Histone acetylation and deacetylation
Histone acetylation and deacetylation are the processes by which the lysine residues within the N-terminal tail protruding from the histone core of the nucleosome are acetylated and deacetylated as part of gene regulation. Histone acetylation and deacetylation are essential parts of gene regulation. These reactions are typically catalysed by enzymes with ""histone acetyltransferase"" (HAT) or ""histone deacetylase"" (HDAC) activity. Acetylation is the process where an acetyl functional group is transferred from one molecule (in this case, Acetyl-Coenzyme A) to another. Deacetylation is simply the reverse reaction where an acetyl group is removed from a molecule.Acetylated histones, octameric proteins that organize chromatin into nucleosomes and ultimately higher order structures, represent a type of epigenetic marker within chromatin. Acetylation removes the positive charge on the histones, thereby decreasing the interaction of the N termini of histones with the negatively charged phosphate groups of DNA. As a consequence, the condensed chromatin is transformed into a more relaxed structure that is associated with greater levels of gene transcription. This relaxation can be reversed by HDAC activity. Relaxed, transcriptionally active DNA is referred to as euchromatin. More condensed (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.