Nucleosome-remodeling complexes

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Chromatin is composed of nucleosomes, where each comprise 147 base pairs of DNA wrapped around an octamer oftwo copies of each histone H2A, H2B, H3, and H4. Nucleosomes are folded into higher-order structures that are stabilized by linker histones. Chromatin structure can be altered by enzymes that posttranslationally modify histones (e.g., through phosphorylation, acetylation, methylation, or ubiquitination) or by ATP-driven chromatin-remodeling complexes that alter nucleosome position and/or composition. 

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Figure 1. Schematics of the effect of mH2A on transcription, (a) The mobiUzation of conventional promoter nucleosome by the chromatin remodeling complex generated a nucleosome-free promoter, which allowed the binding of the transcription factors and transcription to proceed, (b) MacroH2A nucleosome cannot be mobilized by die chromatin remodeling complex and die transcriptions factors are unable to bind macroH2A nucleosome containing promoter, which results in inhibition of the initiation of transcription. (See Colour Plate 8.)...

Figure 2. Effect of H2A.Bbd on transcription. The presence of H2A.Bbd confers lower stability and more loose structure to the nucleosomes, which allows the transcription factors binding to this variant nucleosome and thereby recruitment of p300 and acetylation of the promoter proximal histones. The remodeling complex can not mobilize the variant nucleosome, but instead helps in the removal of H2A.Bbd-H2B dimer. All these events facilitate transcription. (See Colour Plate 9.)...

In yeast, H2A.X is dephosphorylated by the phosphatase PPH3p after it has been released from chromatin. The release of y-H2A.X from repaired chromatin is independent of DNA replication, and it therefore must be assumed that a chromatin remodeling complex actively exchanges 7-H2A.X from nucleosomes. Thus far, several candidate remodeling complexes have been identified that specifically target nucleosomes containing 7-H2A.X. 

In addition to a potential role of HMGB proteins in nucleosome assembly, it has recently become apparent that many chromatin remodeling complexes either contain, or can associate with, a polypeptide containing an HMG-box homologous to the HMGBl B domain (Table 2). Examples of such complexes include the BAF (a mammalian SWI/SNF related complex [106]) and the Drosophila BRM (brahma)... 

Fig. 4. Possible mechanism for HMGB proteins in facilitating the unwrapping of nucleosomal DNA. The model postulates that the binding of an HMGB protein at an exit/entry point for the wrapped DNA increases the accessibility of the wrapped DNA both at the exit/entry points adjacent to the bound HMGB protein and also at internal positions. Such a conformational change would facilitate the binding both of sequence-specific regulatory proteins and of remodeling complexes. Adapted from Ref. [119] with permission.

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