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In Gloeocapsa, NH4 + formed by N2 fixation was assimilated by the glutamine synthetase-glutamate synthase pathway. The inhibition of acetylene reduction following addition of NH4 + to N2-fixing cultures was not caused by NH4 + 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 NH4 + assimilation, these analogues probably acted by inhibiting nitrogenase synthesis. Certain analogues of tryptophan and phenylalanine, which have been reported to overcome NH4 + inhibition of cell differentiation and, possibly, nitrogenase synthesis in heterocystous cyanobacteria growing on combined nitrogen, did not prevent NH4 + from inhibiting acetylene reduction in cultures of Gloeocapsa. In contrast, l-methionine-dl-sulphoximine, which similarly counteracts the effect of NH4 + on heterocyst differentiation, also prevented NH4 + from inhibiting acetylene reduction in Gloeocapsa.
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