ABSTRACT
For a long time, the nonhistone proteins of chromatin presented a considerable challenge to biochemists concerned with the characterization of nuclear proteins. Elgin and Bonner isolated several fractions of chromatin nonhistone proteins in a purity sufficient to warrant their analysis. A special class of nuclear nonhistone proteins are the phosphoproteins. The group of nuclear phosphoproteins may indeed prove to be the fraction of nuclear nonhistone proteins containing genetic activator and repressor proteins similar to those found in bacteria. The most convincing evidence of tissue specificity of the chromatin nonhistone proteins was presented by Chytil and Spelsberg. If the nonhistone proteins of chromatin play a role in maintaining the restricted state of DNA in differentiated cells, they most likely function as variable repressors or activators as contrasted with the permanently restricting histones. While histone biosynthesis is tightly coupled to DNA replication, the nonhistone proteins are most actively labeled outside the period of the cell cycle.
