Ammonia at a concentration of 1×10−3Mcompletely inhibited nitrogenase activity, as measured by acetylene reduction, in the blue-green algaAnabaena cylindrica. Free ammonia was undetectable in cells grown either on N2or ammonia within the limits of precision of the method used. Glutamic acid formed a major amino acid pool in N2-grown cells, and basic amino acids, i.e. lysine, histidine and arginine were abundant in ammonia-grown cells. A 10-fold increase in the amounts of labile amino compound(s) was observed when N2-grown cells were exposed to ammonia.When cells were incubated under anaerobic conditions, the acetylene-reducing activity increased 2-fold or more; ammonia had no effect. Oxygen was required for ammonia to inhibit acetylene reduction.Modes of inhibition by ammonia on acetylene reduction were compared with those by chloramphenicol, puromycin, cycloheximide, DCMU and CCCP. On the basis of these comparisons we concluded that ammonia not only acts as a suppressor of nitrogenase synthesis but also inhibits acetylene-reducing activity by lowering the supply of reductant and/or of energy for the nitrogenase syst
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