Salt-induced structural changes of nucleosome core particles

Abstract

The effects of sodium chloride concentration on the structure of chicken erythrocyte nucleosome core particles have been studied by the use of fluorescently labelled histones. Histone H3 was modified with two sulfhydryl-specific dyes and reconstituted into core nucleosomes. Between 10 −4 m and 0.6 M-NaCl four different states were observed by the fluorescent techniques of collisional quenching, polarization and energy transfer. Below 5 × 10 −4 m-NaCl the nucleosome is flexible, with the single cysteine residues of the two H3 species about 48 Å apart and somewhat exposed. Between 5 × 10 −3 m and 10 −1 m-NaCl the nucleosome is rigid and non-spherical. The cysteine residues are close together and buried. Between 10 −1 m and 4 × 10 −1 m-NaCl, the cysteines become slightly more exposed but remain close together. At 6 × 10 −1 m-NaCl the nucleosome is very flexible. The cysteines are more than 70 Å apart and are quite exposed. The dramatic structural changes that are observed in core nucleosomes are consistent with the variety of functions in which they must participate in the cell.