Transient unfolding of trypsin-digested chromatin core particles.

Limited digestion of nucleosome core particles with trypsin caused cleavage and removal of N-terminal histone sequences of 10-30 amino acids. The proteolyzed core particles exhibited salt-dependent structural transitions revealed by sedimentation, circular dichroism and nuclease-cutting assays, while the intact nucleosome cores were not affected under the experimental conditions. The results obtained indicate that the observed transitions correspond to the transient unfolding of terminal segments of core particle nucleoprotein caused by the increase of its net negative charge. The excision of the N-terminal histone domains therefore leads to partial destabilization but not to irreversible disruption of the compact nucleosome structure.