Regulation and biochemical characterization of the glutamine synthetase of azotobacter vinelandii

Diazotrophic Growth Allows Azotobacter vinelandii To Overcome the Deleterious Effects of a glnE Deletion

Overcoming the inhibitory effects of excess environmental ammonium on nitrogenase synthesis or activity and preventing ammonium assimilation have been considered strategies to increase the amount of fixed nitrogen transferred from bacterial to plant partners in associative or symbiotic plant-diazotroph relationships. The GlnE adenylyltransferase/adenylyl-removing enzyme catalyzes reversible adenylylation of glutamine synthetase (GS), thereby affecting the posttranslational regulation of ammonium assimilation that is critical for the appropriate coordination of carbon and nitrogen assimilation. Since GS is key to the sole ammonium assimilation pathway of Azotobacter vinelandii, attempts to obtain deletion mutants in the gene encoding GS (glnA) have been unsuccessful. We have generated a glnE deletion strain, thus preventing posttranslational regulation of GS. The resultant strain containing constitutively active GS is unable to grow well on ammonium-containing medium, as previously observed in other organisms, and can be cultured only at low ammonium concentrations. This phenotype is caused by the lack of downregulation of GS activity, resulting in high intracellular glutamine levels and severe perturbation of the ratio of glutamine to 2-oxoglutarate under excess-nitrogen conditions. Interestingly, the mutant can grow diazotrophically at rates comparable to those of the wild type. This observation suggests that the control of nitrogen fixation-specific gene expression at the transcriptional level in response to 2-oxoglutarate via NifA is sufficiently tight to alone regulate ammonium production at levels appropriate for optimal carbon and nitrogen balance.IMPORTANCE In this study, the characterization of the glnE knockout mutant of the model diazotroph Azotobacter vinelandii provides significant insights into the integration of the regulatory mechanisms of ammonium production and ammonium assimilation during nitrogen fixation. The work reveals the profound fidelity of nitrogen fixation regulation in providing ammonium sufficient for maximal growth but constraining energetically costly excess production. A detailed fundamental understanding of the interplay between the regulation of ammonium production and assimilation is of paramount importance in exploiting existing and potentially engineering new plant-diazotroph relationships for improved agriculture.

In vitro effects of divalent metal ions and metabolic modulators on purified glutamine synthetase from brain of teleostean fish

The purified brain glutamine synthetase of a teleostean fish, Clarias batrachus has been examined under the influence of divalent metal ions activation and in vitro studies using different amino acids and metabolic modulators. All observations of the enzyme activity are based on transferase reaction at optimum temperature and pH. The enzyme exhibits an absolute requirement for Mn2+ (most potent) Mg2+ and Co2+. The activity is markedly inhibited by leucine, asparagine, isoleucine, carbamyl phosphate, uridine monophosphate and glutamate and activated by alpha-ketoglutarate. The significance of these results has been discussed in brief with emphasis on its involvement in teleostean physiology and metabolism.

Isolation of ntrA-like mutants of Azotobacter vinelandii

A number of chlorate-resistant mutants of Azotobacter vinelandii affected in a general control of nitrogen metabolism were isolated. These mutants could not utilize dinitrogen, nitrate, or nitrite as a nitrogen source. The reason for this inability is that they were simultaneously deficient in nitrogenase and nitrate and nitrite reductase activities. They were complemented by a cosmid carrying a DNA fragment of A. vinelandii able to complement ntrA mutants of Escherichia coli, so they seemed to be ntrA-like mutants.

Purification and characterization of glutamine synthetase from Clostridium pasteurianum

Glutamine synthetase from Clostridium pasteurianum grown on molasses as the sole carbon source and ammonium chloride as the nitrogen source has been purified to homogeneity (45-fold) with 32% recovery. The procedure involves ammonium sulfate precipitation and chromatography on a combined Sepharose 4B/DEAE-Sephadex A-50 column. The purified enzyme being very unstable was stabilized by the addition of 25% (v/v) glycerol. The enzyme has an unusually high molecular weight of 1 X 10(6) and 20 subunits of Mr 50 000 each, as determined by gel filtration and sodium dodecyl sulfate gel electrophoresis, respectively. It has an absorption maximum at 280 nm and a fluorescence emission maximum at 380 nm when excited at 280 nm. Its substrate binding pattern as studied by fluorescence quenching studies is different from that of the Escherichia coli enzyme. Both the gamma-glutamyltransferase and synthetase activities reside in the same protein as the ratio of the two activities at each step of purification remains constant and the enzyme exhibits optimal transferase and synthetase activities at the same pH (7.2) and temperature (50 degrees C). The thermal stabilities of both activities were also similar, and decay of both the activities at 50 degrees C ran parallel. The enzyme shows stabilization by substrates, as L-glutamate, Mg2+, and ATP + Mg2+ protected both the synthetase and gamma-glutamyltransferase activities against thermal inactivation. Storage in 25% (v/v) glycerol enhanced the thermal stability of glutamine synthetase. Metal ion requirement and substrate specificity of the enzyme have been examined. Maximum synthetase activity occurs when [Mg2+]: [ATP] = 2. The Km app values are as follows (in parentheses): ATP (0.34 mM), NH2OH (0.4 mM in the synthetase reaction and 4.1 mM in the transferase reaction), glutamine (14.7 mM), ADP (3.8 X 10(-4) mM), arsenate (2.5 mM), and L-glutamate (3.4 mM, 22.2 mM). The enzyme exhibits negative cooperativity in the binding of glutamate. Amino acids such as L-serine, glycine, L-alanine, and L-aspartic acid inhibit the enzyme.

Purification and regulation of glutamine synthetase in a collagenolytic Vibrio alginolyticus strain

Glutamine synthetase (EC 6.3.1.2) has been purified from a collagenolytic Vibrio alginolyticus strain. The apparent molecular weight of the glutamine synthetase subunit was approximately 62,000. This indicates a particle weight for the undissociated enzyme of 744,000, assuming the enzyme is the typical dodecamer. The glutamine synthetase enzyme had a sedimentation coefficient of 25.9 S and seems to be regulated by adenylylation and deadenylylation. The pH profiles assayed by the gamma-glutamyltransferase method were similar for NH4-shocked and unshocked cell extracts and isoactivity point was not obtained from these curves. The optimum pH for purified and crude cell extracts was 7.9. Cell-free glutamine synthetase was inhibited by some amino acids and AMP. The transferase activity of glutamine synthetase from mid-exponential phase cells varied greatly depending on the sources of nitrogen or carbon in the growth medium. Glutamine synthetase level was regulated by nitrogen catabolite repression by (NH4)2SO4 and glutamine, but cells grown in the presence of proline, leucine, isoleucine, tryptophan, histidine, glutamic acid, glycine and arginine had enhanced levels of transferase activity. Glutamine synthetase was not subject to glucose, sucrose, fructose, glycerol or maltose catabolite repression and these sugars had the opposite effect and markedly enhanced glutamine synthetase activity.