Fractionation of purified nucleosomes on the basis of aggregation properties.

Monomeric nucleosomes from micrococcal nuclease digest of rat liver nuclei have been purified on Sepharose columns in the presence of divalent cations and 0.6 M NaCl. The particles contain histones H2A, H2B, H3, H4, and a complement of nonhistone chromosomal proteins. In 0.6 M NaCl, the nucleosome mixture sedimented at 10 S; however, when the NaCl was removed approximately 30% of the particles aggregated and precipitated, and the remaining soluble fraction sedimented at 11 S. The aggregation phenomenon was divalent cation-dependent and reversible. Characterization of the macromolecular components of the subfractions of nucleosomes showed that the subfractions differed in composition of species of histone H3 as well as of several nonhistone chromosomal proteins but not in the size of the DNA fragment present. The aggregation properties of the isolated nucleosomes showed similarities to the divalent cation-dependent differences in the extent of chromatin condensation in the intact eukaryote nucleus.