Modification of histone binding in calf thymus chromatin and in the chromatin-protamine complex by acetic anhydride.

A relationship between side-chain modification of histones and their displaceability from DNA has been investigated using calf thymus chromatin which was chemically acetylated with acetic anhydride. When the chromatin is treated with increasingly higher concentrations of the reagent, histones become acetylated to an increasingly greater extent, attaining the modification at 23-24 sites for histone I, 5-6 for IIb1, 9-10 for IIb2, 5-6 for III and 3-4 for IV. As the chromatin becomes more acetylated, NaCl concentrations required for histone removal are lowered. Saturation binding of protamine does not bring about either an increase in the number of acetylation sites of histones in chromatin or a decrease of the NaCl requirement for dissociation of the acetylated chromatins. A comparison of the present results with the extents of histone acetylation known to occur enzymatically in vivo indicates that the complete removal of somatic histones during transformation of chromatin in spermiogenesis cannot be explained on the basis of decreased binding of the histone to DNA by acetylation or by a combination of acetylation and protamine binding, suggesting that the displacement process may require some additional processes.