Regulation of Nitrite Reductase Cellular Levels in the Cyanobiont Nostoc ANTH

Abstract

The cellular nitrite reductase activity in response to different nitrogen sources has been studied in the cyanobiont Nostoc ANTH isolated from the liverwort Anthoceros. De novo protein synthesis of nitrite reductase occurred even in the absence of added nitrogen source, although enzyme activity was higher when nitrite or nitrate served as the sole nitrogen source. In tungstate-treated cells, nitrate did not show any positive effect on nitrite reductase activity, suggesting that the stimulatory effect was not due to nitrate itself but it requires its reduction to nitrite. Thus, nitrite may be the actual inducer in this case. Ammonium-grown cells showed reduced level of nitrite reductase activity. The nitrite reductase activity was freed from ammonium repression by L-methionine-DL-sulphoximine (MSX), an irreversible inhibitor of glutamine synthetase (GS). Thus ammonium metabolism through GS is required for the ammonium repression of nitrite reductase to occur.