Rotational Diffusion of Nucleosomes: Role of the N-Terminal Histone Domains in Structural Transitions

摘要

We have measured the rotational diffusion of nucleosome core particles using the fluorescence anisotropy decay of ethidium intercalated into the core particle DNA as a function of ionic strength. The "native" form of the core particle (ionic strength from 0.001 to 0.1 M) has a rotational correlation time (Φ{sup}(max)) of ～170 ns. At higher salt concentrations Φ{sup}(max) rises slowly from ～170 ns at 0.1 M NaCl to a value of ～230 ns at 0.35 M NaCl, a point just above physiological ionic strength; we call this change the moderate-salt transition. Φ{sup}(max) remains constant at ～230 ns until the onset of the "high-salt dissociation" which occurs above 0.7 M NaCl. This dissociation begins with the release of H2A-H2B dimers; increasing DNA flexibility in this salt range prohibits accurate measurement of the rotational correlation time beyond this point. Light treatment of the core particles with trypsin to remove the N-terminal histone domains abolishes the plateau in (Φ{sub}(max) between 0.35 and 0.65 M NaCl. Thus the moderate-salt transition derives from the release of these N-terminal ends from the body of the core particle. The anisotropy decays show no evidence for DNA release from the core particle at salt concentrations below 0.65 M.