Dissimilatory nitrate metabolism by eukaryotic microorganisms and cytochrome P-450.

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1. I found that the fungus Fusarium orum could reduce nitrate or nitrite anaerobically to form nitrous oxide (N_2O). This is the first finding that a fungus exhibits a potent denitrifying activity, and would be an important result in microbiology.2. I isolated and identified two components involved in the dissimilatory nitrite reduction. A cytochrome c(c-553)was shown to exhibit nitrite reductase activity, and a cytochrome P-450(P-450)that had already been isolated (P-450_<dNIR>) was shown to be a nitric oxide reductase. The fungal denitrification system was thus clarit'llien in part at molecular level. The function of P-450_<dNIR> is quite unique, since P-450 has been known as a group of hemeproteins that exhibit monooxygenase activities. P-450_<dNIR> has thus proposed highly intriguing problems with respect to structure, function, and evolutional relationships between members of P-450 superfamily.3. Many fungi other than F. oxysporum were shown to exhibit denitrifying activities. The main product was N_2O in these fungal denitrification as in the case of Forum. Several fungi could not reduce nitrate. A few fungi, such as Fusarium solani and Cylindrocarpon tonkinense, formed N_2 as well as N_2O from nitrite. It was shown that only one nitrogen atom in the product N_2 was derived from nitrite, whereas both nitrogen atoms in N_2O were derived from nitrite. It was thus demonstrated that nitrogen atoms in compounds other than nitrate or nitrite were utilized as a counterpart of nitrogen atoms in the product N. . Such a phenomenon was also observed with F. oxysporum. Nitrogen atoms in azide or salicylhyd?oxamic acid was incorporated into N_2O as a counterpart, although these compounds were not capable of inducing denitrifying activity. Such "co-denitrification" is a unique feature of fungal denitrification.