Size-dependence of a stable higher-order structure of chromatin

Abstract

We have recently reported a study of the formation of higher-order structures of chromatin with increasing ionic strength, in which we measured sedimentation coefficients of long nucleosome oligomers. We found that somewhere in the range of 30 to 60mer the sedimentation coefficient developed a jump of about 10% between ionic strengths of 45 m m and 55 m m, which persisted for larger oligomers (Butler & Thomas, 1980). This posed the question of whether the jump observed represented a greater relative compaction at high ionic strength on the part of long polymers, or a relatively lesser resistance to hydrodynamic shear forces at low ionic strength. We now define the dependence of the jump upon oligomer size in detail, the critical size being 50 nucleosomes. We also show that it occurs because the sedimentation of a large oligomer appears “slow” for its size at lower ionic strength, but “normal” at higher ionic strengths. We interpret this as the consequence of insufficient axial interaction to stabilize the helical coiling of long nucleosome filaments at low ionic strength, leading to a more open and slowly sedimenting structure.