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

The cyanotrophic bacterium Pseudomonas pseudoalcaligenes CECT5344 responds to cyanide by defence mechanisms against iron deprivation, oxidative damage and nitrogen stress

Two-dimensional (2-D) electrophoresis approach has been used to test protein expression changes in response to cyanide in the alkaliphilic bacterium Pseudomonas pseudoalcaligenes CECT5344. This is a cyanide-assimilating strain which also grows in media containing cyanide-enriched effluent from the jewellery industry. The bacterium efficiently uses this residue as the sole nitrogen source for aerobic growth under alkaline pH with negligible nitrogen losses as HCN. Cell-free extracts isolated from P. pseudoalcaligenes grown with a jewellery residue, free cyanide or ammonium chloride as nitrogen source were subjected to 2-D electrophoresis and the spot patterns were examined to determine differential protein expression. Electrophoretic plates exhibiting an average of 1000 spots showed significant differences in the expression of about 44 proteins depending on the nitrogen source. Some of these protein spots were analysed by Matrix-assisted laser desorption/ionization-time of flight-mass spectrometry (MALDI-TOF-MS). Characterization of five of these proteins reveals that cyanide shock induces proteins related to iron acquisition, regulation of nitrogen assimilation pathways and oxidative stress repairing and protection.

Halotolerance of the Phototrophic Bacterium Rhodobacter capsulatus E1F1 Is Dependent on the Nitrogen Source

Phototrophic growth of the moderate halotolerant Rhodobacter capsulatus strain E1F1 in media containing up to 0.3 M NaCl was dependent on the nitrogen source used. In these media, increased growth rates and growth levels were observed in the presence of reduced nitrogen sources such as ammonium and amino acids. When the medium contained an oxidized nitrogen source (dinitrogen or nitrate), increases in salinity severely inhibited phototrophic growth. However, the addition of glycine betaine promoted halotolerance and allowed the cells to grow in 0.2 M NaCl. Inhibition of diazotrophic growth by salinity was due to a decrease in nitrogenase activity which was no longer synthesized and reversibly inactivated, both effects being alleviated by the addition of glycine betaine. In R. capsulatus E1F1, inhibition of cell growth in nitrate by salt was due to a rapid inhibition of nitrate uptake, which led to a long-term decrease in nitrate reductase activity, probably caused by repression of the enzyme. Addition of glycine betaine immediately restored nitrate uptake, but the recovery of nitrate reductase activity required several hours. Neither ammonium uptake nor ammonium assimilation through the glutamine synthetase-glutamate synthase pathway was affected by NaCl.

Arginine catabolism in the phototrophic bacterium Rhodobacter capsulatus E1F1. Purification and properties of arginase

The phototrophic bacterium Rhodobacter capsulatus E1F1 grew with L-arginine or L-homoarginine as nitrogen source under light/anaerobiosis. However, when L-arginine was used as the only source of both carbon and nitrogen, the bacterium exhibited weak growth levels and the excess of nitrogen was excreted to the medium as ammonia. By contrast, L-ornithine was used under phototrophic conditions as either nitrogen or carbon source. Other compounds of the arginine catabolic pathways, such as putrescine or proline, also supported phototrophic growth of this bacterium. Under heterotrophic/dark conditions, R. capsulatus always showed a low growth rate with those nitrogen compounds. Cells growing on media containing L-arginine, L-homoarginine or L-ornithine induced an Mn(2+)-dependent arginase activity regardless of the presence of ammonium ions and other readily utilizable nitrogen sources. Arginase activity was strongly inhibited by Zn2+, Cu2+, borate, L-cysteine, L-ornithine and gamma-guanidinobutyrate. Mercurials also inactivated arginase, the activity being partially restored by the presence of thiols. Arginase was purified to electrophoretic homogeneity and found to consist of four identical subunits of 31 kDa. The molecular parameters and kinetic constants of arginase from R. capsulatus E1F1 resembled those previously described for the Saccharomyces cerevisiae enzyme rather than those of bacterial arginases.

Low- and high-activity forms of glutamine synthetase from Rhodospirillum rubrum: sensitivity to feed-back effectors and activation of the low-activity form

Glutamine synthetase from Rhodospirillum rubrum can be isolated in two forms, with low and high activity, respectively, depending on the concentration of combined nitrogen in the medium before harvest. The two forms have been studied with respect to their dependence on Mn2+ and Mg2+ in both the transferase and the biosynthetic assay. There is no difference in pH optimum between the forms in the biosynthetic assay. In addition the pH-optima for the two cations studied are very close, 7.4 (Mg2+) and 7.2 (Mn2+). It also shows that the activity of the low-activity form is higher than that of the high-activity form in the Mn(2+)-dependent biosynthetic assay. The two forms of Rsp. rubrum glutamine synthetase have also been studied with respect to their sensitivity towards feed-back effectors. In the transferase assay both forms are inhibited to essentially the same degree by alanine, glycine, histidine, AMP, CTP and UTP, CTP being the most effective of the nucleotides and of the amino acids alanine causes the highest inhibition. In the biosynthetic assay these effectors show different degrees of inhibition on the two different forms; the high-activity form being the most sensitive. The results are discussed in relation to properties of glutamine synthetase from Escherichia coli and other phototropic bacteria in which regulation of glutamine synthetase is known to be due to adenylylation. It is also shown that the low-activity form of Rsp. rubrum glutamine synthetase can be activated in crude extracts in a reaction that is inhibited by glutamine.

