Nucleosome conformation: pH and organic solvent effects

Abstract

Monomer nucleosomes (v ) from chicken erythrocyte nuclei were examined in aqueous buffers (8>pH>3) and in solvent mixtures (i.e., water and ethanol, ethylene glycol, dioxane, dimethyl sulfoxide, 2-methyl-2,4-pentanediol, polyethylene glycol, sucrose, or urea). Circular INTRODUCTIONOur present understanding of nucleosome structure is based primarily upon studies conducted in vitro, Recent biochemical studies suggest, however, that nucleosome conformation is altered in chromatin that is active in transcription (1-3). As an approach to identifying the conformational states or transitions of nucleosomes, extensive studies of the effects of urea and ionic strength on nucleosome structure were made previously (4-6).In this paper we report the effects of pH and various organic solvents on nucleosome conformations. Circular dichroism and loser Raman spectroscopy were applied to identify changes of secondary structure in histones and DNA. The fluorescence of N-(3-pyrene)maleimide (NPM), which was covalently labeled to a cysteine residue at position 110 in chicken erythrocyte H3, has also been used effectively as a sensitive probe of nuclesome conformation.

On the de novo formation of compact oligonucleosomes at high ionic strength. Evidence for nucleosomal sliding in high salt

“…We have formerly utilized such nuclei and found that this depletion does not destroy the typical nuclear chromatin structure on the nucleosomal level (13). Our findings do not comply with results yielded by investigating isolated nucleosomes at low pH under equilibrium conditions (27). It seems, therefore, justified to assume that the chromatin gained by lysing such nuclei offers the potential for complementing a number of studies in which a distinction between the effect of high ionic strength and the properties of the H 1-deficient chromatin eo ipso would appear desirable.…”

Nucleosome Structure

“…As discussed earlier, the sulfhydryl group of histone H3 becomes reactive over the same range of urea concentrations (121,123). A potentially important generalization has been made from the denatura tion of core particles by urea and by othel' denaturants, namely that the DNA conformation changes coincidently with a change in the alpha-helix content of the core histones (174).…”

Disruption of Reconstituted Nucleosomes

“…The nucleosome core is a fragile object (van Holde, 1988). Systematic study of the stability of nucleosome cores isolated from the nuclei of somatic cells has determined the temperature, pH, and salt concentrations at which histone-DNA interactions are disrupted (Zama et al, 1978;Gordon et al, 1979;Libertini and Small, 1980;Burch and Martinson, 1980;Burton et al, 1978;Walker and Wolffe, 1984). An important aspect of nucleosome core stability is the sensitivity of histone-DNA interactions to dilution.…”