Ammonia switch-off is the immediate inactivation of nitrogen fixation that occurs when a superior nitrogen source is encountered.<br><br>In certain bacteria switch-off occurs by reversible covalent ADP-ribosylation of the dinitrogenase reductase protein, NifH.<br><br>Ammonia switch-off occurs in diazotrophic species of the methanogenic Archaea as well.<br><br>Nitrogenase activity resumed within a short time after ammonia was removed from a switched-off culture, suggesting that whatever the mechanism, it is reversible.<br><br>We demonstrate the physiological importance of switch-off by showing that it allows growth to accelerate substantially when a diazotrophic culture is switched to ammonia.<br><br>Nitrogenase activity resumed within a short time after ammonia was removed from a<br>switched-off culture, suggesting that whatever the mechanism, it is reversible
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