Comparative biochemical characterization of mutants at the nitrate reductase/molybdenum cofactor loci cnxA,cnxB, and cnxC of Nicotiana plumbaginifolia

Mutations in any of the three gene loci cnxA, cnxB, cnxC can lead to a total loss of nitrate reductase activity in Nicotiana species. The cnx loci are involved in synthesis and processing of the molybdenum cofactor, which is an essential structural constituent of nitrate reductase. The biochemical properties of cnxA, cnxB and cnxC mutant cell lines of Nicotiana plumbaginifolia were examined further. The cnxA line (N×9) was found to possess a catalytically defective but dimerization-active and under in vivo/in vitro-conditions repairable molybdenum cofactor, thus, resembling the properties of N. tabacum cnxA lines. The cnxB (N×24) and cnxC (N×21) mutants. however, show a phenotype very different from cnxA. This new phenotype is characterised by an irreversible loss of both the catalytic function and dimerization ability of the molybdenum cofactor which makes it likely that the molybdopterin moiety of the cofactor is defective or lacking in these mutants. In this report we summarize and compare the phenotypic data presently available for the Nicotiana loci cnxA, cnxB and cnxC. Possible functions of the gene products of these loci will be discussed.