SUMMARY: H production by nitrogenase was investigated in O-, N-, C- or SO -limited continuous cultures of . H evolution occurred under air and was augmented when Ar replaced N. Pretreatment of each culture with 40% acetylene in air or Ar/O mixtures inhibited the H-uptake hydrogenase and increased H evolution. H production in each culture was O-dependent, and, like acetylene reduction by nitrogenase, it increased to a maximum at an optimum O concentration and was inhibited by excess O. The molar ratio of H produced to N reduced was least at the optimum O concentration and increased sharply under O-limiting or O-inhibiting conditions in vivo. The minimum value achieved was approximately 1 in O-, N- or C-limited cultures, or with purified nitrogenase components assayed in vitro, and 0·5 in SO -limited cultures. These differences may reflect different mechanisms of N reduction in vivo. Estimates of the levels of nitrogenase component proteins indicated Ac1: Ac2 ratios of 1 or less. However, Ac2 activity may be inhibited under O-limiting or O-inhibiting conditions. The maximum ratio of the number of electrons transferred to nitrogenase and to O was 0·1 in assays with O-, C- or SO -limited cultures; thus, recycling by the H-uptake hydrogenase of H produced by nitrogenase could contribute up to 7% of the total energy produced by respiration.


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