STRUCTURAL-CHANGES OF NUCLEOSOMES IN LOW-SALT CONCENTRATIONS

We report transient electric dichroism studies of the unfolding of 140 base pair calf thymus nucleosomes in the salt concentration range from 26 mM down to 0.1 mM. A single unfolding transition was found, occurring within the range 0.3-3 mM, with a midpoint at ~1.3 mM. A concentration of 100 μM Mg2+ is sufficient to reverse completely the unfolding, yielding the native structure. Nucleosomes cross-linked with dimethyl suberimidate do not undergo unfolding in low-salt solution. The unfolding transition is characterized by an increase in the negative limiting reduced dichroism from 0.29 to 0.48 and an increase in the field-induced, viscosity-limited, rotational orientation time from 0.8 to 1.9 μs. The results imply a model for the low-salt structure consisting of a 178-Å diameter disk, 60 A in thickness, containing 140 base pair deoxyribonucleic acid (DNA) wound in 0.9 superhelical turn. The low-salt structure orients by a permanent dipole mechanism, with a dipole moment directed along the C2 symmetry axis of 2600 D, compared to 1200 D for the native 140 base pair nucleosome. At least two to three counterions are released when the nucleosome unfolds. We propose that electrostatic repulsion between adjacent DNA sections in native nucleosomes is primarily responsible for unfolding. This repulsion is relieved in the structure having only 0.9 superhelical turn since the overlap of 0.4-0.75 turn of DNA is lost. We estimate that the free energy of forming the low-salt unfolded structure under physiological conditions may be as small as 7 kcal mol-1, making it an energetically plausible candidate for an enzyme-induced intermediate in functional unfolding of nucleosomes. © 1979, American Chemical Society. All rights reserved.