Purification and Properties of an Endonuclease from the Mitochondrion of Saccharomyces cerevisiae

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Abstract
An endonuclease, which is found only in the mitochondrion of the yeast S. cerevisiae, was purified. The protein has a sedimentation coefficient of 6.3 S, equivalent to a MW of 105,000. The enzyme is active at pH 7.6, when it degrades signle-stranded DNA about 10-times faster than double-stranded DNA, but at pH 5.4 only double-stranded DNA is degraded. In both cases the enzyme acts endonucleolytically, breaking a single phosphodiester bond at a random location within the DNA substrate. Mn2+ or Mg2+ are required for activity; Ca2+ and Zn2+ are ineffective cofactors. Enzyme activity at pH 7.6 is severely inhibited by low concentrations of NaCl or KCl, while activity at pH 5.4 is unaffected by salt. Ethidium bromide inhibits both the DNase activity at pH 5.4 and the activity with single-stranded DNA at pH 7.6 but has no effect on the DNase activity with double-stranded DNA at pH 7.6.