Effect of histone composition on the stability of chromatin structure

The mode of fragmentation of chromatin by micrococcal nuclease has been studied in nuclei from different sources at physiological ionic strength and low temperature. During digestion, the size of chromatin was reduced until an average S value of 95-100 (hen erythrocyte) or 60-65 (rat liver) was attained. The accumulation of these structures correlated with the period of maximum solubility (80%), indicating that the bulk of chromatin behaved in this manner. Further digestion did not result in a corresponding decrease in S value but in a bimodal sedimentation pattern. As opposed to this behavior, chromatin containing actively acetylated core histones showed a continuous variation in size during the digestion. Indirect immunoprecipitation of chromatin by anti-H5 antibody and sheep anti-rabbit antibody revealed that the acetylated chromatin is partially depleted of H5.