Summary: The effects of different nitrogen sources on the H-uptake hydrogenase activity were studied in batch-grown and carbon- or sulphate-limited chemostat cultures of Hydrogenase activity was significantly higher in N-fixing than in NH - or NO -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 NH - to N-dependent growth in carbon-limited cultures. A possible explanation is that H produced by nitrogenase induced hydrogenase synthesis. Carbon-limited N-fixing cultures possessed higher hydrogenase activity than did sulphate- or O-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 H act when such repression is removed.


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