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The effect of the nitrogen source on the cellular activity level of assimilatory nitrate reductase in the cyanobacteria Anabaena variabilis (ATCC29413) and Synechocystis sp. (PCC6714) has been examined. In the filamentous N2-fixing A. variabilis, nitrate behaved as a nutritional inducer of nitrate reductase, with ammonium acting (via products of its assimilation) as an antagonist with regard to nitrate. Ammonium-promoted repression of nitrate reductase was also evident in the unicellular non-nitrogen fixer Synechocystis, but in this strain nitrate was not required as an obligatory inducer.
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Herrero et al. (1985) studied this question.
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