Tightly Bound Non‐histone Proteins in Nucleosomes from Pig‐Liver Chromatin

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Abstract
Core particles prepared by micrococcal nuclease digestion of pig liver chromatin were adsorbed on hydroxyapatite and dissociated by gradual increase in ionic strength and finally by urea and guanidine. By this method non-histone proteins were associated with the core particles. Proteins tightly bound to the core particle DNA (i.e., dissociated only by urea and guanidine) were also found; these are proteins with a limited heterogeneity, with respect to their MW, since only 6 components are present with MW ranging from 71,000 to 20,000. They show a peculiar amino acid composition. Other tightly bound proteins were present only in the spacer regions. The existence of 2 different classes of tightly bound proteins probably reflects different modes of binding to the DNA, which are compatible or incompatible, respectively, with the simultaneous binding of the histone octamer.