We examined the denitrification system of the fungus Cylindrocarpon tonkinense and found several properties distinct from those of the denitrification system of Fusarium oxysporum. C. tonkinense could form N2O from nitrite under restricted aeration but could not reduce nitrate by dissimilatory metabolism, Nitrite-dependent N2O formation was accompanied by distinct cell growth. N2O formation and/or cell growth during the anaerobic culture was not affected by further addition of ammonium ions but was suppressed by respiration inhibitors such as rotenone or antimycin, suggesting that denitrification plays a physiological role in respiration, Dissimilatory nitrite reductase and nitric oxide reductase (Nor) activities could be detected in cell extracts of the denitrifying cells. The Nor activity was purified and found to depend on two isozymes of cytochrome P-450nor (P-450nor), which were designated P-450nor1 and P-450nor2. These isozymes differed in the N-terminal amino acid sequence, isoelectric point, specificity to the reduced pyridine nucleotide (NADH or NADPH), and the reactivity to the antibody to P-450nor off. oxysporum, The difference between the specificities to NADH and NADPH suggests that P-450nor1 and P-450nor2 play different roles in anaerobic energy acquisition.
