Response to Hypoxia, Reduction of Electron Acceptors, and Subsequent Survival by Filamentous Fungi

Filamentous fungi usually inhabit normoxic environments by utilizing oxygen as a substrate for respiration and for the biosynthesis of some essential cellular components. This review examines the metabolic mechanisms used by filamentous fungi tinder oxygen-limited (hypoxic) conditions. Denitrification is one mechanism through which Fusarium oxysporum and other fungi reduce nitrate or nitrite to nitrous oxide, generating nitric oxide as a reaction intermediate. The involvement of cytochrome P450nor as a nitric oxide reductase is a unique feature of fungal denitrification, as opposed to cytochrome bc-type nitric oxide reductase, which is unique to the bacterial mechanism. Ammonia fermentation is the mechanism through which nitrate is reduced to ammonium, and it allows fungal growth tinder hypoxic conditions. Studies of the model filamentous fungus Aspergillus nidulans have revealed that niaD and niiA encoding NAD(P)H-dependent nitrate and nitrite reductases are essential for producing ammonia. Since niaD and niiA have been identified as genes for nitrate utilization by the fungus, ammonia fermentation and nitrate utilization mechanisms probably share a nitrate-reducing mechanism. I also discuss recent progress in studies of the hypoxic response of A. nidulans.