The reaction of the Ca-Mg endonuclease with the A-sites of rat nucleoprotein.

A quantitative study has been made on the action of rat nuclear Ca-Mg endonuclease on rat liver nuclei. In a standard 30 minute digest 0.5-1.5% of the DNA was rendered acid soluble, 1.5-4% of the chromatin was rendered buffer soluble, 50-60% of the potential cleavage sites were actually cleaved, and these cleavages were distributed evenly throughout the bulk of the genome. During these standard digests there was no significant loss of histones either in aggregate or relative to each other. Trypsin digestion of the nuclei to a trypsin resistant core did not lower the specificity of the Ca-Mg endonuclease cleavages or expose other sites to its action. Evidence is presented that indicates Ca-Mg endonuclease and micrococcal nuclease attack the same sites rather than different sites with the same spacing.