An Endonuclease from Mitochondria of Neurospora crassa

Endonuclease activity has been found to be closely associated with mitochondria of Neurospora crassa. The evidence for this association is that: (a) the activity sediments with mitochondria even after extensive washing; (b) purified nuclease is not bound by freshly prepared mitochondria; (c) nuclease and cytochrome oxidase activities cosediment through a sucrose gradient; and (d) the mitochondria must be disrupted in order to observe significant activity. The possibility has not been eliminated, however, that this association represents the binding to mitochondria of a nuclease of extramitochondrial origin. The enzyme has been purified from both mitochondria and crude mycelial extracts. It degrades both deoxyribonucleic and ribonucleic acids to small oligonucleotides terminated by 5'-phosphomonoester groups, is most active at 37°, between pH values of 6.0 and 7.5, requires a divalent cation (Mg++, Mn++, or Co++), is inhibited by ethylenediaminetetraacetic acid, and is relatively unaffected by β-mercaptoethanol. These catalytic properties distinguish it from the extramitochondrial nuclease previously described in N. crassa.