Internal architecture of the core nucleosome: fluorescence energy transfer studies at methionine-84 of histone H4.

Chicken histone H4, labeled separately at Met-84 with N-[[(iodoacetyl)amino]ethyl]-5-naphthylamine-1-sulfonic acid and 5-(iodoacetamido)fluorescein, was reassociated with unlabeled histones H2A, H2B, and H3 and 146 base pairs of DNA to produce fluorescently labeled nucleosomes having physical characteristics virtually the same as those of native core particles. Four types of particles were prepared containing respectively unlabeled H4, dansylated H4, fluoresceinated H4, and a mixture of the two labeled H4 molecules. Quantitative singlet-singlet energy-transfer measurements were carried out to determine changes in the distance between the two Met-84 H4 sites within the same nucleosome following conformational transitions which we have reported earlier. In the ionic strength range 0.1-100 mM NaCl, the distance between these sites is less than 2 nm except at 1 mM. Between 100 and 600 mM monovalent salt the distance separating the donor and acceptor fluors at Met-84 H4 increases to 3.8 nm. The conformational change centered around 200 mM NaCl is cooperative. Our results and those of others indicate that there is little unfolding of the histone octamer, at least around Met-84 H4, in the entire ionic strength range studied. A mechanism involving the rotation of the globular portion of H4 is proposed to account for this transition which occurs at physiological ionic strengths.