Heterogeneity in nucleosome spacing.

The organization of spacer DNA connecting 160 base-pair cores of bovine thymus polynucleosomes has been studied by a combination of biochemical and electron microscopic techniques. The results reveal that a major fraction of chromatin consists of a spectrum of repeating units; these differ from each other by up to 20 base-pairs due to variation in spacer DNA length. Those polynucleosomes which have larger spacer DNA lengths are processed by micrococcal nuclease to mononucleosomes more rapidly than those organized with shorter spacers. Although a distribution of spacer DNA lengths exists, a significant proportion of spacers that are nearest neighbors share common DNA lengths. These findings imply that functional roles may be related to the manners in which spacers are organized along chromatin fibers.