Trypsin digestion of core chromatin

Chicken erythrocyte core chromatin was digested with trypsin for 18 h. Five major limit peptides were produced with mol.wts. from 10 800 to 8000 which arose from the central regions of each core histone. The basic amino- and carboxyl-terminal regions of each core histone were digested to small peptides having an average size of less than six amino acids. The small basic peptides, 25% by weight of the total histone, dissociated from the complex and could be removed by dialysis. The five major limit peptides remained bound to the DNA and contained all the secondary structure originally present in the native histones. Trypsin digestion decreased the supercoiling of the DNA in the complex and perturbed the tertiary structure of the histones. By contrast, there were no changes in the secondary structure of the large degraded histone fragments. However, when these were dissociated from the DNA, the secondary structure, which is predominantly alpha-helix, decreased by 50%. It is concluded that DNA binds strongly to the central regions of the core histones via alpha-helical segments on the polypeptide chains.