The So‐Called Non‐Histones from Acid‐Treated Calf Thymus Chromatin

When chromatin from calf thymus is treated with dilute hydrochloric acid under mild conditions variable proportions of histones become bound to the DNA, thus becoming insoluble and appearing as non‐histones. This is confirmed by model experiments in which artificial mixtures of pure histones and DNA were treated with acid. The acid insoluble residual proteins formed are strongly bound (presumably covalently) to the DNA, but can be separated by density gradient centrifugation at pH 11.5. Some properties of these “non‐histones” are discussed. Their amino acid analyses are different from those of the histones. The differences, also recognizable in model experiments must originate from the mild acid treatment in presence of DNA and suggest modifications of amino acids. These “non‐histones” are considered to be masked histones, because after strong alkali treatment definite histone fractions can be recognized by electrophoreses. The conclusion is drawn that mild acid treatment of chromatin artificially produces “non‐histones”.