Glutamine synthetase of pseudomonads: some biochemical and physicochemical properties.

The glutamine synthetases from several Pseudomonas species were purified to homogeneity, and their properties were compared with those reported for the enzymes from Escherichia coli and other gram-negative bacteria. The glutamine synthetase from Pseudomonas fluorescens was unique because it was nearly precipitated quantitatively as a homogeneous protein during dialysis of partially purified preparations against buffer containing 10 mM imidazole (pH 7.0) and 10 mM MnCl2. The glutamine synthetases from Pseudomonas putida and Pseudomonas aeruginosa were purified by affinity chromatography on Affi-blue gel. Dodecamerous forms of the E. coli and P. fluorescens glutamine synthetases had identical mobilities during polyacrylamide gel electrophoresis. Their dissociated subunits, however, migrated differently and were readily separated by electrophoresis on polyacrylamide gels containing 0.1% sodium dodecyl sulfate. This difference in subunit mobilities is not related to the state of adenylylation. Regulation of the Pseudomonas glutamine synthetase activity is mediated by an adenylylation-deadenylylation cyclic cascade system. A sensitive procedure was developed for measuring the average number of adenylylated subunits per enzyme molecule for the glutamine synthetase from P. fluorescens. This method takes advantage of the large differences in transferase activity of the adenylylated and unadenylylated subunits at pH 6.0 and of the fact that the activities of both kinds of subunits are the same at pH 8.45.     

Adenylylation and catalytic properties of Mycobacterium tuberculosis glutamine synthetase expressed in Escherichia coli versus mycobacteria

Bacterial glutamine synthetases (GSs) are complex dodecameric oligomers that play a critical role in nitrogen metabolism, converting ammonia and glutamate to glutamine. Recently published reports suggest that GS from Mycobacterium tuberculosis (MTb) may be a therapeutic target (Harth, G., and Horwitz, M. A. (2003) Infect. Immun. 71, 456-464). In some bacteria, GS is regulated via adenylylation of some or all of the subunits within the aggregate; catalytic activity is inversely proportional to the extent of adenylylation. The adenylylation and deadenylylation of GS are catalyzed by adenylyl transferase (ATase). Here, we demonstrate via electrospray ionization mass spectrometry that GS from pathogenic M. tuberculosis is adenylylated by the Escherichia coli ATase. The adenylyl group can be hydrolyzed by snake venom phosphodiesterase to afford the unmodified enzyme. The site of adenylylation of MTb GS by the E. coli ATase is Tyr-406, as indicated by the lack of adenylylation of the Y406F mutant, and, as expected, is based on amino acid sequence alignments. Using electrospray ionization mass spectroscopy methodology, we found that GS is not adenylylated when obtained directly from MTb cultures that are not supplemented with glutamine. Under these conditions, the highly related but non-pathogenic Mycobacterium bovis BCG yields partially ( approximately 25%) adenylylated enzyme. Upon the addition of glutamine to the cultures, the MTb GS becomes significantly adenylylated ( approximately 30%), whereas the adenylylation of M. bovis BCG GS does not change. Collectively, the results demonstrate that MTb GS is a substrate for E. coli ATase, but only low adenylylation states are accessible. This parallels the low adenylylation states observed for GS from mycobacteria and suggests the intriguing possibility that adenylylation in the pathogenic versus non-pathogenic mycobacteria is differentially regulated.     

Glutamine synthetase of Klebsiella aerogenes: genetic and physiological properties of mutants in the adenylylation system.

Mutations resulting in defects in the adenylylation system of glutamine synthetase (GS) affect the expression of glnA, the structural gene for GS. Mutants with lesions in glnB are glutamine auxotrophs and contain repressed levels of highly adenylylated GS. Glutamine-independent revertants of the glnB3 mutant have acquired an additional mutation at the glnE site. The glnE54 mutant is incapable of adenylylating GS and produces high levels of enzyme, even when ammonia is present in the growth medium. The fact that mutations in glnB and glnE simultaneously disturb both the normal adenylylation and repression patterns of GS in Klebsiella aerogenes indicates that the adenylylation system, or adenylylation state, of GS is critical for the regulation of synthesis of GS.     

Biochemical parameters of glutamine synthetase from Klebsiella aerogenes.

