Fluorescence studies on the conformational changes of nucleosome core particles

Abstract

This paper begins with a review of our measurements of core particle transitions using the steady state intrinsic tyrosine fluorescence. The transitions described here include the opening of the particle at very low ionic strength, a pH-induced transition at physiological ionic strengths near neutral pH, and the high salt dissociation. The second section of the paper describes measurements of the rotational diffusion of the core particle using fluorescence anisotropy decays of ethidium intercalated into the DNA. We find that the low salt transition consists of two main stages as the ionic strength is lowered. Just below 1 mM, rearrangements in the protein core give rise to a drop in the steady state anisotropy. This first change is not accompanied by a measurable change in the rotational correlation time. Below ~0.2 mM an irreversible change sets in, and the average correlation time jumps to a much higher value. We believe that this structure is highly elongated. The pH-induced change does not give rise to a change in the rotational correlation time. At very high ionic strengths above 0.6 M, where the protein begins to dissociate from the DNA, the average rotational correlation time also increases to much higher values. There is no indication of disruption of the core particle below this ionic strength.