Summary: Sulphate-limited continuous cultures of showed a proportional repression of nitrogenase activity with increasing concentrations of ammonium ion in the influent medium. A fully repressed population had a 50 % greater bacterial density than a fully derepressed one. On derepression, synthesis of nitrogenase lagged for 90 min after exhaustion of NH+ from the medium but was complete within one doubling time. Casamino acids did not decrease the pre-fixation lag obtained under sulphate-limited conditions in the chemostat. Longer lags were obtained when NH+ was exhausted under N-limited conditions. Such lags were decreased by Casamino acids, yeast extract, or L-aspartate. Aspartate-N completely repressed nitrogenase under sulphate- or carbon-limited conditions but not under nitrogen limitation. In the initial stages of repression of nitrogenase synthesis by NH+, apparent production of active enzyme continued for a short time in the absence of protein synthesis ; nitrogenase was then diluted out as the organisms multiplied. Repression by NH+ was at the level of mRNA transcription according to studies with rifampicin and chloramphenicol. ‘Coding capacity’ for nitrogenase synthesis declined with a half-life of about 4.5 min following inhibition of RNA synthesis with rifampicin. NH+ did not influence the decay rate but stimulated translation of such nitrogenase-specifying mRNA as had been initiated.


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