Thermostability of chromatin and variations in chromosomal proteins isolated under different ionic conditions.

Chromatin was prepared from calf thymus by three different methods differing in the ionic composition of the solutions used. The contents of histones and nonhistones per DNA unit were dependent on the preparative method utilized. Variations in protein content were analyzed by SDS-polyacrylamide gel electrophoresis. Although the amount of all histones varied with the isolation method, the abundance of H1 histone in chromatin was the most sensitive to the presence of magnesium ions during the isolation procedure. The variation in protein content effects the thermal denaturation profiles of chromatin. The amount of low-temperature melting chromatin is most sensitive to the preparation method used. Additionally, a very high-temperature denaturation step, designated delta, is detected for chromatin prepared in the absence of magnesium.