Chromatin
Compaction of the large DNA molecules and the control of gene expression in eukaryotes are achieved by having the DNA complexed with a set of special proteins to form the protein–DNA complex called chromatin. The DNA-binding proteins of chromatin fall into the following two classes.
1. Histones - This class of DNA-binding proteins includes five types of protein, whose properties are outlined in Table 28.1. All histones are small, very basic proteins rich in lysine and arginine. Some have been remarkably well conserved in amino acid sequence throughout evolution. Histone H4, for example, shows only two substitutions between humans and peas and only eight substitutions between humans and yeast. The histones are the basic building blocks of chromatin structure. The nucleoids of prokaryotic cells also have proteins associated with DNA, but these proteins are quite different from the histones and do not seem to form a comparable chromatin structure. Thus, a histone-containing chromatin structure is a uniquely eukaryotic feature. In all kinds of eukaryotic nuclei, the histones are present in an equal weight amount with DNA, and histones H2A, H2B, H3, and H4 are always found in equimolar quantities.
2. Nonhistone chromosomal proteins - The histones are accompanied by a much more diverse group of DNA-binding proteins called nonhistone chromosomal proteins. The total amount of these proteins varies greatly from one cell type to another, ranging from about 0.05 to 1 g/g DNA. They include a bewildering variety of proteins, such as polymerases and other nuclear enzymes, hormone receptor proteins, and regulatory proteins of many kinds. It is possible to count, on two-dimensional gels, approximately 1000 different nonhistone chromosomal proteins in a typical eukaryotic nucleus.
The precise role of histones was not understood until about 1974. Then, research in a number of laboratories showed that these proteins combine in a specific way to form a repeating element of chromatin structure, called the nucleosome.
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