Chromatin Structure and the Role of Histone Phosphorylation in the Cell Cycle

Abstract

Chromatin contains 3 major components at the molecular level: DNA, histones and non-histones. RNA and, possibly, lipids are present in smaller quantities. Histones are basic proteins strongly bound to the DNA and they are currently thought to be responsible for the overall structure of DNA in chromatin. Non-histones are the other chromatin proteins and include structural proteins, enzymes, putative specific controllers of gene expression and RNA “packaging” proteins. DNA and histones are normally present in approximately equal amounts but the proportion of non-histones varies with the tissue and the preparation method. In general, rather little is known about the non-histone component. The histones from almost all organisms fall into five groups, Fl, F2A1, F2A2, F2B and F3, with some exceptions notably in the highly repressed avian erythrocyte chromatin where F2C appears to replace Fl. The other common nomenclatures for histones are given in Table 1. The four histones, F2A1, F2A2, F2B, F3, are extremely similar in different organisms, F2A1 in particular showing only minor sequence differences between pea and calf. All the amino acid sequences show a characteristic non-random distribution of residues, giving rise to highly basic regions and hydrophobic regions.