Nitrogen fixation by Anabaena cylindrica

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.

Transport and accumulation of nickel ions in the cyanobacterium Anabaena cylindrica

The uptake of nickel ions by the cyanobacterium Anabaena cylindrica was studied. Nickel transport was dependent on the membrane potential of the cells and the rate of uptake was decreased in the dark or by the addition of inhibitors, including uncouplers and electron transport inhibitors, which decreased or abolished the membrane potential of cells. The transport process obeyed hyperbolic kinetics, with a high affinity (apparent Km = 17 +/- 11 (SEM) nM) and low turnover number (maximum velocity = 22.3 +/- 5.4 (SEM) pmol h-1 mg dry wt-1 of cells or flux rate of 3.1 nmol h-1 m-2 of plasma membrane surface area). The process was also apparently specific for Ni2+, the rate being unaffected by the presence of a range of other metal ions in large excess. Equilibrium experiments showed that, over a range of nickel ion concentrations, the cells concentrated Ni2+ by a factor of 2700 +/- 240 (SEM)-fold, corresponding to a chemical diffusion potential for Ni2+ of 101 mV. It was concluded that the cells transport nickel ions by a carrier-facilitated transport process with the concentration factor for the ions being determined by the cell membrane potential according to the Nernst equation.

The use of nickel to probe the role of hydrogen metabolism in cyanobacterial nitrogen fixation

The hydrogenase activities of the heterocystous cyanobacteria Anabaena cylindrica and Mastigocladus laminosus are nickel dependent, based on their inability to consume hydrogen with various electron acceptors or produce hydrogen with dithionite-reduced methyl viologen, after growth in nickel-depleted medium. Upon addition of nickel ions to nickel-deficient cultures of A. cylindrica, the hydrogenase activity recovered in a manner which was protein synthesis-dependent, the recovery being inhibited by chloramphenicol. We have used the nickel dependence of the hydrogenase as a probe of the possible roles of H2 consumption in enhancing nitrogen fixation, and particularly for protecting nitrogenase against oxygen inhibition. Although at the usual growth temperatures (25 degrees for A. cylindrica and 40 degrees for M. laminosus), the cells consume H2 vigorously in an oxyhydrogen reaction after growth in the presence of nickel ions, we have not found that the reaction confers any significant additional protection of nitrogenase, either at aerobic pO2 (for both organisms) or at elevated pO2 (for A. cylindrica). However, at elevated temperatures (e.g., 40 degrees for A. cylindrica and 48 degrees for M. laminosus) a definite protective effect was observed. At these temperatures both organisms rapidly lost acetylene reduction activity under aerobic conditions. When hydrogen gas (10%) was present, the cells retained approximately 50% of the nitrogenase activity observed under anaerobic conditions (argon gas phase). No such protection by hydrogen gas was observed with nickel-deficient cells. Studies with cell-free extracts of A. cylindrica showed that the predominant effect of temperature was not due to thermal inactivation of nitrogenase.

Comparative characterization of two distinct hydrogenases from Anabaena sp. strain 7120

Two distinct hydrogenases, hereafter referred to as "uptake" and "reversible" hydrogenase, were extracted from Anabaena sp. strain 7120 and partially purified. The properties of the two enzymes were compared in cell-free extracts. Uptake hydrogenase was largely particulate, and although membrane bound, it could catalyze an oxyhydrogen reaction. Particulate and solubilized uptake hydrogenase could catalyze H2 uptake with a variety of artificial electron acceptors which had midpoint potentials above 0 mV. Reversible hydrogenase was soluble, could donate electrons rapidly to electron acceptors of both positive and negative midpoint potential, and could evolve H2 rapidly when provided with reduced methyl viologen. Uptake hydrogenase was irreversibly inactivated by O2, whereas reversible hydrogenase was reversibly inactivated and could be reactivated by exposure to dithionite or H2. Reversible hydrogenase was stable to heating at 70 degrees C, but uptake hydrogenase was inactivated with a half-life of 12 min at this temperature. Uptake hydrogenase was eluted from Sephadex G-200 in a single peak of molecular weight 56,000, whereas reversible hydrogenase was eluted in two peaks with molecular weights of 165,000 and 113,000. CO was competitive with H2 for each enzyme; the Ki's for CO were 0.0095 atm for reversible hydrogenase and 0.039 atm for uptake hydrogenase. The pH optima for H2 evolution and H2 uptake by reversible hydrogenase were 6 and 9, respectively. Uptake hydrogenase existed in two forms with pH optima of 6 and 8.5. Both enzymes had very low Km's for H2, and neither was inhibited by C2H2.

Aerobic hydrogenase activity in Anacystis nidulans. The oxyhydrogen reaction

1. The oxyhydrogen reaction of Anacystis nidulans was studied manometrically and polarographically in whole cells and in cell-free preparations; the activity was found to be associated with the particulate fraction. 2. Besides O2, the isolated membranes reduced artificial electron acceptors of positive redox potential; the reactions were unaffected by O2 levels less than 10--15%; aerobically the artificial acceptors were reduced simultaneously with O2. 3. H2-supported O2 uptake was inhibited by CO, KCN and 2-n-heptyl-8-hydroxyquinoline-N-oxide. Inhibition by CO was partly reversed by strong light. Uncouplers stimulated the oxyhydrogen reaction. 4. The kinetic properties of O2 uptake by isolated membranes were the same in presence of H2 and of other respiratory substrates. 5. Low rates of H2 evolution by the membrane preparations were found in presence of dithionite; methyl viologen stimulated the reaction. 6. The results indicate that under certain growth conditions Anacystis synthesizes a membrane-bound hydrogenase which appears to be involved in phosphorylative electron flow from H2 to O2 through the respiratory chain.

