Abstract
It is known that the lysine‐rich histone H1 induces both higher orders of folding in chromatin and donut shapes in DNA. However, these phenomena occur only on the high‐salt side of a narrow transition range located at about 0.02__M__ salt. Previous theoretical analyses of the ionic‐strength dependencies of DNA persistence length and denaturation rate have provided the information that the basic rigid‐rod unit in high‐molecular‐weight DNA is a segment 60 base pairs in length and that if the phosphate charge is neutralized, this segment will spontaneously adopt a bent conformation with radius of curvature 170 Å. On the assumption that an H1 molecule does not completely neutralize the DNA charge in its vicinity, the theory has been extended here to determine the onset of spontaneous bending as a function of salt concentration and extent of phosphate neutralization. A salt transition of the kind observed has been found for the realistic value of 82% charge neutralization, with the actual value likely to be in the neighborhood of 90%, as suggested by the measurements of Wilson and Bloomfield.^1^ It is recalled that the spacer DNA length in chromatin is of about the same length as the DNA rigid‐rod unit. If binding of H1 to the spacer induces, as predicted, a bent conformation of radius about 170 Å, then the observed value of about 150 Å for the outer radius of the solenoid presently thought to be the basic mode of folding for a nucleosome chain can be understood as a reflection of the inherent maximum curvature of DNA in aqueous salt solution.