MOLYBDENUM COFACTOR BIOSYNTHESIS IN Neurospora crassa: BIOCHEMICAL CHARACTERIZATION OF PLEIOTROPIC MOLYBDOENZYME MUTANTS nit‐7, nit‐8, nit‐9A, B and C*

Abstract— 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‐9 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.