Exchange of H1 histone depends on aggregation of chromatin, not simply on ionic strength

Chromatin solubility was observed at several concentrations of various cations. Spermine and spermidine precipitated (50%) chromatin at about 0.2 mM, Ca2+ and Mg2+ at about 1-2 mM, and Na+ at about 100 mM. Further increases in cation concentration induced more aggregation, but eventually excess cation increased chromatin solubility so that 50% solubility was observed again at 60 mM Mg2+ and 180 mM Na+. H1 histone was 50% released by 80 mM MgCl2 or 425 mM NaCl. Combinations of MgCl2 and NaCl showed that Mg2+ and Na+ are synergistic in the induction of aggregation in lower concentrations (less than 2 mM) of Mg2+ but antagonistic at higher concentrations, and a similar effect of NaCl on spermidine-induced precipitation was shown below and above about 0.2 mM spermidine. At 5 mM, MgCl2 proved capable of precipitating chromatin depleted of H1 histone, but no concentration of NaCl was capable of doing so. These phenomena can be rationalized by supposing that neutralization of chromatin by any cation (including H1 histone) favors aggregation and also that cross-linking of chromatin fibers by multivalent cations (including H1 histone) is also critically important. The exchange of H1 histone between chromatin fragments was tested in various concentrations of different salts. H1 exchange was correlated with chromatin aggregation rather than with ionic strength and thus appears to depend on fiber to fiber contact. Under conditions where H1 exchanges between chromatin fibers that are permitted to make contact with each other, no H1 exchange occurred between chromatin inside the nucleus and chromatin outside, even though H1 histone is capable of passage through the nuclear membrane.