SUMMARY: Metronidazole (2-methyl-5-nitroimidazole-l-ethanol) was used to select nitrogen fixation (Nif) mutants of , either by enrichment with metronidazole, or by direct selection on agar medium containing the drug. By the latter method, approximately 50% of the isolates obtained were Nif. Several Nif mutants showed the phenotype expected for mutants defective in electron transport to nitrogenase: their nitrogenase activity (determined as rate of acetylene reduction) relative to that of the wild-type strain was higher when assayed in toluene-treated cells in the presence of dithionite, than when assayed in resting cell suspensions with -malate as source of electrons. Glutamate-grown cells of one such mutant (RC5) resembled N-starved cells of the wild-type, and differed from glutamate-grown cells of the wild type in that nitrogenase activity in toluene-treated cells was unaffected by the addition of Mn and the rate of H production by resting cell suspensions was significantly higher than the rate of acetylene reduction. These observations implied that glutamate-grown cells of the mutant contained predominantly nitrogenase A, the form of nitrogenase which is independent of the Mn-dependent membrane-bound activating factor, suggesting that the mutant may be affected in the interconversion of nitrogenase between the regulatory forms A and R.


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