SUMMARY: In , NH+ formed by N fixation was assimilated by the glutamine synthetase-glutamate synthase pathway. The inhibition of acetylene reduction following addition of NH+ to N-fixing cultures was not caused by NH+ itself but was, most probably, related to increased intracellular levels of glutamine.

Analogues of several amino acids inhibited acetylene reduction. Apart from glutamine analogues, which interfered with NH+ assimilation, these analogues probably acted by inhibiting nitrogenase synthesis. Certain analogues of tryptophan and phenylalanine, which have been reported to overcome NH+ inhibition of cell differentiation and, possibly, nitrogenase synthesis in heterocystous cyanobacteria growing on combined nitrogen, did not prevent NH+ from inhibiting acetylene reduction in cultures of . In contrast, -methionine--sulphoximine, which similarly counteracts the effect of NH+ on heterocyst differentiation, also prevented NH+ from inhibiting acetylene reduction in .


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