Physical and biochemical studies of histone H1 and its globular domain

The interaction of the histone H5 globular domain with DNA and chromatin was studied using several physical and chemical techniques, including electron microscopy, dynamic light scattering, equilibrium density and velocity ultracentrifugation, and chemical crosslinking.The globular domain of histone H5 was employed in reconstitution experiments designed to test its ability to fold chromatin compared with intact H5. Binding and protein analyses showed that GH5 binds to chromatin in native stoichiometric amounts. Nuclease digestion experiments suggested that GH5 positions itself properly in chromatin and protects spacer DNA from digestion. Electron microscopy and dynamic light scattering studies showed that when GH5 is present in native or two-fold stoichiometric amounts in reconstituted chromatin, chromatin folding is not induced.Several lines of evidence show that GH5 forms cooperative complexes with DNA. Electron microscopy of GH5-DNA mixtures showed that 2 distinct classes of DNA coexist: free DNA and DNA complexed with GH5. Free DNA and complexes were separated into two nonoverlapping components on CsCl density gradients. Chemical crosslinking studies demonstrated that GH5 molecules were in close proximity on GH5-DNA complexes. Despite having very different sedimentation coefficients, complexes and free DNA could not be completely resolved on sucrose gradients. Many of the fractionated complexes were not completely coated with GH5, and free DNA cosedimented with complexes. This behavior was also seen in a comparison study of H1-DNA complexes, and requires that previous conclusions based on measured stoichiometries be reinterpreted. The results of this study lead to new conclusions about the mechanism of H1 action in chromatin, and they point to important new avenues for future research.U of I OnlyETDs are only available to UIUC Users without author permissio