Dissimilatory nitrate reduction metabolisms and their control in fungi.

Most fungi grow under aerobic conditions by generating ATP through oxygen respiration. However, they alternatively express two pathways of dissimilatory nitrate reduction in response to environmental oxygen tension when the oxygen supply is insufficient. The fungus Fusarium oxysporum expressed the pathway of respiratory nitrate denitrification that is catalyzed by the sequential reactions of nitrate reductase and nitrite reductase. These enzymes are coupled with ATP generation through the respiratory chain and produce nitric oxide. Fungal nitric oxide reductase uses NADH as the direct electron donor in contrast to bacterial systems and thus might function in regeneration of NAD+ and detoxification of the toxic radical, nitric oxide. Another pathway of nitrate dissimilation by fungi reduces nitrate to ammonium and couples acetogenic reaction with substrate-level phosphorylation. This metabolic mechanism is also in feature of a variety of fungi and it is called ammonia fermentation. Thus, fungi adapt to various aerated conditions using these pathways of nitrate dissimilation in addition to conventional oxygen respiration.