NITRATE REDUCTASE-DEFICIENT MUTANT-CELL LINES OF NICOTIANA-TABACUM - FURTHER BIOCHEMICAL-CHARACTERIZATION

The wild-type line and 14 nitrate reductase-deficient mutant cell lines of N. tabacum were tested for the presence of nitrate reductase partial activities, and for nitrite reductase and xanthine dehydrogenase activity. Data characterizing the electron donor specificity of nitrate reductase (EC 1.6.6.1., NADH:nitrate oxidoreductase) and nitrite reductase (EC 1.7.7.1., ferredoxin:nitrite oxidoreductase) of the wild-type line are presented. Three lines (designated cnx) simultaneously lack NADH-, FADH2-, reduced benzyl viologen-nitrate reductase and xanthine dehydrogenase activities, but retain the nitrate reductase-associated NADH-cytochrome c reductase activity. These mutants are, therefore, interpreted to be impaired in gene functions essential for the synthesis of an active Mo-containing cofactor. For cnx-68 and cnx-101, the sedimentation coefficient of the defective nitrate reductase molecules does not differ from that of the wild-type enzyme (7.6 S). In 11 lines (designated nia) xanthine dehydrogenase activity is, unaffected and the loss of NADH-nitrate reductase is accompanied by a loss of all partial activities, including NADH-cytochrome c reductase. However, 1 line (nia-95) possessed a partially active nitrate reductase molecule, retaining its FADH2- and reduced benzyl viologen nitrate reductase activity. It is likely that nia-95 is a mutation in the structural gene for the apoprotein. Both the nia and cnx mutant lines exhibit nitrite reductase activity, being either nitrate-inducible or constitutive. Evidently, in N. tabacum, nitrate, without being reduced to nitrite, is an inducer of the nitrate assimilation pathway.