The role of molybdenum in the synthesis of Neurospora nitrate reductase

The enzyme complex exhibiting NADPH-nitrate reductase activity in Neurospora crassa has associated with it nitrate-inducible NADPH-cytochrome c reductase activity and reduced benzyl viologen-nitrate reductase activity. Induction of Neurospora mycelia by nitrate under molybdenum-deficient or tungstate-toxic conditions results in low NADPH-nitrate reductase and reduced benzyl viologen-nitrate reductase activities, but the level of NADPH-cytochrome c reductase activity is the same as under normal conditions. By means of sucrose density gradient centrifugation, it is shown that the NADPH-cytochrome c reductase induced under molybdenum deficiency or tungstate toxicity has the same s20, w value as the nitrate reductase complex induced under normal conditions, but has some other properties that are different. It is proposed that during molybdenum deficiency or tungstate toxicity, the component proteins of the nitrate reductase complex are all synthesized, but owing to the non-availability of molybdenum, the molybdenum-binding site on the enzyme is folded in vivo in such a way that it can no longer bind this metal in vitro. The enzyme consequently has no nitrate reductase activity, but retains its cytochrome c reductase activity which does not require molybdenum.