Purification and properties of Azotobacter vinelandii glutamine synthetase.

A procedure is described for the purification of glutamine synthetase from the nitrogen-fixing organism Azotobacter vinelandii. Electron micrographs of the enzyme reveal a dodecameric arrangement of its subunits in two superimposed hexagonal rings similar to the glutamine synthetase of Escherichia coli. Disc eleetrophoresis in the presence of sodium dodecyl sulfate and sedimentation studies show a subunit molecular weight of 56,500 and a sedimentation coefficient (s_20,w) of the native enzyme of 20.0 S. Like the E. coli enzyme, the glutamine synthetase of A. vinelandii is regulated by adenylylation/deadenylylation. This finding was derived from (a) studies on the effect of snake venom phosphodiesterase treatment on the catalytic and spectral properties of enzyme isolated from cells grown on a nitrogen-rich medium, (b) the identification of the AMP released by the phosphodiesterase by thin-layer chromatography, (c) the selective precipitation of adenylylated enzyme with antibodies directed against adenylylated bovine serum albumin, and (d) the in vitro incorporation of radioactivity from [¹⁴C]ATP into deadenylylated enzyme in the presence of either crude extract from A. vinelandii or partially purified adenylyl transferase from E. coli. The state of adenylylation appears to have a similar influence on the catalytic properties of A. vinelandii glutamine synthetase as on those of the E. coli enzyme, with the exception that the deadenylylated form of the A. vinelandii glutamine synthetase is almost inactive in the Mn-dependent transferase reaction.     

Characterization of the gene encoding glutamine synthetase I (glnA) from Bradyrhizobium japonicum.

We have isolated the Bradyrhizobium japonicum gene encoding glutamine synthetase I (glnA) from a phage lambda library by using a fragment of the Escherichia coli glnA gene as a hybridization probe. The rhizobial glnA gene has homology to the E. coli glnA gene throughout the entire length of the gene and can complement an E. coli glnA mutant when borne on an expression plasmid in the proper orientation to be transcribed from the E. coli lac promoter. High levels of glutamine synthetase activity can be detected in cell-free extracts of the complemented E. coli. The enzyme encoded by the rhizobial gene was identified as glutamine synthetase I on the basis of its sedimentation properties and resistance to heat inactivation. DNA sequence analysis predicts a high level of amino acid sequence homology among the amino termini of B. japonicum, E. coli, and Anabaena sp. strain 7120 glutamine synthetases. S1 nuclease protection mapping indicates that the rhizobial gene is transcribed from a single promoter 131 +/- 2 base pairs upstream from the initiation codon. This glnA promoter is active when B. japonicum is grown both symbiotically and in culture with a variety of nitrogen and carbon sources. There is no detectable sequence homology between the constitutively expressed glnA promoter and the differentially regulated nif promoters of the same B. japonicum strain.     

Role of glutamine synthetase in the uptake and metabolism of methylammonium by Azotobacter vinelandii.

Methylammonium is a substrate for the ammonium transport system of Azotobacter vinelandii. During cellular uptake methylammonium is rapidly converted to a less polar metabolite (E. M. Barnes, Jr., and P. Zimniak, J. Bacteriol. 146:512-516, 1981). This metabolite has been isolated from A. vinelandii and identified as gamma-glutamylmethylamide by mass spectroscopy, 1H nuclear magnetic resonance spectroscopy, and cochromatography with the authentic compound. Escherichia coli also accumulated gamma-glutamylmethylamide during methylammonium uptake. The biosynthesis of gamma-glutamylmethylamide in vitro required methylammonium, ATP, L-glutamate, and a soluble cell extract from A. vinelandii. The enzyme responsible for gamma-glutamylmethylamide synthesis was glutamine synthetase. In a crude extract, L-methionine-DL-sulfoximine was equipotent in inhibiting the activities for gamma-glutamyltransferase and for the synthesis of glutamine and gamma-glutamylmethylamide. Likewise, an antiserum against the glutamine synthetase of E. coli precipitated the transferase and both synthetic activities at similar titers. During repression by growth of cells on ammonium medium, the synthesis of glutamine and gamma-glutamylmethylamide in vitro was also inhibited coordinately. A partially purified preparation of glutamine synthetase from A. vinelandii utilized methylammonium as substrate (Km = 78 mM, Vmax = 0.30 mumol/min per mg), although less efficiently than ammonium (Km = 0.089 mM, Vmax = 1.1 mumol/min per mg). The kinetic properties of glutamine synthetase with methylammonium as substrate as well as the insensitivity of this activity to inhibition by T1+ were strikingly different from methylammonium translocation. Thus, methylammonium (ammonium) translocation and intracellular trapping as glutamylamides are experimentally distinguishable processes.     

