text: Nuclear DNA is normally wrapped tightly around histones, making it hard or impossible for the transcription machinery to access the DNA. This association is due primarily to the electrostatic attraction between the DNA and histones as the DNA phosphate backbone is negatively charged and histones are rich in lysine residues, which are positively charged. The tight DNA-histone association prevents the transcription of DNA into RNA.
  Many coactivators have histone acetyltransferase (HAT) activity meaning that they can acetylate specific lysine residues on the N-terminal tails of histones. In this method, an activator binds to an enhancer site and recruits a HAT complex that then acetylates nucleosomal promoter-bound histones by neutralizing the positively charged lysine residues. This charge neutralization causes the histones to have a weaker bond to the negatively charged DNA, which relaxes the chromatin structure, allowing other transcription factors or transcription machinery to bind to the promoter (transcription initiation). Acetylation by HAT complexes may also help keep chromatin open throughout the process of elongation, increasing the speed of transcription.
  Acetylation of the N-terminal histone tail is one of the most common protein modifications found in eukaryotes, with about 85% of all human proteins being acetylated. Acetylation is crucial for synthesis, stability, function, regulation and localization of proteins and RNA transcripts.
  HATs function similarly to N-terminal acetyltransferases (NATs) but their acetylation is reversible unlike in NATs. HAT mediated histone acetylation is reversed using histone deacetylase (HDAC), which catalyzes the hydrolysis of lysine residues, removing the acetyl group from the histones. This causes the chromatin to close back up from their relaxed state, making it difficult for the transcription machinery to bind to the promoter, thus repressing gene expression.
  Examples of coactivators that display HAT activity include CARM1, CBP and EP300.