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Abstract
The effects of different nitrogen sources on the H2-uptake hydrogenase activity were studied in batch-grown and carbon- or sulphate-limited chemostat cultures of Azotobacter chroococcum. Hydrogenase activity was significantly higher in N2-fixing than in NH4 +- or NO3 −-dependent cultures under carbon limitation or in early-stationary phase batch growth but was not influenced by the nitrogen source in sulphate-limited cultures. Hydrogenase activity developed nearly two generation times later than nitrogenase activity during the transition from NH4 +- to N2-dependent growth in carbon-limited cultures. A possible explanation is that H2 produced by nitrogenase induced hydrogenase synthesis. Carbon-limited N2-fixing cultures possessed higher hydrogenase activity than did sulphate- or O2-limited cultures. It appears that carbon substrates or metabolites act as primary repressors of hydrogenase activity and that secondary effectors, such as nitrogen sources or H2, act when such repression is removed.
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