Arrangement of subunits in assembled histone H4 fibers.

The structure of assembled histone H4 fibers has been studied by analysis of electronmicrographs, including optical diffraction. An individual fiber has the appearance of an 80-A wide ribbon, twisted at intervals of about 330 A. Thicker fibers which have been observed seem to be bundles of ribbons. In diffraction patterns from both kinds of fiber, layer lines at axial spacings of about 1/55 A-1, 1/37 A-1, and 1/27 A-1 were most consistently observed. The possible arrangements of molecules within the twisted ribbons have been deduced and are found to be fairly closely related. The ribbons appear to consist of two parallel, unstaggered rows of repeating units, which are probably H4 dimers. The similarity between the observed layer line spacings of the H4 fibers and the spacings of the maxima in x-ray diffraction patterns from whole chromatin suggests that the H4 fibers have a structure related to that of chromatin. Since homogeneous preparations of histones H2A, H2B, and H3, or any mixture of these four histones, can form similar structures, it seems likely that the basic organization of chromatin is determined by a fibrous histone core around which the DNA is wrapped.