Purification and partial characterization of glutamate synthase from Rhodospirillum rubrum grown under nitrogen-fixing conditions

Glutamate synthase, a key enzyme in ammonia assimilation, has been purified from the photosynthetic bacterium Rhodospirillum rubrum. The purification procedure involves ion-exchange chromatography, affinity chromatography and gel filtration. The recovery in the procedure is high (62%) and the specific activity is 21 mumol of NADPH oxidized/min per mg. The enzyme is specific for its substrates, and no activity was demonstrated with NADH or NH4+ ions substituting for NADPH and glutamine respectively. The enzyme is composed of two dissimilar subunits with molecular masses of 53 and 152 kDa, and it is shown that Cl- ions have an effect on the aggregation of the enzyme. Km values for the substrates are: NADPH, 16 microM; 2-oxoglutarate, 10 microM; and glutamine, 65 microM. The enzyme is inhibited by amidotransferase inhibitors at micromolar concentrations. The role of the enzyme in the metabolic regulation of nitrogenase is discussed.

Purification and properties of L-alanine dehydrogenase of the phototrophic bacterium Rhodobacter capsulatus E1F1

In the phototrophic nonsulfur bacterium Rhodobacter capsulatus E1F1, L-alanine dehydrogenase aminating activity functions as an alternative route for ammonia assimilation when glutamine synthetase is inactivated. L-Alanine dehydrogenase deaminating activity participates in the supply of organic carbon to cells growing on L-alanine as the sole carbon source. L-Alanine dehydrogenase is induced in cells growing on pyruvate plus nitrate, pyruvate plus ammonia, or L-alanine under both light-anaerobic and dark-heterotrophic conditions. The enzyme has been purified to electrophoretic and immunological homogeneity by using affinity chromatography with Red-120 agarose. The native enzyme was an oligomeric protein of 246 kilodaltons (kDa) which consisted of six identical subunits of 42 kDa each, had a Stokes' radius of 5.8 nm, an s20.w of 10.1 S, a D20,w of 4.25 x 10(-11) m2 s-1, and a frictional quotient of 1.35. The aminating activity was absolutely specific for NADPH, whereas deaminating activity was strictly NAD dependent, with apparent Kms of 0.25 (NADPH), 0.15 (NAD+), 1.25 (L-alanine), 0.13 (pyruvate), and 16 (ammonium) mM. The enzyme was inhibited in vitro by pyruvate or L-alanine and had two sulfhydryl groups per subunit which were essential for both aminating and deaminating activities.

Organic nitrogen metabolism of phototrophic bacteria

Recent reviews dealing with phototrophic bacteria are concerned with bioenergetics, nitrogen fixation and hydrogen metabolism, synthesis of the photosynthetic apparatus and phylogeny/taxonomy. The organic N-metabolism of these phylogenetically diverse bacteria has last been reviewed in 1978. However, amino acid utilization and biosynthesis, ammonia assimilation, purine and pyrimidine metabolism and biosynthesis of delta-aminolevulinic acid as precursor of bacteriochlorophylls and hemes are topics of vital importance. This review focuses on utilization of amino acids as N- and C/N-sources, the pathways of purine and pyrimidine degradation, novel aspects of amino acid biosynthesis (with emphasis on branched-chain amino acids and delta-aminolevulinic acid) and some aspects of ammonia assimilation and glutamate synthesis by purple bacteria, green sulfur bacteria and Chloroflexus aurantiacus.

Purification and partial characterization of glutamine synthetase from the photosynthetic bacterium Rhodospirillum rubrum

Glutamine synthetase (L-glutamate: ammonia ligase (ADP-forming), EC 6.3.1.2) from the photosynthetic bacterium Rhodospirillum rubrum grown under nitrogen fixing conditions has been purified to homogeneity. The purification procedure involves affinity chromatography on ADP-agarose type 2 as the major purification step. The recovery in the purification is 70%. The specific activity of the purified enzyme is about 10-times higher in the gamma-glutamyl transferase assay than in the coupled biosynthetic assay. The molecular weight was determined to 530,000 by native gradient polyacrylamide gel electrophoresis and to 500,000 by gel filtration. The subunits have an apparent molecular weight of 52,000. Glutamine synthetase isolated from Rsp. rubrum which had been exposed to ammonium ions ('switch-off') before harvest had about 20% of the transferase activity compared with the enzyme purified from nitrogen-starved cells. The low-activity form showed two bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

Molecular aspects of nitrogen fixation by photosynthetic prokaryotes

The photosynthetic prokaryotes possess diverse metabolic capabilities, both in carrying out different types of photosynthesis and in their other growth modes. The nature of the coupling of these energy-generating processes with the basic metabolic demands of the cell, such as nitrogen fixation, has stimulated research for many years. In addition, nitrogen fixation by photosynthetic prokaryotes exhibits several unique features; the oxygen-evolving cyanobacteria have developed various strategies for protection of the oxygen-labile nitrogenase proteins, and some photosynthetic bacteria have been found to regulate their nitrogenase (N2ase) activity in a rapid response to fixed nitrogen, thus saving substantial amounts of energy. Recent advances in the biochemistry, physiology, and genetics of nitrogen fixation by cyanobacteria and photosynthetic bacteria are reviewed, with special emphasis on the unique features found in these organisms. Several major topics in cyanobacterial nitrogen fixation are reviewed. The isolation and characterization of N2ase and the isolation and sequence of N2ase structural genes have shown a great deal of similarity with other organisms. The possible pathways of electron flow to N2ase, the mechanisms of oxygen protection, and the control of nif expression and heterocyst differentiation will be discussed. Several recent advances in the physiology and biochemistry of nitrogen fixation by the photosynthetic bacteria are reviewed. Photosynthetic bacteria have been found to fix nitrogen microaerobically in darkness. The regulation of nif expression and possible pathways of electron flow to N2ase are discussed. The isolation of N2ase proteins, particularly the covalent modification of the Fe protein, the nature of the modifying group, properties of the activating enzyme, and regulating factors of the inactivation/activation process are reviewed.