DENITRIFICATION, A NOVEL TYPE OF RESPIRATORY METABOLISM IN FUNGAL MITOCHONDRION

Subcellular localization and coupling to ATP synthesis were investigat ed with respect to the denitrifying systems of two fungi, Fusarium oxy sporum and Cylindrocarpon tonkinense. Dissimilatory nitrate reductase of F. oxysporum or nitrite reductase of C. tonkinense could be detecte d in the mitochondrial fraction prepared from denitrifying cells of ea ch fungus. Fluorescence immunolocalization, cofractionation with mitoc hondrial marker enzymes, and cytochromes provided evidence that the de nitrifying enzymes are co-purified with mitochondria. Respiratory subs trates such as malate plus pyruvate, succinate, and formate were effec tive donors of electrons to these activities in the mitochondrial frac tions, Moreover, nitrite and nitrate reduction were shown to be couple d to the synthesis of ATP with energy yields (P:NO3- or P:2e ratios) o f 0.88 to 1.4, depending upon whether malate/pyruvate or succinate wer e provided as substrates, Nitrate or nitrite reductase activity was in hibited by inhibitors such as rotenone, antimycin A, and thenoyltriflu oroaeetone. Thus, fungal denitrification activities are localized to m itochondria and are coupled to the synthesis of ATP. The existence of these novel respiration systems are discussed with regard to the origi n and evolution of mitochondria.