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Physical studies on the H3/H4 histone tetramer.

High-resolution proton magnetic resonance spectroscopy (270 MHz), circular dichroism, and infrared spectroscopies and ultracentrifugation studies have been carried out on the salt-extracted (H3/H4)2 tetramer from calf thymus. The tetramer contains about 29% alpha helix and no beta structure. It is denatured in 6 M urea but can be renatured simply by dialysis to water. The proton spectrum shows a number of perturbed resonances which are not observed in the spectra of either H3 or H4 alone. The observation of these resonances demonstrates that the tetramer contains some elements of tertiary structure. The overall appearance of the spectrum however is close to that of a partially denatured protein. Sedimentation velocity studies show the tetramer to have a frictional ratio of 1.99 in 50 mM acetate/50 mM bisulfite and thus to be hydrodynamically quite different from a globular protein. Two possible structural models compatible with the data are discussed.