Previously, we showed that anaerobic induction of respiratory nitrate reductase (NR) activity in Bradyrhizobium sp. (Lupinus) USDA 3045 is strongly enhanced by nitrate or nitrite through de novosynthesis. Here, multiple NR-active soluble forms, ranging from 75kDa to 190kDa, were observed under anaerobic conditions. Electrophoretic activity band patterns differed depending on the level and the type of the N oxyanion added. The intensity of the membrane-bound NR activity band of 230kDa changed with time along with consumption of 2 mM nitrate. It was associated with a parallel 5-fold increase and then 2-fold reduction in the amount of membrane-bound NR protein. In contrast, on 4 mM nitrate, the level of NR protein was much more stable, apparently due to slower nitrate depletion. Moreover, in cells anaerobically grown without nitrate addition, a 42-kDa derivative of NR degradation was immunodetected, which was not observed if nitrate was present in the medium. These findings suggest that the amount of the respiratory NR protein could be negatively regulated by endogenous proteases in relation to the level of nitrate available. It seems, therefore, that multiple native forms might be not different isoenzymes but immature complexes or derivatives of the enzyme protein turnover. This report adds to a modest list of bacterial enzymes apparently regulated by proteolysis, such as GS, MurAA, EnvA, GdhA, and MetA.
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