The glutamine synthetase (GS) from Klebsiella aerogenes is similar to that from Escherichia coli in several respects: (i) it is repressed by high levels of ammonia in the growth medium; (ii) its biosynthetic activity is greatly reduced by adenylylation; and (iii) adenylylation lowers the pH optimum and alters the response of the enzymes to various inhibitors in the gamma-glutamyl transferase (gammaGT) assay. There are, however, several important differences: (i) the isoactivity point for the adenylylated and non-adenylylated forms in the gammaGT assay occurs at pH 7.55 in K. aerogenes and at pH 7.15 in E. coli; (ii) the non-adenylylated form of the GS from K. aerogenes is stimulated by 60 mM MgCl2 in the gammaGT assay at pH 7.15. A biosynthetic reaction assay that correlates well with number of non-adenylylated enzyme subunits, as determined by the method of Mg2+ inhibition of the gammaGT assay, is described. Finally, we have found that it is necessary to use special methods to harvest growing cells to prevent changes in the adenylylation state of GS from occurring during harvesting.     

Role of glutamine synthetase adenylylation in the self-protection of Pseudomonas syringae subsp. "tabaci" from its toxin, tabtoxinine-beta-lactam

Selected pathovars of Pseudomonas syringae produce an extracellular phytotoxin, tabtoxinine-beta-lactam, that irreversibly inhibits its known physiological target, glutamine synthetase (GS). Pseudomonas syringae subsp. "tabaci" retains significant amounts of glutamine synthetase activity during toxin production in culture. As part of our investigation of the self-protection mechanism(s) used by these pathovars, we have determined that GS becomes adenylylated after toxin production is initiated and that the serine released from the zinc-activated hydrolysis of tabtoxin is a factor in the initiation of this adenylylation. The adenylylation state of this GS was estimated to range from E5.0-7.5. The irreversible inactivation by tabtoxinine-beta-lactam of unadenylylated and adenylylated glutamine synthetase purified from P. syringae subsp. "tabaci" was investigated. Adenylylated GS was inactivated by tabtoxinine-beta-lactam at a slower rate than was unadenylylated enzyme. Adenylylated GS (E7.5-10.5) was significantly protected from this inactivation in the presence of the enzyme effectors, AMP, Ala, Gly, His, and Ser. Thus, the combination of the adenylylation of GS after toxin production is initiated and the presence of the enzyme effectors in vivo could provide part of the self-protection mechanism used by subsp. "tabaci".     

Hydrophobic and ionic effects upon the electrophoretic mobilities of the subunits of coupling factor 1 from mitochondria

A sodium dodecyl sulfate (SDS)-urea polyacrylamide gel system was used to investigate certain properties of the subunits of the beef heart mitochondrial ATPase, (native F1, nF1). By examining the affects of urea concentration and acrylamide concentration upon the electrophoretic mobilities of the polypeptides comprising the nF1 enzyme, we have obtained conditions under which all five subunits are simultaneously resolved when the discontinuous buffer system of Laemmli is used (U. K. Laemmli (1970) Nature (London) 277, 680-685). The determination of the apparent molecular weights by analysis of Ferguson plots (K. A. Ferguson (1964) Metabolism 13, 985-1002) revealed that the addition of urea to the SDS gels resulted in a decrease in the apparent molecular weight of the beta subunit. A dramatic increase in the apparent molecular weight of the delta subunit was also brought about by the presence of urea in the SDS gels. In addition, the apparent molecular weight of both the alpha and the beta subunits was dependent upon the acrylamide concentration used, indicating that these subunits contain either areas highly resistant to denaturation by the combined action of urea and SDS, or covalent modifications leading to anomalous electrophoretic mobility. The results of experiments in which urea analogs were used indicate that the interactions of urea with the beta subunit involve the formation of hydrogen bonds between urea and regions of this subunit. On the other hand, the interactions of urea with the delta subunit are primarily of a hydrophobic nature, suggesting that these interactions could involve domains of the delta subunit required for binding of the coupling factor to the mitochondrial membrane.     

