MOLYBDENUM COFACTOR BIOSYNTHESIS IN NEUROSPORA-CRASSA - BIOCHEMICAL-CHARACTERIZATION OF PLEIOTROPIC MOLYBDOENZYME MUTANTS NIT-7, NIT-8 NIT-9A, NIT-9B AND NIT-9C

Available mutants of molybdenum cofactor (MoCo) biosynthesis of Neurospora crassa were studied for converting factor activity and for in vitro molybdate repair of nitrate reductase (NR) activity. Mutant nit-7 was found to contain an activity that fits the functional definition of converting factor activity in Escherichia coli. Its high molecular weight fraction converts a low molecular weight compound from nit-1 and nit-8 into biologically active molybdopterin (MPT). Like nit-1, mutant nit-8 is devoid of this activity. Mutants nit-9 A, B and C contain a protein-bound precursor form of MoCo, which is presumed to be MPT bound to apo-NR. It is converted into active MoCo as part of NR in the presence of reduced glutathione and high exogenous molybdate concentrations. The NR apoenzyme of nit-1 is needed to detect the total amount of MoCo after molybdate repair, because mutants nit-g A, B and C build no detectable content of functional NR apoenzyme. Evidence is presented for the transfer of MPT from demolybdo-NR to free NR apoenzyme.