Studies on histone oligomers. IV. Reassociation of chromatin from histones of various conformations.
Chicken erythrocyte chromatin, whole or (H1,H5)-depleted, was dissociated in 2 M NaCl in the presence or absence of 5 M urea at pH 8 or pH 5, and reconstituted by decreasing the concentration of NaCl and urea. The reassociated products were characterized by their solubilities in 0.1 mM EDTA, micrococcal nuclease digestion, cross-linking of histones, and fractionation of histone oligomers by solubility in ammonium sulfate solution. In the absence of urea, the nucleosome structure and the histone octamer were reconstituted perfectly at both pH 5 and pH 8. When chromatin was exposed to urea, no nucleosome structure or histone octamer was obtained at pH 5 either decreasing the concentration of salt first or that of urea first. At pH 8, the chromatin structure was regained fairly well by decreasing the concentration of urea first, but only partially by decreasing the concentration of salt first. Solubility in 0.1 mM EDTA was found to be a good criterion for monitoring the proper reassociation of chromatin.