固氮粪产碱菌谷氨酰胺合成酶的分离纯化及其特性

联合固氮细菌粪产碱菌（Ａｌｃａｌｉｇｅｎｅｓｆａｅｃａｌｉｓ）Ａ１５０１菌体经超声破碎后,无细胞粗提液以ＰＥＧ－６０００分级沉淀,丙酮沉淀,再经蓝琼脂糖（ＢｌｕｅＳｅｐｈａｒｏｓｅＣＬ－６８）亲和层析分离、纯化。获得的纯谷氨酰胺合成酶（ＧＳ）在ＳＤＳ－ＰＡＧＥ和４－３０％梯度ＰＡＧＥ上均呈均一的一条带。ＧＳ亚基及整酶分子量分别为５５ｋＤ和６４５ｋＤ,亚基由４５６个氨基酸残基组成。ＧＳ的Ｋｍ值,在以Ｇｌｕ为氮源的介质中培养时分别为２０ｍｍｏｌ／Ｌ（Ｇｌｕ）,５０ｍｍｏｌ／Ｌ（ＡＴＰ）和４５ｍｍｏｌ／Ｌ（ＮＨ~＋_４）;在以ＮＨ~＋_４为氮源的介质中培养时则分别为７０ｍｍｏｌ／Ｌ（Ｇｌｕ）,４９ｍｍｏｌ／Ｌ（ＡＴＰ）和８０ｍｍｏｌ／Ｌ（ＮＨ~＋_４）,表明ＮＨ~＋_４培养下形成高度腺苷化的ＧＳ对Ｇｌｕ及ＮＨ~＋_４的亲和力有所下降。     

Close linkage of genes encoding glutamine synthetases I and II in Frankia alni CpI1.

Frankia alni CpI1 has two glutamine synthetases (GSs), GSI and GSII. The GSI gene (glnA) was isolated from a cosmid library of F. alni CpI1 DNA by heterologous probing with glnA from Streptomyces coelicolor. The glnA gene was shown to be located upstream of the GSII gene (glnII) by DNA-DNA hybridization. The nucleotide sequences of the 1,422-bp CpI1 glnA gene and of the 449-bp intervening region between glnA and glnII were determined, and the glnA amino acid sequence was deduced. In common with GSIs from other organisms, CpI1 GSI contains five conserved regions near the active site and a conserved tyrosine at the adenylylation site. F. alni CpI1 glnA complemented the glutamine growth requirement of the Escherichia coli glnA deletion strain YMC11 but only when expressed from an E. coli lac promoter. While the functional significance of maintaining two GSs adjacent to one another remains unclear, this arrangement in F. alni provides support for the recently proposed origin of GSI and GSII as resulting from a gene duplication early in the evolution of life.     

Involvement of the product of the glnF gene in the autogenous regulation of glutamine synthetase formation in Klebsiella aerogenes.

Mutations in a site, glnF, linked by P1-mediated transduction of argG on the chromosome of Klebsiella aerogenes, result in a requirement for glutamine. Mutants in this gene have in all media a level of glutamine synthetase (GS) corresponding to the level found in the wild-type strain grown in the medium producing the strongest repression of GS. The adenylylation and deadenylylation of GS in glnF mutants is normal. The glutamine requirement of glnF mutants could be suppressed by mutations in the structural gene for GS, glnA. These mutations result in altered regulation of GS synthesis, regardless of the presence or absence of the glnF mutation (GlnR phenotype). In GlnR mutants the GS level is higher than in the wild-type strain when the cells are cultured in strongly repressing medium, but lower than in the wild-type strain when cells are cultured in a derepressing medium. Heterozygous merodiploids carrying a normal glnA gene as well as a glnA gene responsible for the GlnR phenotype behave in every respect like merodiploids carrying two normal glnA genes. These results confirm autogenous regulation of GS synthesis and indicate that GS is both a repressor and an activator of GS synthesis. The mutation in glnA responsible for the GLnR phenotype has apparently resulted in the formation of a GS that is incompetent both as repressor and as activator of GS synthesis. According to this hypothesis, the product of the glnF gene is necessary for activation of the glnA gene by GS.     

Autogenous regulation of the synthesis of glutamine synthetase in Klebsiella aerogenes.

We isolated an F' episome of Escherichia coli carrying the glnA+ gene from K. aerogenes and an F' episome of E. coli carrying the glnA4 allele from K. aerogenes responsible for the constitutive synthesis of glutamine synthetase. Complementation tests with these episomes showed that the glnA4 mutation (leading to the constitutive synthesis of active glutamine synthetase) was in the gene identified by mutations glnA20, glnA51, and glnA5 as the structural gene for glutamine synthetase. By using these merodiploid strains we were able to show that the glnA51 mutation lead to the synthesis of a glutamine synthetase that lacked enzymatic activity but fully retained its regulatory properties. Finally, we discuss a model that explains the several phenotypes associated with mutations such as glnA4 located within the structural gene for glutamine synthetase leading to constitutive synthesis of active glutamine synthetase.     

