Five types of histones have been identified: H1, H2A, H2B, H3 and H4. Four of the histones, H2A, H2B, H3 and H4, are involved in forming the nucleosome structure with DNA, and are referred to as core histones. Histone H3 and H4 are also involved in histone acetylation.
Acetylation adds acetyl (COCH3) groups to the amino acid lysine in the N terminus of histones H3 and H4. The reaction is catalyzed by Histone AcetylTransferases (HATs). Acetylation removes the positive charge of the lysine side chain, removing lysine binding to DNA, which causes diminished histone binding.
Histone acetylation reduces the histone binding on the DNA structure, making this section of DNA available for the binding of transcription factors and RNA Polymerase to the promoter region of genes to start transcription.