MoBio Histones and Nucleosomes Chapter 3

Histones are the proteins closely associated with DNA molecules. They are responsible for the structure of chromatin and play important roles in the regulation of gene expression. Five types of histones have been identified: H1 (or H5), H2A, H2B, H3 and H4. H1 and its homologous protein H5 are involved in higher-order structures of chromatin. The other four types of histones associate with DNA to form nucleosomes. H1 (or H5) has about 220 residues. Other types of histones are smaller, each consisting of 100-150 residues.

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Figure 3-D-2. Each nucleosome consists of 146 bp DNA and 8 histones: two copies for each of H2A, H2B, H3 and H4. The DNA is wrapped around the histone core, making nearly two turns per nucleosome.

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Figure 3-D-3. The sequence of H4 from a cow. Lysine residues (red color) at the N terminus play a major role in the regulation of gene transcription.

An important feature about histones is that they contain a few lysine (K) residues at the N terminus. Under normal cellular conditions, the R group of lysine is positively charged, which can interact with the negatively charged phosphates in DNA. The positive R group of lysine may be neutralized by acetylation, reducing the binding force between histones and DNA. Such mechanism has been demonstrated to play a major role in the regulation of gene transcription (Chapter 4 Section G).