Biochemical characterization of the molybdenum cofactor mutants of Neurospora crassa: in vivo and in vitro reconstitution of NADPH-nitrate reductase activity.

Molybdenum cofactor (MoCo) mutants of Neurospora crassa lack both NADPH-nitrate reductase and xanthine dehydrogenase activity. In vivo and in vitro studies to further characterize these mutants are now reported. The MoCo mutants nit-9A and nit-9B are capable of growing, albeit poorly, with nitrate as the sole nitrogen source, provided high levels of molybdate are present. The MoCo mutants nit-9A, nit-9B and nit-9C, but not nit-1, nit-7 or nit-8, have significant levels of NADPH-nitrate reductase when grown in nitrate medium containing 30 mM molybdate. In vitro reconstitution experiments using cell free extracts of the N. crassa MoCo mutants and E. coli HB101 as a source of wild-type MoCo were performed. MoCo from E. coli was capable of reconstituting NADPH-nitrate reductase activity to nit-1, nit-7 and nit-8. Molybdate is required for the in vitro reconstitution of NADPH-nitrate reductase activity. It was not possible to in vitro reconstitute NADPH-nitrate reductase activity in the MoCo mutants nit-9A, nit-9B or nit-9C.