The higher order structure of chromatin and histone H1

The basic chromatin fibre (the 10 nm diameter fibre) is a linear repeating array of nucleosomes, or nucleosome filament. The core of each disk-like nucleosome is a wedge-shaped protein octamer containing two molecules of each of the four core histones (H3, H4, H2A and H2B) around which two turns of DNA are wound in a left-handed superhelix with about 80 base-pairs per turn. The two turns are sealed by a molecule of the fifth histone H1 (or H5 in nucleated erythrocytes). The linker DNA that connects one two-turn particle to the next varies from essentially zero to about 80 base-pairs in chromatins from different sources. The exact significance of this variation is unclear. Interphase chromatin exists largely in the form of 30 nm fibres. Folding of the nucleosome filament into very similar 30 nm fibres, which is H1-dependent, occurs in vitro in the presence of monovalent cations or much lower concentrations of divalent cations. These higher-order structures probably arise by helical coiling of the nucleosome filament into a solenoid. Systematic studies of chromatin folding in solution, for a range of chromatin fragment sizes and ionic strengths, reveal two discontinuities in behaviour that reflect two structural transitions. One is interpreted as the formation of a turn of a solenoid with about six nucleosomes, at ionic strength 25 mM, and is a common feature of chromatin from three different sources, which differ in DNA repeat length, and type and amount of H1. The other transition is interpreted in terms of hydrodynamic shearing of long solenoids at low ionic strengths (below approximately 45 mM). It suggests a more stable higher-order structure for chicken erythrocyte chromatin than for rat liver chromatin (attributed largely to the presence of H5), and may prove to be a useful general assay for the relative stabilities of different chromatins, which might be relevant to their ease of unravelling for transcription. A study of short (165 base-pair) repeat chromatin from cerebral cortex neurons has led to the suggestion that in the general case the linker DNA might be located, perhaps with H1, in the central hole in the solenoid. H1 molecules in both extended and condensed chromatin (although not when dissociated from it) are close enough to be chemically cross-linked with reagents of span 2-12A.(ABSTRACT TRUNCATED AT 400 WORDS)