Endodeoxyribonuclease from nuclei of bovine small intestinal mucosa: further studies on intranuclear localization and cleavage mechanism of the enzyme.

Most of the activity of endodeoxyribonuclease was extracted from isolated chromatin with buffer containing 0.6 M NaCl, indicating that the endonuclease is present as a chromatin-bound form. When nuclei or chromatin of calf thymus or rat liver was digested with the bovine nuclear enzyme in the presence of 2 mM EGTA, to suppress endogenous Ca, Mg-dependent DNase activity, discrete DNA bands with integral multiples of 200 base pairs in length were produced, but no acid-soluble nucleotide was detected. The enzyme made single-strand breaks in pBR322 DNA and degraded it to fragments of limited size. The size of the final products of DNA's of Micrococcus luteus, rat liver nuclei, and calf thymus nuclei was about 3,000, 200, and 160 base pairs, respectively, but the enzyme showed no base specificity. Thus the endonuclease seems preferentially to recognize AT-rich regions of double-stranded DNA and to make single-strand breaks.