Ionic strength dependent structural changes of nucleosomes

Rat thymus nucleosomes were studied as to their sedimentation behaviour and molecular weight Mr in NaCl and (NH)2SO4 solutions of different ionic strengths. The sedimentation coefficient s20,w decreases in two steps at ionic strengths of about 0.45-0.6 and of 1-1.2 and amounts to 11.0 S in salt-free buffer and to 5.5 S in 2 M NaCl-solution. This decrease in s20,w is paralleled by an analogous decrease in Mr. There is, however, an additional decline in Mr from 233 000 to 200 000 between the ionic strengths of 0.005 and 0.4. These changes are accompanied by conformational changes of nucleosomes as shown by varying molar frictional ratios f/f0. The decrease in Mr could be attributed to the successive release of histones as checked by electrophoretic analysis. Furthermore, nucleosomes in (NH4)SO4-solution proved to be more stable against increasing salt concentration than nucleosomes in NaCl-solution.