Histone Acetylation occurs in the N-terminal tail and on the surface of the nucleosome core as part of gene regulation when the histones are acetylated on lysine residues. The reactions are catalyzed by enzymes with histone acetyltransferase (HAT) activity. The source of the acetyl group in histone acetylation is Acetyl-Coenzyme A.

Acetylated histones and nucleosomes represent an epigenetic tag within chromatin. Acetylation brings in a negative charge that acts to neutralize the positive charge on the histones, and decreases the interaction of the N termini of histones with the negatively charged phosphate groups of DNA. Consequentially the condensed chromatin is transformed into a more relaxed structure which is associated with greater levels of gene transcription. This relaxation can be reversed by histone deacetylation HDAC activity. Relaxed, transcriptionally active DNA is referred to as euchromatin. More condensed and tightly packed DNA is referred to as heterochromatin. Condensation can be brought about by processes including deacetylation and methylation; the action of methylation is indirect and has no effect upon charge.

The charge neutralization model of histone acetylation has been subject to to recent studies that challange it. The studies attempt to show that transcriptionally active genes are correlated with rapid turnover of histone acetylation. This requires that the HATs and HDACs must act continuously on the affected histone tail. Methylation at a specific lysine residue (K4) is involved in targeting histone tails for continuous acetylation and deacetylation.

Histone Acetylation & Nucleosome

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