1021 and CE3 turn over nitrogen and carbon from glutamine to ammonium and CO, respectively. Some of the ammonium released is assimilated back into glutamine, indicating that a glutamine cycle similar to that in operates in In addition, a previously unrecognized metabolic pathway in was discovered - namely, conversion of glutamine-carbon to γ-hydroxybutyric acid and β-hydroxybutyric acid. Additionally, some of the 2-oxoglutarate derived from glutamine catabolism in is converted to succinate in glutamine-containing medium. Both 1021 and CE3 oxidize succinate preferentially over glutamine when provided with both carbon sources. In contrast to 1021 and CE3, an double mutant that lacks both glutamine synthetase (GS) I and II preferentially oxidizes glutamine over succinate when supplied with both substrates. GSII activity is induced in wild-type 1021 and CE3 grown in succinate-glutamine medium, and this enzyme participates in the cycling of glutamine-carbon and -nitrogen. On the other hand, GSII activity is repressed in both micro-organisms when glutamine is the only carbon source. These findings show that, in medium containing both glutamine and succinate, glutamine synthesis helps drive the utilization of succinate. When glutamine is in excess as an energy-providing substrate its synthesis is restricted, allowing for more effective utilization of glutamine as an energy source.


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