The Binding of Deoxyribonucleic Acid and Histone in Native Nucleohistone

Abstract

Abstract The dissociation of histones from DNA by salt has been reinvestigated. Previous methods were examined and found to be unsatisfactory for two reasons. (a) An endogenous proteolytic enzyme is highly active during the separation procedures and the resultant histones are severely degraded. (b) Dissociated histones aggregate at elevated salt concentrations and histone can subsequently cosediment with DNA, thus decreasing the apparent level of dissociation. We describe conditions in solutions of sodium phosphate-urea at pH 5.5 which not only alleviate the above problems, but also permit a highly selective extraction of either histone F1 or of histones F1, F2b, and F2a2 from the other histone fractions. Evidence is presented arguing that histones interact with DNA through two kinds of binding. On the one hand there is a substantial involvement of electrostatic ionic bonds and on the other there is a variable contribution from an interaction which has many of the features of the hydrophobic bond. This latter interaction is characterized by (a) a decrease in strength in solutions of urea, (b) a decrease in strength in hydrophobic solvents such as in the presence of 1-propanol or propylurea, and (c) an increase in binding at higher temperatures. The contribution of this additional interaction is most marked for the arginine-rich histones F3 and F2a1 and least so for the lysine-rich histone F1. We have also observed that F3 and F2a1 bind denatured DNA so strongly that they cannot be dissociated at all in high salt or at low pH unless high urea concentrations are also present.