G. diazotrophicus was grown on LGI-P medium. Combinations of citric acid and potassium phosphates were used to obtain media with different pH. G. diazotrophicus strain PAL5 (ATCC 49037) and MAD3A (a nifD' mutant for Hup+ determination) were used. Nitrogenase activity (H2 evolution under Ar:02) was measured in real time at ambient 02 concentrations (Vessey 1992). Hup activity of G. diazotrophicus was assessed by measuring H2 consumption by MAD3A in a closed chamber with wild type PAL5 and the Hup+ Ralstonia eutropha (ATCC 17699) as controls.
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