Relationships between nitrogenase,glutamine synthetase,glutamine, and energy charge in Azotobacter vinelandii

When continuous cultures of Azotobacter vinelandii were supplied with ammonium or nitrate in amounts, which just repressed nitrogenase synthesis completely, both the intracellular glutamine level and the degree of adenylylation of the glutamine synthetase (GS) increased only slightly (from 0.45–0.50 mM and from 2 to 3 respectively), while the total GS level remained unaffected. Higher amounts of ammonium additionally inhibited the nitrogenase activity, caused a strong rise in the intracellular glutamine concentration and adenylylation of the GS, but caused no change in the ATP/ADP ratio. These results are considered as evidence that in A. vinelandii the regulation of nitrogenase synthesis is not linked to the adenylylation state of the GS and to the intracellular glutamine level, and that the inhibition of the nitrogenase activity as a consequence of a high extracellular ammonium level is not mediated via a change in the energy charge.Abbreviations GS glutamine synthetase - GS-S(Mg) Mg²⁺ dependent synthetic activity of GS - GS-T(Mn) Mn²⁺ dependent transferase activity of GS     

Phenotypic changes resulting from distinct point mutations in the Azospirillum brasilense glnA gene,encoding glutamine synthetase

Sequencing the glnA genes of two chemically induced Azospirillum brasilense glutamine synthetase mutants revealed an Arg-->Cys mutation, corresponding to the glutamate binding site, in one mutant and an Asp-->Asn mutation, corresponding to the ammonium binding site, in the second mutant. The phenotypic changes in these mutants are discussed in relation to their genotypes.     

ATP synthetase associated with the nitrogenase of Azotobacter vinelandii.

Preparations of nitrogenase from $\text{Azotobacter vinelandii}$ show an ATP synthetase activity when incubated in the presence of ADP, phosphate, ammonium chloride and an oxidizing agent. The synthesis is linked to an oxidation-reduction and the activity parallels nitrogenase activity through purification and in a step gradient sedimentation. The reductive dephosphorylation of nitrogen fixation may possibly be reversed to yield an oxidative phosphorylation.     

Lipoamide dehydrogenase from Azotobacter vinelandii. Molecular cloning, organization and sequence analysis of the gene

The gene encoding lipoamide dehydrogenase from Azotobacter vinelandii has been cloned in Escherichia coli. Fragments of 9-23 kb from Azotobacter vinelandii chromosomal DNA obtained by partial digestion with Sau3A were ligated into the BamHI site of plasmid pUC9. E. coli TG2 cells were transformed with the resulting recombinant plasmids. Screening for clones which produced A. vinelandii lipoamide dehydrogenase was performed with antibodies raised against the purified enzyme. A positive colony was found which produced complete chains of lipoamide dehydrogenase as concluded form SDS gel electrophoresis of the cell-free extract, stained for protein or used for Western blotting. After subcloning of the 14.7-kb insert of this plasmid the structural gene could be located on a 3.2-kb DNA fragment. The nucleotide sequence of this subcloned fragment (3134 bp) has been determined. The protein-coding sequence of the gene consists of 1434 bp (478 codons, including the AUG start codon and the UAA stop codon). It is preceded by an intracistronic region of 85 bp and the structural gene for succinyltransferase. A putative ribosome-binding site and promoter sequence are given. The derived amino acid composition is in excellent agreement with that previously published for the isolated enzyme. The predicted relative molecular mass is 50223, including the FAD. The overall homology with the E. coli enzyme is high with 40% conserved amino acid residues. From a comparison with the three-dimensional structure of the related enzyme glutathione reductase [Rice, D. W., Schultz, G. E. & Guest, J. R. (1984) J. Mol. Biol. 174, 483-496], it appears that essential residues in all four domains have been conserved. The enzyme is strongly expressed, although expression does not depend on the vector-encoded lacZ promoter. The cloned enzyme is, in all the respects tested, identical with the native enzyme.     

