IDENTIFICATION BY MUTATIONAL ANALYSIS OF 4 CRITICAL RESIDUES IN THE MOLYBDENUM COFACTOR DOMAIN OF EUKARYOTIC NITRATE REDUCTASE

The nucleotide sequence of the nitrate reductase (NR) molybdenum cofac tor (MoCo) domain was determined in four Nicotiana plumbaginifolia mut ants affected in the NR apoenzyme gene. In each case, missense mutatio ns were found in the MoCo domain which affected amino acids that were conserved not only among eukaryotic NRs but also in animal sulfite oxi dase sequences. Moreover an abnormal NR molecular mass was observed in three mutants, suggesting that the integrity of the MoCo domain is es sential for a proper assembly of holo-NR. These data allowed to pinpoi nt critical residues in the NR MoCo domain necessary for the enzyme ac tivity but also important for its quaternary structure.