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[Native and trypsin-treated histone oligomers--tetramer (H3-H4)2 and dimer H2a-h2b].

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.