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Histone acetylation

Eukaryotic cells package their DNA by tightly wrapping it around proteins called histones. Histones are a family of small, positively charged proteins. Human histones are H1, H2A, H2B, H3, and H4. Having a negative charge due to the phosphate-sugar backbone, DNA tightly binds to positively charged histones. The resulting DNA-protein complex is called chromatin. The basic unit of chromatin is a nucleosome, which is comprised of nine histone proteins and 166 base pairs of DNA.

The unacetylated histones viewed from the side are a group of small spheres kept all together by a thread, which is representing double helix DNA strands. There is a zoom in the nucleosome from one of the ends. It is illustrated as four small spheres together surrounded by a DNA double helix. The histones have a tail that is not surrounded by the DNA and each of them has a few amino acids available for chemical modification, represented by small dots. Some of these tails are cleaved out resulting in free histone tails. An arrow is pointing from the unacetylated histones to acetylated histones. There is more separation between histones and are all connected by the DNA, which has some acetylated groups, showed as small spheres on the chains.

Figure 1. Histone Acetylation

The structure of chromatin not only serves as a packaging mechanism, but also as a gene expression regulator. The position of DNA can affect the accessibility of the DNA transcription apparatus. When DNA is located in highly condensed chromatin (heterochromatin), it becomes physically inaccessible to the DNA transcription apparatus, such as transcription factors and RNA polymerase.

Each histone has a 20 amino acid-long “tail” at its N-terminus that protrudes outward. These histone tails are accessible to various modifying enzymes that remove or catalyze additional specific chemical groups such as acetyl (-COCH3), methyl, and phosphate groups. Histone acetyltransferase can add acetyl groups to the positively charged histone tail, thus changing the charge. By reducing the positive charge of a histone tail, the affinity of DNA to histones decreases; therefore opening up the compacted chromatin and promoting gene expression. Another enzyme called histone deacetylase does the opposite of histone acetyltransferase: this enzyme removes the acetyl group from histone tails, thus repressing transcription.

Histone acetylation is part of gene regulation

Referred from:

Article
Chromatin modification
Article
Mechanism to Generate iPS