| Sumario: | The effect of the nitrogen source on the cellular activity of ferredoxin-nitrate reductase in different cyanobacteria was examined. In the unicellular species Anacystis nidulans, nitrate reductase was repressed in the presence ofammonium but de novo enzyme synthesis took place in media containing either nitrate or no nitrogen source, indicating that nitrate was not required as an obligate inducer. Nitrate reductase in A. nidulans was freed from ammonium repression by Lmethionine-D,L-sulfoximine, an irreversible inhibitor of glutamine synthetase. Ammonium-promoted repression appears therefore to be indirect; ammonium has to be metabolized through glutamine synthetase to be effective in the repression of nitrate reductase. Unlike the situation in A. nidulans, nitrate appeared to play an active role in nitrate reductase synthesis in the filamentous nitrogen-fixing strains Anabaena sp. strain 7119 and Nostoc sp. strain 6719, with ammonium acting as an antagonist with regard to nitrate.
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