Glutamine biosynthesis and the utilization of succinate and glutamine by Rhizobium etli and Sinorhizobium meliloti

Abstract

Sinorhizobium meliloti 1021 and Rhizobium etli CE3 turn over nitrogen and carbon from glutamine to ammonium and CO2, respectively. Some of the ammonium released is assimilated back into glutamine, indicating that a glutamine cycle similar to that in Neurospora operates in Rhizobium. In addition, a previously unrecognized metabolic pathway in Rhizobium 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 Rhizobium is converted to succinate in glutamine-containing medium. Both S. meliloti 1021 and R. etli CE3 oxidize succinate preferentially over glutamine when provided with both carbon sources. In contrast to Sinorhizobium meliloti 1021 and Rhizobium etli CE3, an S. meliloti 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 S. meliloti 1021 and R. etli 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.

Key Role of Bacterial NH 4 + Metabolism in Rhizobium-Plant Symbiosis

“…Amino acid utilization requires active uptake, deamination/deamidation, common metabolic pathways, such as the citric acid cycle and gluconeogenesis, and NH 4 ϩ excretion. R. etli mutants showing very low levels of glutaminase, asparaginase, and aspartase activity are Nod ϩ /Fix ϩ (46,47,50,79). Enzymes involved in gluconeogenesis are expressed in BTs of R. leguminosarum, S. meliloti, and Rhizobium sp.…”

Glutamine Suppresses Airway Neutrophilia by Blocking Cytosolic Phospholipase A2 via an Induction of MAPK Phosphatase-1

“…The nonessential amino acid L-glutamine (Gln) is the most abundant amino acid in plasma (13) and is an energy substrate for most cells (14,15). Extensive research in animal models of endotoxin shock, including severe injury models, has demonstrated that supplementation with Gln improves survival, enhances immune and gut barrier function, decreases bacteremia, and inhibits gut mucosal atrophy (16,17).…”

Glutamine Protects Mice from Lethal Endotoxic Shock via a Rapid Induction of MAPK Phosphatase-1

“…T he nonessential amino acid L-glutamine (Gln) 4 is the most abundant amino acid in plasma (1), and Gln is an energy substrate for most cells (2,3). Consequently, Gln plays an important role in nitrogen and carbon-skeleton exchange among different tissues, where this amino acid fulfills many different physiological functions (4).…”