Acetylated histones transcriptionally repressive state

In summary, the reciprocal decrease in histone acetylation coupled with an increase in histone HI expression could, in principle, promote the conversion of transcriptionally permissive chromatin into a transcriptionally repressed form and hence serve as the foundation for the development of the transcriptionally repressed state that is first observed at the two-cell stage. The second round of DNA replication may also contribute to the development of this state by displacing transcriptionally productive complexes and thus providing an opportunity for the assembly of a mature, transcriptionally repressive chromatin structure. 

The amino terminal tads of the four core histones contain lysines that are acetylated by HATs and deacetylated by HDACs. The histone octamer (H2A, H2B, H3, H4)2 is represented as a cylinder wrapped by DNA. It is thought that neutrahzation of the positive charges on the histone tads results in alterations of the nucleosome structure that may lead to a higher mobdity of the nucleosome and/or an improved accessibdity of the bound DNA, with accompanying changes in chromatin structure, chromatin hierarchy and transcription. In most, but not ad cases, deacetylation correlates with the repressed state and acetylation correlates with the transcriptionady active state. 

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