transcription from nucleosomal templates in vitro in an acetyl coenzyme A-dependent fashion. Unlike NuA4, however, sucrose gradient analyses of the ciliate enzyme, following sequential denaturation and renaturation, estimate the molecular size of the catalytically active subunit to be approximately 80 kDa, consistent with the notion that a single polypeptide or a stable subcomplex is sufficient for this H2A/H4 nucleosomal HAT activity. Together, these data document the importance of this novel HAT activity for transcriptional activation from chromatin templates and suggest that a second catalytic HAT subunit, in addition to p55/Gcn5p, is conserved between yeast and Tetrahymena. (+info)
(3/3798) Stable remodeling of tailless nucleosomes by the human SWI-SNF complex.
The histone N-terminal tails have been shown previously to be important for chromatin assembly, remodeling, and stability. We have tested the ability of human SWI-SNF (hSWI-SNF) to remodel nucleosomes whose tails have been cleaved through a limited trypsin digestion. We show that hSWI-SNF is able to remodel tailless mononucleosomes and nucleosomal arrays, although hSWI-SNF remodeling of tailless nucleosomes is less effective than remodeling of nucleosomes with tails. Analogous to previous observations with tailed nucleosomal templates, we show both (i) that hSWI-SNF-remodeled trypsinized mononucleosomes and arrays are stable for 30 min in the remodeled conformation after removal of ATP and (ii) that the remodeled tailless mononucleosome can be isolated on a nondenaturing acrylamide gel as a novel species. Thus, nucleosome remodeling by hSWI-SNF can occur via interactions with a tailless nucleosome core. (+info)
(4/3798) Histone octamer transfer by a chromatin-remodeling complex.
RSC, an abundant, essential chromatin-remodeling complex related to SWI/SNF complex, catalyzes the transfer of a histone octamer from a nucleosome core particle to naked DNA. The newly formed octamer-DNA complex is identical with a nucleosome in all respects. The reaction requires ATP and involves an activated RSC-nucleosome intermediate. The mechanism may entail formation of a duplex displacement loop on the nucleosome, facilitating the entry of exogeneous DNA and the release of the endogenous molecule. (+info)
(5/3798) Localization of curved DNA and its association with nucleosome phasing in the promoter region of the human estrogen receptor alpha gene.
We determined DNA bend sites in the promoter region of the human estrogen receptor (ER) gene by the circular permutation assay. A total of five sites (ERB-4 to -1, and ERB+1) mapped in the 3 kb region showed an average distance of 688 bp. Most of the sites were accompanied by short poly(dA) x poly(dT) tracts including the potential bend core sequence A2N8A2N8A2 (A/A/A). Fine mapping of the ERB-2 site indicated that this A/A/A and the 20 bp immediate flanking sequence containing one half of the estrogen response element were the sites of DNA curvature. All of the experimentally mapped bend sites corresponded to the positions of DNA curvature as well as to nucleosomes predicted by computer analysis. In vitro nucleosome mapping at ERB-2 revealed that the bend center was located 10-30 bp from the experimental and predicted nucleosome dyad axes. (+info)
(6/3798) ISWI is an ATP-dependent nucleosome remodeling factor.
The ATPase ISWI is a subunit of several distinct nucleosome remodeling complexes that increase the accessibility of DNA in chromatin. We found that the isolated ISWI protein itself was able to carry out nucleosome remodeling, nucleosome rearrangement, and chromatin assembly reactions. The ATPase activity of ISWI was stimulated by nucleosomes but not by free DNA or free histones, indicating that ISWI recognizes a specific structural feature of nucleosomes. Nucleosome remodeling, therefore, does not require a functional interaction between ISWI and the other subunits of ISWI complexes. The role of proteins associated with ISWI may be to regulate the activity of the remodeling engine or to define the physiological context within which a nucleosome remodeling reaction occurs. (+info)
(7/3798) Reconstitution of a core chromatin remodeling complex from SWI/SNF subunits.
Protein complexes of the SWI/SNF family remodel nucleosome structure in an ATP-dependent manner. Each complex contains between 8 and 15 subunits, several of which are highly conserved between yeast, Drosophila, and humans. We have reconstituted an ATP-dependent chromatin remodeling complex using a subset of conserved subunits. Unexpectedly, both BRG1 and hBRM, the ATPase subunits of human SWI/SNF complexes, are capable of remodeling mono-nucleosomes and nucleosomal arrays as purified proteins. The addition of INI1, BAF155, and BAF170 to BRG1 increases remodeling activity to a level comparable to that of the whole hSWI/SNF complex. These data define the functional core of the hSWI/SNF complex. (+info)
(8/3798) Interleukin-2 (IL-2) regulates the accessibility of the IL-2-responsive enhancer in the IL-2 receptor alpha gene to transcription factors.
Interleukin-2 (IL-2) responsiveness of T lymphocytes is controlled through transcription of the IL-2 receptor (IL-2R) alpha subunit by antigen and by IL-2 itself. IL-2 induces IL-2Ralpha transcription via an IL-2-responsive enhancer (IL-2rE), whose activity depends on the cooperative binding of IL-2-induced STAT5 to two sites and of constitutively active Elf-1 to a third one. Here we describe the changes in IL-2rE chromatin that occur in normal T lymphocytes upon activation of IL-2Ralpha expression. In cells induced to transiently express IL-2Ralpha with concanavalin A (which mimics antigen), none of the IL-2rE sites is occupied despite the presence of Elf-1 and STAT1, which bind to the IL-2rE in vitro. The two STAT binding sites are occupied rapidly upon IL-2 stimulation, concomitantly with STAT5 activation. Occupation of the Elf-1 binding site is delayed, although Elf-1 concentration and binding activity are not modified by IL-2. Digestion of T-cell chromatin with DNase I and micrococcal nuclease shows that IL-2 induces the appearance of nuclease-hypersensitive sites flanking the IL-2rE. Thus IL-2, in addition to activating STAT5, appears to regulate IL-2Ralpha transcription by making IL-2Ralpha chromatin accessible to transcription factors. (+info)