Twenty-five mutants unable to utilize nitrate as sole nitrogen source were isolated from 41. These mutations mapped at four different sites, and were located between and on the chromosome. mutants were affected in assimilatory nitrate reduction but not in‘respiratory’ nitrate reduction and had methyl viologen-coupled nitrate reductase activity. mutants were affected in both assimilatory and ‘respiratory’ nitrate reduction and lacked methyl viologen-coupled nitrate reductase activity. and mutants were impaired not only in assimilatory and ‘respiratory’ nitrate reduction but lacked xanthine dehydrogenase activity as well. Acid-treated crude extracts of these two mutant classes were unable to restore NADPH-coupled nitrate reductase activity to the mutant of , indicating the lack of active molybdenum cofactor. All mutants tested were effective in symbiotic plant tests and had normal nitrogenase activity, indicating that nitrogenase and nitrate reductase do not share the same molybdenum cofactor.


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