Genetical and biochemical studies on physiological function of intracellular nuclease of Aspergillus oryzae

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Aspergillus oryzae has nuclease O and its specific inhibitor protein in the cell. The nuclease and the inhibitor combine to make an inactive complex with a Ki of 3.2 x 10^<-12> (T.Uozumi et al., J.Biol. Chem., 251, 2808 (1976)). In the course of senescence of the fungi, especially under the deficiency of oxygen, the inhibitor protein is decomposed by protease, resulting in the activation of the nuclease, which in turn digests cellular nucleic acids. This study was intended to clarify the physiological function of intracellular nucleases including nuclease O.We reported the cloning of nuclease O gene (nucO)(M.Sano et al., Curr. Genet., 30, 312 (1996)). In the present study, a mutant strain of A.oryzae with disrupted nucO gene was constructed. The mutant showed an apparently normal growth comparable to the wild strain, which suggested that nuclease O is not essential for growth of the fungi.Recently, a caspase-activated DNase (CAD) and the specific inhibitor (ICAD) have been reported in animal cells. In the process of apoptosis, caspase degrades ICAD leading to the activation of CAD, which in turn degrades the cellular DNA.Physical properties of nuclease O (isoelectric point, pI=10.0) and the inhibitor (heat-stable, pI=4.1) are very similar to those of CAD and ICAD.So, there is a high possibility that the phenomenon of autolysis of A.oryzae corresponds to apoptosis of animal cells. More researches should be done to clarify the possibility.