Anaerobic hydrogenase activity in Anacystis nidulans. H2-dependent photoreduction and related reactions

1. Anaerobic hydrogenase activity in whole cells and cell-free preparations of H2-induced Anacystis was studied both manometrically and spectrophotometrically in presence of physiological and artificial electron acceptors. 2. Up to 90% of the activity measured in crude extracts were recovered in the chlorophyll-containing membrane fraction after centrifugation (144 000 X g, 3 h). 3. Reduction of methyl viologen, diquat, ferredoxin, nitrite and NADP by the membranes was light dependent while oxidants of more positive redox potential were reduced also in the dark. 4. Evolution of H2 by the membranes was obtained with dithionite and with reduced methyl viologen; the reaction was stimulated by detergents. 5. Both uptake and evolution of H2 were sensitive to O2, CO, and thiolblocking agents. The H2-dependent reductions were inhibited also by the plastoquinone antagonist dibromothymoquinone, while the ferredoxin inhibitor disalicylidenepropanediamine affected the photoreduction of nitrite and NADP only. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea did not inhibit any one of the H2-dependent reactions. 6. The results present evidence for a membrane-bound 'photoreduction' hydrogenase in H2-induced Anacystis. The enzyme apparently initiates a light-driven electron flow from H2 to various low-potential acceptors including endogenous ferredoxin.

Hydrogen Production by the Thermophilic Alga Mastigocladus laminosus : Effects of Nitrogen, Temperature, and Inhibition of Photosynthesis

Hydrogen production by nitrogen-limited cultures of a thermophilic blue-green alga (cyanobacterium), Mastigocladus laminosus , was studied to develop the concept of a high-temperature biophotolysis system. Biophotolytic production of hydrogen by solar radiation was also demonstrated. Hydrogen consumption activity in these cultures was relatively high and is the present limiting factor on both the net rate and duration of hydrogen production.

Purification and properties of nitrogenase from the cyanobacterium, Anabaena cylindrica

The nitrogenase complex was isolated from nitrogen-starved cultures of Anabaema cylindrica. Sodium dithionite, photochemically reduced ferredoxin, and NADPH were found to be effective election donors to nitro genase in crude extracts whereas hydrogen and pyruvate were not. The Km for acetylene in vivo is ten-fold higher than the Km in vitro, whereas this pattern does not hold for the non-heterocystous cyanobacterium, Plectonema boryanum. This indicates that at least one mechanism of oxygen protection in vivo involves a gas diffusion barrier presented by the heterocyst cell wall. The Mo-Fe component was purified to homogeneity. Its molecular weight (220,000), subunit composition, isoelectric point (4.8), Mo, Fe, and S2- content (2, 20 and 20 mol/mol component), and amino acid composition indicate that this component has similar properties to Mo-Fe-containing components isolated from other bacterial sources. The isolated components from A. cylindrica were found to cross-react, to varying degrees, with components isolated from Azotobacter vinelandii, Rhodospirillum rubrum, and P. boryanum.

The differential action of metronidazole on nitrogen fixation, hydrogen metabolism, photosynthesis and respiration in Anabaena and Scenedesmus

Metronidazole (2-methyl-5-nitroimidazole-1-ethanol) at 1--2 mM levels has been shown to be a selective inhibitor of nitrogenase activity in Anabanena. Two constitutive hydrogenases and photosynthesis are insensitive to metronidazole at these same concentrations. At higher concentrations metronidazole inhibits photosynthesis in Anabaena while photoreduction and to a lesser extent photohydrogen production are retarded in Scenedesmus. Respiration is slightly stimulated at high metronidazole levels in both algae. The reductant source for nitrogenase in Anabaena and photohydrogen production and photoreduction electron transport in Scenedesmus are discussed. Due to the activity to metronidazole as a selective inhibitor of ferredoxin-associated processes, it should prove to be useful in N2 fixation studies and in distinguishing between ferredoxin-linked reactions of different sensitivities and other activities not associated with low reduction potential components.

Hydrogen metabolism in blue-green algae

This manuscript reviews the literature on hydrogen metabolism in blue-green algae and reports some new data from this laboratory. H2-formation by intact cells is found to be catalyzed exclusively by nitrogenase. Its rate appears to be variable from strain to strain used byt is--in our hands--very small. Therefore, blue-green algae are presumably of limited value in projects of solar energy conversion to form molecular hydrogen. These organisms are also able to consume the gas in a reaction catalysed by hydrogenase. Hydrogen is mainly consumed in an oxygen dependent reaction, as in aerobic nitrogen fixing bacteria. It can also serve as an electron donor for nitrogen fixation under certain physiological conditions. In experiments with a cell-free preparation, hydrogenase is found to be membrane-bound. The enzyme is characterized with respect to its specifity towards electron donors and acceptors.

The hydrogenase-nitrogenase relationship in the blue-green algaAnabaena cylindrica

Nitrogen-fixingAnabaena cylindrica cells are found to evolve hydrogen in high quantities in the presence of CO plus C2H2. Studies with the inhibitors dichlorophenyldimethylurea (DCMU), disalicylidenepropanediamine (DSPD), dibromothymoquinone (DBMIB), undecylbenzimidazole (UDB) and chloro-carbonyl-cyanide-phenylhydrazone (CCCP) and also withAnabaena grown on nitrate- and ammonia-nitrogen show that the H2-formation is due to the ATP-dependent H3O(+)-reduction catalysed by nitrogenase. In control experiments CO plus C2H2 inhibited the activities of a cell-free hydrogenase fromClostridium pasteurianum. It is concluded that Anabaena has a hydrogenase whose natural function is to recycle the H2 lost by the action of nitrogenase.