Genetic and Biochemical Analysis of Mutants Affected in Nitrate Reduction in Rhizobium meliloti

Abstract

Mutants (25) unable to utilize nitrate as sole N source were isolated from R. meliloti 41. These mutations mapped at 4 different sites, narA, narB, narC and narD; narB, C and D were located between trp-15 and ade-15 on the chromosome. NarA mutants were affected in assimilatory nitrate reduction but not in respiratory nitrate reduction and had methyl viologen-coupled nitrate reductase activity. NarB mutants were affected in assimilatory and respiratory nitrate reduction and lacked methyl viologen-coupled nitrate reductase activity. NarC and narD mutants were impaired in assimilatory and respiratory nitrate reduction and lacked xanthine dehydrogenase activity. Acid-treated crude extracts of these 2 mutant classes were unable to restore NADPH-coupled nitrate reductase activity to the nit-1 mutant of Neurospora crassa, indicating the lack of active Mo cofactor. All mutants tested were effective in symbiotic plant [Medicago sativa] tests and had normal nitrogenase activity, indicating that nitrogenase and nitrate reductase do not share the same molybdenum cofactor.