Iron-regulated phenylalanyl-tRNA synthetase activity in Azotobacter vinelandii

Azotobacter vinelandii strain UA22 was produced by pTn5luxAB mutagenesis, such that the promoterless luxAB genes were transcribed in an iron-repressible manner. Tn5luxAB was localized to a fragment of chromosomal DNA encoding the thrS, infC, rpmI, rplT, pheS and pheT genes, with Tn5 inserted in the 3'-end of pheS. The isolation of this mutation in an essential gene was possible because of polyploidy in Azotobacter, such that strain UA22 carried both wild-type and mutant alleles of pheS. Phenylalanyl-tRNA synthetase activity and PHES::luxAB reporter activity was partially repressed under iron-sufficient conditions and fully derepressed under iron-limited conditions. The ferric uptake regulator (Fur) bound to a DNA sequence immediately upstream of luxAB, within the pheS gene, but PHES::luxAB reporter activity was not affected by phenylalanine availability. This suggests there is novel regulation of pheST in A. vinelandii by iron availability.     

Molecular cloning of nif DNA from Azotobacter vinelandii.

Two clones which contained nif DNA were isolated from a clone bank of total EcoRI-digested Azotobacter vinelandii DNA. The clones carrying the recombinant plasmids were identified by use of the 32P-labeled 6.2-kilobase (kb) nif insert from pSA30 (which contains the Klebsiella pneumoniae nifK, nifD, and nifH genes) as a hybridization probe. Hybridization analysis with fragments derived from the nif insert of pSA30 showed that the 2.6-kb insert from one of the plasmids (pLB1) contains nifK whereas the 1.4-kb insert from the other plasmid (pLB3) contains nifD. Marker rescue tests using genetic transformation indicated that the 2.6-kb A. vinelandii nif fragment contains the wild-type alleles for the nif-6 and nif-38 mutations carried by Nif- strains UW6 and UW38. The 1.4-kb insert contains the wild-type allele for the nif-10 mutation carried by Nif- strain UW10.     

Molecular weights of nitrogenase components from Azotobacter vinelandii.

Meniscus depletion sedimentation equilibrium ultracentrifuge experiments were performed on purified MoFe and Fe proteins of $\text{Azotobacter vinelandii}$. The MoFe protein was found to have a molecular weight of 245,000, using an experimentally confirmed partial specific volume of 0.73. The MoFe protein formed one band on sodium dodecyl sulfate gel electrophoresis and had a subunit molecular weight of 56,000. The subunit molecular weight from ultracentrifuge experiments in 8 M urea was 61,000. The molecular weight of the Fe protein was calculated to be 60,500 in meniscus depletion experiments. Similar experiments in 8 M urea solvent indicated a subunit molecular weight of 30,000. A subunit molecular weight of 33,000 was obtained from sodium dodecyl sulfate gel electrophoresis experiments.     

Organization and nucleotide sequence of the glutamine synthetase (glnA) gene from Lactobacillus delbrueckii subsp. bulgaricus.

A 3.3-kb BamHI fragment of Lactobacillus delbrueckii subsp. bulgaricus DNA was cloned and sequenced. It complements an Escherichia coli glnA deletion strain and hybridizes strongly to a DNA containing the Bacillus subtilis glnA gene. DNA sequence analysis of the L. delbrueckii subsp. bulgaricus DNA showed it to contain the glnA gene encoding class I glutamine synthetase, as judged by extensive homology with other prokaryotic glnA genes. The sequence suggests that the enzyme encoded in this gene is not controlled by adenylylation. Based on a comparison of glutamine synthetase sequences, L. delbrueckii subsp. bulgaricus is much closer to gram-positive eubacteria, especially Clostridium acetobutylicum, than to gram-negative eubacteria and archaebacteria. The fragment contains another open reading frame encoding a protein of unknown function consisting of 306 amino acids (ORF306), which is also present upstream of glnA of Bacillus cereus. In B. cereus, a repressor gene, glnR, is found between the open reading frame and glnA. Two proteins encoded by the L. delbrueckii subsp. bulgaricus gene were identified by the maxicell method; the sizes of these proteins are consistent with those of the open reading frames of ORF306 and glnA. The lack of a glnR gene in the L. delbrueckii subsp. bulgaricus DNA in this position may indicate a gene rearrangement or a different mechanism of glnA gene expression.