Optical model studies of salt-induced conformational transitions in the nucleosome.

Optical modeling of the DNA conformation has been used to interpret the results of highly sensitive flow birefringence and extinction angle studies on nucleosome core particles over a range of counterion strength (using KCl as the supporting electrolyte) from less than 0.15 mM to greater than 0.6 M. Results are consistent with an oblate disk or wedge of axial ratio p-1 congruent to 2 over an intermediate salt concentration range from about 1.5 to 450 mM. Below approximately 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 approximately 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.