Condensation of polynucleosome by histone H1 binding

The cooperative binding of histone H1 to polynucleosome was studied quantitatively. The equilibrium and kinetic data were satisfactorily described in terms of the large ligand model. The binding constant K and the cooperativity parameter q showed remarkable salt effects: K = 7.5 X 10(7) M-1 and q = 1.3 X 10(4) at 0.2 M NaCl, pH 7.5 and 20 degrees C. This considerably strong cooperativity reveals that the polynucleosome state, condensed or not, is sensitive to small changes in the free histone H1 concentration around the value of 1/K. The association rate constant was of the order of 10(7) M-1 X s-1 and had a small salt concentration dependence, indicating a diffusion-controlled association process.