The higher-order structure of chromatin: evidence for a helical ribbon arrangement.

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Abstract
Both intact and nuclease-isolated chromatin fibers were examined at different degrees of salt-induced compaction, using a variety of preparation techniques. The initial folding step in nucleosome packing may involve the formation of a zig-zag ribbon as was proposed by others, and that subsequent compaction occurs by coiling of the ribbon to form a double helical structure. This type of folding generates a fiber in which the nucelosome-nucleosome contacts established in the zig-zag ribbon are maintained and in which the histone H1 molecules occupy equivalent sites. The diameter of the fiber is not dependent upon the nucleosome repeat length. Direct mass values for individual isolated fibers obtained from electron scattering measurements showed that the mass per length was dependent on ionic strengh, and ranged from 6.0 .times. 104 daltons/nm at 10 mM NaCl to 27 .times. 104 daltons/nm at 150 mM salt. These values are equivalent to 2.5 nucleosomes/11 nm at 10 mM NaCl and to 11.6 nucleosomes/11 nm at 150 mM salt, and are consistent with the range of packing ratios for the proposed helical ribbon.