Sequence of the Bacillus subtilis glutamine synthetase gene region

The nucleotide sequence of the glutamine synthetase (GS) region of Bacillus subtilis has been determined and found to contain several unique features. An open reading frame (ORF) upstream of the GS structural gene is part of the same operon as GS and is involved in regulation. Two downstream ORFs are separated from glnA by an apparent Rho-independent termination site. One of the downstream ORFs encodes a very hydrophobic polypeptide and contains its own potential RNA polymerase and ribosome-binding sites. The derived amino acid (aa) sequence of B. subtilis GS is similar to that of several other prokaryotes, especially to the GS of Clostridium acetobutylicum. The B. subtilis and C. acetobutylicum enzymes differ from the others in the lack of a stretch of about 25 aa as well as the presence of extra cysteine residues in a region known to contain regulatory as well as catalytic mutations. The region around the tyrosine residue that is adenylylated in GS from many species is fairly similar in the B. subtilis GS despite its lack of adenylylation.     

Anomalous electrophoretic behaviour of the glutathione S-transferase Ya and Yk subunits isolated from man and rodents. A potential pitfall for nomenclature.

GSH S-transferases are dimeric enzymes. The subunits in the rat are resolved into six types, designated Yf, Yk, Ya, Yn, Yb and Yc, by discontinuous SDS/polyacrylamide-gel electrophoresis [Hayes (1986) Biochem. J. 233, 789-798]. The relative electrophoretic mobility of the Ya and Yk subunits is dependent on the amount of cross-linker (NN'-methylenebisacrylamide) in the resolving gel. At low degrees of cross-linking, CBis 0.6% (w/w), the Yk and Ya subunits possess a faster anodal mobility than do the Yf, Yn, Yb and Yc subunits (i.e. order of mobility Yk greater than Ya greater than Yf greater than Yn greater than Yb greater than Yc), whereas at higher degrees of cross-linking, CBis 5.0% (w/w), Yf subunits possess the fastest mobility (i.e. order of mobility Yf greater than Yk greater than or equal to Yn greater than Yb greater than or equal to Ya greater than Yc). Resolving gels that contain low concentrations of cross-linker [CBis 0.6% (w/w)] allow the resolution of a hitherto unrecognized polypeptide that is isolated by S-hexyl-GSH-Sepharose affinity chromatography. This new polypeptide, which we have designated Yb, is normally obscured by the main Yb band in resolving gels that comprise concentrations of cross-linker of at least CBis 1.6% (w/w). The Ya- and Yb-type subunits in guinea pig, mouse, hamster and man were identified by immuno-blotting and their apparent Mr values in different electrophoresis systems were determined. The Ya subunits in all species studied possess a variable cross-linker-dependent mobility during electrophoresis. Since the transferase subunits are currently classified according to their mobilities during SDS/polyacrylamide-gel electrophoresis, it is apparent that the variable electrophoretic behaviour of the Ya and Yk subunits may lead to the mis-identification of enzymes.     

Molecular and regulatory properties of glutamine synthetase from the phototrophic bacterium Rhodopseudomonas capsulata E1F1.

The glutamine synthetase of the phototrophic bacterium Rhodopseudomonas capsulata E1F1 was purified to homogeneity by a procedure which used a single affinity chromatography step. Like enzymes from other photosynthetic procaryotes, native glutamine synthetase from R. capsulata E1F1 was found to be a dodecameric protein of approximately 660 kilodaltons with identical subunits of about 55 kilodaltons each. The Stokes radius and S20,w of the native enzyme were 8.35 nm and 19.20, respectively. The enzyme exhibited different aggregation states with detectable oligomers of 1, 2, 3, 4, 6, 8, 10, and 12 subunits. Disaggregation of the glutamine synthetase occurred after the native protein was subjected to electrophoresis in polyacrylamide gels, as well as occurring spontaneously at low ionic strength. Glutamine synthetase from R. capsulata E1F1 was regulated by an adenylylation-deadenylylation mechanism, and the adenylylation state of the protein depended on the nitrogen source, growth phase, and light intensity. Ammonia repressed glutamine synthetase, whereas glycine, serine, alanine, valine, and aspartate were noncompetitive inhibitors of the glutamine synthetase biosynthetic activity.     

