Biological Function of the Ammonia‐Induced Inactivation of Glutamine Synthetase in Escherichia coli

In the first 15–30 see after addition of 10 mM NH4+ to Escherichia coli cells, one observes a 20‐fold increase in the concentration of glutamine, an approximate one‐third reduction in the stationary concentration of glutamate, an approximate 90% decrease in the concentration of ATP and a decrease in the activity of glutamine synthetase to a few percent of the initial value. In the subsequent 30 see, glutamine synthetase remains inactive, glutamine concentration decreases, glutamate concentration increases 2 to 3‐fold, and the ATP concentration rises slowly. The above‐mentioned changes after addition of NH4+, together with previous observations on properties of purified glutamine synthetase and adenylyl transferase, make the following sequence of events probably: NH4+ enters the cells and is quickly incorporated into glutamate yielding glutamine, with a concomitant utilization of ATP. The accumulated glutamine stimulates the adenylyl transferase resulting in an inactivation of glutamine synthetase. Thus, 15 sec after addition of NH4+, further synthesis of glutamine is prevented. Consequently, glutamate concentration increases, glutamine concentration decreases, and ATP concentration increases. On the basis of these findings and conclusions, the following biological functions of NH4+ inactivation of glutamine synthetase are discussed: (a) prevention of a too high level of glutamine and (b) prevention of a sustained decrease in the ATP concentration which would be harmful to the cell.