High order packing of DNA as revealed by autodigestion of chromatin in small intestine cell nuclei.

Small intestine cell nuclei incubated in sucrose media released large fractions of DNA into the culture medium. This effect was partially or completely suppressed when incubation was carried out in the presence of a protease inhibitor, 10 to 30 mM NaHSO3. The DNA released in sucrose media containing NaHSO3 was precipitated as a DNA-protein complex by increasing the bivalent ion concentration to 10 mM Ca2+ or 20 mM Mg2+. Most of the released DNA was not precipitated by Ca2+ or Mg2+ when incubation was performed without NaHSO3. As determined by viscosity measurements the mean molecular weight of the DNA released in the absence of NaHSO3 was from 3.5-8.0 x 10(5) and increased to about 11 x 10(5) (corresponding to 8 nucleosomes) when the incubation mixture contained NaHSO3. End group analysis indicated that the DNA segments were terminated by 3'-OH groups. It is suggested that fragmentation of DNA in chromatin was produced by a endogenous alkaline endonuclease activity which was present in the fraction of released DNA. The data support the view that the third-order repeat structure of chromatin consists of subunits containing 8 nucleosomes.