Regulation of the Synthesis and Activity of Ammonia Monooxygenase in Nitrosomonas europaea by Altering pH To Affect NH3 Availability

The obligately ammonia-oxidizing bacterium Nitrosomonas europaea was incubated in medium containing 50 mM ammonium. Changes in the concentration of nitrite, the pH, and the NH4 and NH2OH-dependent O2 uptake activities of the cell suspension were monitored. The NH4 -dependent O2 uptake activity doubled over the first 3 h of incubation and then slowly returned to its original level over the following 5 h. The extent of stimulation of NH4 -dependent O2 uptake activity was decreased by lowering the initial pH of the medium. Radiolabeling studies demonstrated that the stimulation of NH4 -dependent O2 uptake activity involved de novo synthesis of several polypeptides. Under O2-limited conditions, the stimulated NH4 -dependent O2 uptake activity was stabilized. Rapid, controlled, and predictable changes in this activity could be caused by acidification of the medium in the absence of ammonia oxidation. These results indicate that the NH4 dependent O2 uptake activity in N. europaea is strongly regulated in response to NH3 concentration.