Aggregation of nucleosomes by divalent cations

Conditions of precipitation of nucleosome core particles (NCP) by divalent cations (Ca2+ and Mg2+) have been explored over a large range of nucleosome and cation concentrations. Precipitation of NCP occurs for a threshold of divalent cation concentration, and redissolution is observed for further ad dition of salt. The phase diagram looks similar to those obtained with DNA and synthetic polyelectrolytes in the presence of multivalent cations, whic h supports the idea that NCP/NCP interactions are driven by cation condensa tion. In the phase separation domain the effective charge of the aggregates was determined by measurements of their electrophoretic mobility. Aggregat es formed in the presence of divalent cations (Mg2+) remain negatively char ged over the whole concentration range. They turn positively charged when a ggregation is induced by trivalent (spermidine) or tetravalent (spermine) c ations. The higher the valency of the counterions, the more significant is the reversal of the effective charge of the aggregates. The sign of the eff ective charge has no influence on the aspect of the phase diagram. We discu ss the possible reasons for this charge reversal in the light of actual the oretical approaches.