Selective dissociation of histones from calf thymus nucleoprotein

The extent of the dissociation of histones from nucleohistone by increasing concentrations of sodium chloride and sodium perchlorate has been measured. Dissociated histones were separated from nucleohistone in a given salt medium by differential centrifugation. The histone fractions were identified by gel electrophoresis, or, in some preliminary experiments, by column chromatography. Histone fraction I is dissociated from nucleohistone by NaCl in the range of 0.4 to 0.5 m. Histone II is dissociated by 0.8 to 1.2 m-NaCl. Histone III-IV consists of two components B₁ and A, by gel electrophoresis. The B₁ component is dissociated from native nucleohistone in the range 0.8 to 1.2 m-NaCl, the A component in the range 0.9 to 1.6 m. The NaClO₄ studies are not as detailed, but the general result is that the concentration of NaClO₄ needed to dissociate a certain histone fraction is about half the concentration of NaCl needed. The dissociation of a histone fraction is somewhat co-operative and takes place over a rather small salt concentration range. These results imply that both electrostatic and non-electrostatic interactions contribute to the strength of the binding between histones and DNA. The results of the sedimentation separations were confirmed by electrophoretic separations in different salt concentrations. Furthermore, and as expected, the more histone dissociated from a nucleohistone, the greater the electrophoretic velocity of the latter. The optical melting profiles of partially dissociated nucleohistones in a medium of low salt concentration are intermediate between those of fully covered nucleohistone and DNA. The melting curves for the partially covered materials are broader than those for fully covered nucleohistone or for DNA, but not clearly biphasic. The results therefore suggest that the histones are somewhat heterogeneously distributed along the DNA chain. Fully covered native nucleohistone is insoluble at sodium chloride concentrations in the range 0.15 to 0.30 m. Nucleohistone, from which histone I has been removed by extraction with 0.6 m-NaCl, is soluble in 0.15 to 0.30 m-NaCl.