Fractionation of individual, biologically active factor VIII multimers

We have designed an electrophoretic system for the fractionation of individual, biologically active multimers of factor VIII. Human factor VIII, purified by gel filtration on Sepharose CL-2B from plasma cryoprecipitate, was submitted to electrophoresis without SDS on 2.0% polyacrylamide gels in 0.04 M Tris/0.06 M Tes buffer, pH 7.5. Staining with Coomassie blue revealed a series of protein bands. Measurement of electrophoretic mobility showed constant size intervals between adjacent bands. Electrophoresis in a second dimension, in the presence of SDS, resulted in an identical order of mobilities, suggesting that the different migration rates of factor VIII proteins in the first electrophoretic system were size- and not charge-dependent. After electrophoresis in the absence of SDS both factor VIII coagulant and ristocetin cofactor activities as well as factor VIII-related antigen were recovered by elution from gel slices. The distribution of activity peaks resembled that of Coomassie-stained factor VIII proteins found in control gels. We thus demonstrate that an electrophoretic fractionation of factor VIII multimers is possible even at neutral pH where factor VIII activities are retained.     

Purification of RNAase II by preparative polyacrylamide gel electrophoresis.

Purification of RNAase II to electrophoretic homogeneity is described. The exonuclease is activated by K+ and Mg2+ and hydrolyses poly(A) to 5'-AMP, exclusively as described by Nossal and Singer (1968, J. Biol. Chem. 243, 913--922). To separate RNAase II from ribosomes, DEAE-cellulose chromatography was used. Two additional chromatographic steps give a preparation that yields 10 bands after analytical polyacrylamide gel electrophoresis. Preparative polyacrylamide gel electrophoresis resulted in a final preparation which on analytical polyacrylamide gels gives a single band. A molecular weight of 76 000 +/- 4000 was obtained from Sephadex G-200 chromatography, with three bands from sodium dodecyl sulfate (SDS) denaturation and SDS gel electrophoresis. The subunits have a molecular weight of 40 000 +/- 2000, 33 000 +/- 2000, and 26 000 +/- 1000. The enzyme thus appears to consist of three dissimilar subunits.     

Relationship between epitope density and immunoprecipitation of multivalent antigens by bivalent antibody: immunoprecipitation of adenylylated glutamine synthetase by anti-AMP antibodies

Escherichia coli glutamine synthetase (GS) preparations composed of 12 adenylylated subunits (GS₁₂^?) are almost completely precipitated by sheep Anti-AMP immunoglobulin G (IgG), whereas glutamine synthetase preparations containing 6 adenylylated subunits (GS₆^?) are only partially precipitated by the antibodies (R.J. Hohman, S.G. Rhee, and E.R. Stadtman, 1980, Proc. Nat. Acad. Sci. USA77, 7410–7414). By means of ¹²⁵I-labeled anti-AMP antibodies and double immunoprecipitation techniques, in which rabbit antiserum to sheep IgG or anti-GS antibodies were used to precipitate soluble immune complexes, it was demonstrated that under optimal conditions, both the soluble and insoluble immune complexes obtained with either GS₆^? or GS₁₂^? contain 0.5 mol antibody/mol adenylylated subunit. In agreement with the lattice theory of immuno-precipitation, soluble immune complexes are formed in antibody excess. Scatchard plots of binding data indicate that under conditions of antibody excess, one antibody molecule is bound to each AMP moiety of GS₁₂^?, whereas GS₆^? binds a maximum of only 0.68 antibody molecule/adenylylated subunit. We propose that with some species of GS₆^?, the distribution of adenylylated subunits favors monogamous interactions of the bivalent antibody with two subunits within the same GS molecule and thereby leads to the formation of small, soluble, immune complexes. Other explanations are considered. Only 30% of the antibody population that recognizes unconjugated 5′-AMP binds to the AMP moiety of adenylylated GS. Anti-AMP antiserum can be fractionated on a GS₁₂^?-Sepharose matrix into two subpopulations of antibody with strikingly different immunoprecipitation characteristics. Conversely, species of GS with various states of adenylylation ranging from 0 to 8 were separated from a GS₆^? preparation by means of affinity chromatography on an anti-AMP antibody-Sepharose matrix. Under optimal conditions, antibodies purified by affinity chromatography precipitated a smaller fraction of a GS₆^? preparation than did unfractionated antiserum. Competence of the purified antibody was nearly restored to that of the unfractionated serum by the addition of an enhancement factor present in the IgG fraction of nonimmune serum. The enhancement factor was not required for complete precipitation of GS^?₁₂ by purified antibodies. Contrary to most antibody-antigen reactions, immunoprecipitation of GS₆^? with anti-AMP antibodies is greater at 30 °C than at 4 °C.