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A protein complex, extracted from calf thymus chromatin with 2 M NaCl, pH 7.5,
containing approximately equal molar ratios of histones H2A, H2B, H3, and H4,
has been characterized in this study. Gel filtration, sedimentation velocity,
and sedimentation equilibrium experiments demonstrate that this complex, known
as the core complex, has a molecular weight near that expected for a histone
octamer (108 000 for a unit containing two each of the four inner histones) and
far exceeding that of a histone tetramer (54 400). This finding suggests that
the histone octamer, postulated to be the fundamental histone unit in chromatin,
is stable in 2 M NaCl, pH 7.5, in the absence of DNA or chemical cross-linking
reagents. In the second part of this study, we demonstrate that the bonds
maintaining the octameric complex in 2 M NaCl are weak and distinctly different
from the forces stabilizing the H2A-H2B dimer or H3-H4 tetramer. The octamer is
dissociated into two H2A-H2B dimers and one H3-H4 tetramer by (i) increasing
temperature; (ii) decreasing NaCl concentration; (iii) adding low concentrations
of urea or guanidine hydrochloride; and (iv) lowering the pH below 7 or raising
it above 10. These findings indicate that the octamer is assembled by two
sets-of protein-protein interactions. The first set involves mostly hydrophobic
interactions and yields the H2A-H2B dimer and the H3-H4 tetramer subunits. The
second set involves the weak association of one H3-H4 tetramer with two H2A-H2B
dimers to form an octamer. We suggest that these weak interactions might be
derived predominantly from histidine-lysine or histidine-tyrosine hydrogen bonds
between the dimer and tetramer subunits.
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