SUMMARY: The effect of the nitrogen source on the activity of ferredoxin-dependent nitrite reductase has been studied in the cyanobacterium synthesis of nitrite reductase occurred in the absence of an added nitrogen source, although enzyme activity was higher when the medium contained either NO or NO . The positive effect of NO on nitrite reductase was also evident in tungstate-treated , which lacked an active nitrate reductase, indicating that the stimulatory effect was due to NO itself and not to the NO resulting from its intracellular reduction. NH acted as a repressor, overriding any positive effect of NO or NO . Nitrite reductase synthesis was freed from NH repression by -methionine--sulphoximine, an irreversible inhibitor of glutamine synthetase. NH must therefore be metabolized through glutamine synthetase before repressing nitrite reductase. The kinetics of nitrate reductase and nitrite reductase development were similar in cells transferred from NH - to NO -containing media, suggesting a coordinate regulation of synthesis.


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