Physical Studies of Chromatin

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Abstract
Experiments were carried out to define celarly which histone combinations can induce a higher order structure when combined with DNA, the criterion for a higher order structure being the series of low-angle X-ray diffraction maxima nominally at 5.5 nm, 3.7 nm, 2.7 nm and 2.2 nm. Such a pattern, with resolution similar to that of H1-depleted chromatin, is readily attainable by recombining histones H2A + H2B + H3 + H4 with DNA using a salt-gradient dialysis method. However, the use of urea in the recombination procedure is shown to be detrimental to the production of a higher order structure. Low-angle ring patterns are not obtained by recombining DNA with single pure histones or any combination of histone pairs except H3 + H4. The diffraction maxima from the latter are, however, weaker than those from chromatin and there are pronounced semi-equatorial arcs. The presence of a 3rd histone, either H2A or H2B in the H3 + H4 recombination mixture tends to distort the recognised low-angle pattern. The histone pair H3 + H4 is probably essential for the formation of a regular higher order structure in chromatin, although for a complete structural development the presence of H2A + H2B is also required.