Résumé
In
cnx mutants of
Nicotiana plumbaginifolia, the over-expression of nitrate reductase (NR)-associated NADH cytochrome
c reductase (Ccr) activity appears to be correlated with increased levels of NR mRNA. However, in ELISA tests,
cnxB, C, D, E and F extracts present low levels of cross-reacting material, whereas
cnxA extracts exhibit very high levels, indicating an altered NR structure in all
cnx mutants except
cnxA, due to the absence of a functional molybdenum cofactor (MoCo). Detection of dimeric NR in all
cnx mutants, either by gel filtration or native gel electrophoresis, shows that the cofactor is not required for enzyme dimerization, but probably for the stability of the dimer. Similarities between results obtained with
cnxB through F mutants, and those
nia mutants impaired in the molybdopterin domain, suggest a related NR structure.