The histones are a group of water and dilute acid soluble basic proteins found associated with DNA in chromosomes. They are characterized by relatively high levels of lysine and arginine. Although histones are classified into a limited number of types of fractions (see below) with each particular fraction having a fundamentally distinct amino acid composition and sequence, numerous subfractions are observed due to the acetylation, methylation, and phosphorylation of various amino acid residues. Microheterogeneity or alteration of structure is dynamic such that the histones of a single cell type are found to vary during development. They are believed to play a role in gene activity and cellular metabolism. See "The Structure and Function of Chromatin," Ciba Foundation Symposium 28, American Elsevier, N.Y. (1975).
Although the classic nomenclature for histones is that of Johns and Butler (1962), the nomenclature of Bradbury (1975) was submitted to the IUPAC.
Characteristics of Histones from Calf Thymus:
| |
Fractions (Nomenclature) |
|
| |
|
|
|
| Class |
Bradbury |
Johns |
Molecular Weight |
| Lysine Rich |
H1 |
f1 |
about 21,500 |
| Slightly Lysine Rich |
H2a |
f2a2 |
14,004 |
| Slightly Lysine Rich |
H2b |
f2b |
13,774 |
| Arginine Rich |
H3 |
f3 |
15,324 |
| Arginine Rich |
H4 |
f2a1 |
11,282 |
from Elgin and Weintraub(1975)
With the exception of H1, the primary structures of the calf thymus histones have been determined. Comparisons with the structures for histones from other sources indicate that the histones rank among the most highly conserved (low mutation rate) proteins in nature.
Naturally occurring histones are often found to be partially acetylated, methylated, or phosphorylated. These modifications may contribute to the electrophoretic microheterogeneity of the histone fractions.
The nucleoprotein complex of histone and deoxyribonucleic acid is referred to as nucleohistone or deoxyribonucleoprotein. It is important as a source of its two components as well as an entity in itself for physical studies. Intracellularly, these complexes may be important factors in chromosomal structure and gene transcription. Kornberg (1974) has proposed a model for chromatin in which 200 DNA base pairs are coiled on the outside of a histone unit composed of (H3)2(H4)2 tetramer and two each of H2a and H2b.
Bradbury (1976) suggests that H1 may be involved in generating higher order chromatin structures and that the initiation of mitosis may in part be mediated by H1 phosphorylation.
Worthington histones are water soluble at pH 7.0 and are characterized by gel electrophoresis and solubility. Stable for years at 2 - 8°C. when stored dry.