SUMMARY: , an N-fixing organism, was grown at constant concentrations of dissolved O When supplied with NH+ aerobically, its doubling time was 1 h; when it fixed N microaerophilically, its doubling time was 5.5 to 7 h and the optimal for growth was 0.005 to 0.007 atm. At its optimal for growth on N, assimilated 8 to 10 mg nitrogen/g carbon substrate used; its efficiency was less at higher levels. Nitrogenase in cell-free extracts required Mg and Mn, and the Fe-protein was activated by activating factor. The nitrogenase had an optimal pH of 7.1 to 7.4 and an apparent for acetylene of 0.0036 atm. Extracts of lost their nitrogenase activity on storage at -18 °, and activity was restored by adding purified Feprotein from other N-fixing bacteria.


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