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The native oligomers of histones isolated from calf thymus nuclear chromatin were investigated. After mild treatment with trypsin, the tetramer, (H3-H4)2, has a molecular weight of 36 000, whereas Mr of the dimer, H2a-H2b, is equal to 25 000. Intact oligomers have Mr of 55 000 (tetramer) and 33 000 (dimer). Analysis of the fluorescence intensity changes indicates that the native tetramer can exist in three, while the dimer in two conformational states. The (H2a-H2b) dimer persists, but the (H3-H4)2 tetramer does not persist these transitions after proteolytic degradation. The trypsin-treated dimer H2a-H2b is highly labile and readily aggregates, while the tetramer (H3-H4)2 loses its aggregation capacity. It is assumed that the conformational features observed during the aggregation of histone oligomers may play a role in the assembly and structural transitions in nucleosomes and chromatin.

The accessibility to trypsin of "core" histones within the dimer (H2A-H2B), tetramer (H3-H4)2, octamer (H2A-H2B-H3-H4)2 and in chromatin was studied. It was shown that the hydrolysis of histones H2A and H2B within the dimer and octamer occurs in essentially the same way. The tetramer (H2-H4)2 becomes more compact with an increase in the ionic strength. Some of the tetramer (H3-H4)2 sites within the octamer are protected against trypsin. It was demonstrated that in terms of the histone accessibility to trypsin chromatin can exist in three states, i.e., tightly packed (in the presence of histone H1 and bivalent cations), intermediate (in the absence of histone H1 or bivalent cations) and folded (in the absence of histone H1 and bivalent cations). The folding of histones in neither of these chromatin states coincides with that within the octamer in 2M NaCl.