Optical model studies of salt-induced conformation transitions in the nucleosome

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Abstract
Optical modeling of the DNA conformation was used to interpret the results of highly sensitive flow birefringence and extinction angle studies on [chicken erythrocyte] nucleosome core particles over a range of counterion strength (using KCl as the supporting electrolyte) from < 0.15 mM to > 0.6 M. Results are consistent with an oblate disk or wedge of axial ratio p-1 .simeq. 2 over an intermediate salt concentration range from about 1.5-450 mM. Below .apprx. 1.5 mM, the particle appears to unfold into an extended prolate or oblate structure which can be modeled as a uniform superhelix of DNA. Above .apprx. 0.45 M, the particle unfolds into a conformation which is hydrodynamically similar to but optically quite different from the low-salt structure. This form can be modeled as a partially unfolded disk in which only the nucleosomal DNA ends become dissociated and the central region remains bound to the histone core. A description of the optical modeling methods is presented, and the results are correlated with hydrodynamic property changes.

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