Effect of ammonia on nitrogen fixation by the blue-green alga Anabaena cylindrica

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Ammonia at a concentration of 1 × 10^<-3>M completely inhibited nitrogenase activity, as measured by acetylene reduction, in the blue-green alga Anabaena cylindrica. Free ammonia was undetectable in cells grown either on N_2 or ammonia within the limits of precision of the method used. Glutamic acid formed a major amino acid pool in N_2-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 N_2-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 system.
日本植物生理学会の論文