Rotational diffusion of nucleosomes: role of the N-terminal histone domains in structural transitions

摘要

The rotational diffusion of nucleosome core particles were measured 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 ($phi$- max$/) of approximately 170 ns. At higher salt concentrations $phi$-max$/ rises slowly from approximately 170 ns at 0.1 M NaCl to a value of approximately 230 ns at 0.35 M NaCl, a point just above physiological ionic strength; we call this change the moderate-salt transition. $phi$-max$/ remains constant at approximately 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 ore particles with trypsin to remove the N- terminal histone domains abolishes the plateau in $phi$-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.