Histones repression

Chew, Y.C., West, J.T., Kratzer, S.J., Ilvarsonn, A.M., Eissenberg, J.C., Dave, B.J., Klinkebiel, D., Christman, J.K., and Zempleni, J., 2008. Biotinylation of histones represses transposable elements in human and mouse cells and cell lines, and in Drosophila melanogaster. Journal of Nutrition. 138 2316-2322. 

Even without addition of RNA-polymerase, DNA purified from histone possessed considerably (five times) more activity as regards RNA synthesis on account of the pol3onerase still remaining in it (Huang and Bonner, 1962). These workers concluded from their findings that histones repress mRNA synthesis on DNA. 

In this way, by providing all the enzymes necessary for synthesis of RNA and by comparing the synthetic activity of DNA before and after deproteinization, it is possible to determine how much of the DNA in the chromatin is in a free state and how much is bound with histone (repressed). However, it was found that these determinations could be carried out very clearly and demonstratively in another way, by a physico-chemical method. Double... 

Repression of genes is associated with reversal of this process under the control of histone deacetylases (HDACs). Deacetylation of histones increases the winding of DNA round histone residues, resulting in a dense chromatin structure and reduced access of transcription factors to their binding sites, thereby leading to repressed transcription of inflammatory genes. 

The core unit of the chromatin, the nucleosome, consists of histones arranged as an octamer consisting of a (H3/ H4)2-tetramer complexed with two histone H2A/H2B dimers. Accessibility to DNA-binding proteins (for replication, repair, or transcription) is achieved by posttranslational modifications of the amino-termini of the histones, the histone tails phosphorylation, acetylation, methylation, ubiquitination, and sumoyla-tion. Especially acetylation of histone tails has been linked to transcriptional activation, leading to weakened interaction of the core complexes with DNA and subsequently to decondensation of chromatin. In contrast, deacetylation leads to transcriptional repression. As mentioned above, transcriptional coactivators either possess HAT activity or recruit HATs. HDACs in turn act as corepressors. 

Methylation of histones is correlated with activation and repression of gene transcription. 

The cis-acting elements that decrease or repress the expression of specific genes have also been identified. Because fewet of these elements have been smdied, it is not possible to fotmulate genetalizations about their mechanism of action—though again, as for gene activation, chromatin level covalent modifications of histones and other proteins by (repressor)-recruited multisubunit corepressors have been imphcated. 

Interestingly, post integration latency in microglial cells seems to be the result of the concerted action of both HDACs as well as HMTs on core histone H3 in nuc-1 (Marban et al. 2007). COUP-TF interacting protein 2 (CTIP2), a transcriptional repressor interacts with HDACl and HDAC2 via its N-terminus to repress... 

Ding HF, Bustin M, Hansen U (1997) Alleviation of histone Hl-mediated transcriptional repression and chromatin compaction by the acidic activation region in chromosomal protein HMG-14. Mol Cell Biol 17 5843-5855... 

Doyen CM, An W, Angelov D, Bondarenko V, Mietton F, Studitsky VM, Hamiche A, Boeder RG, Bouvet P, Dimitrov S (2006) Mechanism of polymerase II transcription repression by the histone variant macroH2A. Mol Cell Biol 26 1156-1164... 

Zhang H, Roberts DN, Cairns BR (2005a) Genome-wide dynamics of Htzl, a histone H2A variant that poises repressed/basal promoters for activation through histone loss. Cell 123 219-231... 

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