Whole cell respiration and nitrogenase activities in Azotobacter vinelandii growing in oxygen controlled continuous culture

Overview of physiological, biochemical, and regulatory aspects of nitrogen fixation in Azotobacter vinelandii

Understanding how Nature accomplishes the reduction of inert nitrogen gas to form metabolically tractable ammonia at ambient temperature and pressure has challenged scientists for more than a century. Such an understanding is a key aspect toward accomplishing the transfer of the genetic determinants of biological nitrogen fixation to crop plants as well as for the development of improved synthetic catalysts based on the biological mechanism. Over the past 30 years, the free-living nitrogen-fixing bacterium Azotobacter vinelandii emerged as a preferred model organism for mechanistic, structural, genetic, and physiological studies aimed at understanding biological nitrogen fixation. This review provides a contemporary overview of these studies and places them within the context of their historical development.

A critical role of an oxygen-responsive gene for aerobic nitrogenase activity in Azotobacter vinelandii and its application to Escherichia coli

Since nitrogenase is irreversibly inactivated within a few minutes after exposure to oxygen, current studies on the heterologous expression of nitrogenase are limited to anaerobic conditions. This study comprehensively identified genes showing oxygen-concentration-dependent expression only under nitrogen-fixing conditions in Azotobacter vinelandii, an aerobic diazotroph. Among the identified genes, nafU, with an unknown function, was greatly upregulated under aerobic nitrogen-fixing conditions. Through replacement and overexpressing experiments, we suggested that nafU is involved in the maintenance of nitrogenase activity under aerobic nitrogenase activity. Furthermore, heterologous expression of nafU in nitrogenase-producing Escherichia coli increased nitrogenase activity under aerobic conditions by 9.7 times. Further analysis of NafU protein strongly suggested its localization in the inner membrane and raised the possibility that this protein may lower the oxygen concentration inside the cells. These findings provide new insights into the mechanisms for maintaining stable nitrogenase activity under aerobic conditions in A. vinelandii and provide a platform to advance the use of nitrogenase under aerobic conditions.

Competitive Biosynthesis of Bacterial Alginate Using Azotobacter vinelandii 12 for Tissue Engineering Applications

This study investigated the effect of various cultivation conditions (sucrose/phosphate concentrations, aeration level) on alginate biosynthesis using the bacterial producing strain Azotobacter vinelandii 12 by the full factorial design (FFD) method and physicochemical properties (e.g., rheological properties) of the produced bacterial alginate. We demonstrated experimentally the applicability of bacterial alginate for tissue engineering (the cytotoxicity testing using mesenchymal stem cells (MSCs)). The isolated synthesis of high molecular weight (M_w) capsular alginate with a high level of acetylation (25%) was achieved by FFD method under a low sucrose concentration, an increased phosphate concentration, and a high aeration level. Testing the viscoelastic properties and cytotoxicity showed that bacterial alginate with a maximal M_w (574 kDa) formed the densest hydrogels (which demonstrated relatively low cytotoxicity for MSCs in contrast to bacterial alginate with low M_w). The obtained data have shown promising prospects in controlled biosynthesis of bacterial alginate with different physicochemical characteristics for various biomedical applications including tissue engineering.

Metabolic Model of the Nitrogen-Fixing Obligate Aerobe Azotobacter vinelandii Predicts Its Adaptation to Oxygen Concentration and Metal Availability

There is considerable interest in promoting biological nitrogen fixation (BNF) as a mechanism to reduce the inputs of nitrogenous fertilizers in agriculture, but considerable fundamental knowledge gaps still need to be addressed. BNF is catalyzed by nitrogenase, which requires a large input of energy in the form of ATP and low potential electrons. Diazotrophs that respire aerobically have an advantage in meeting the ATP demands of BNF but face challenges in protecting nitrogenase from inactivation by oxygen. Here, we constructed a genome-scale metabolic model of the nitrogen-fixing bacterium Azotobacter vinelandii, which uses a complex respiratory protection mechanism to consume oxygen at a high rate to keep intracellular conditions microaerobic. Our model accurately predicts growth rate under high oxygen and substrate concentrations, consistent with a large electron flux directed to the respiratory protection mechanism. While a partially decoupled electron transport chain compensates for some of the energy imbalance under high-oxygen conditions, it does not account for all substrate intake, leading to increased maintenance rates. Interestingly, the respiratory protection mechanism is required for accurate predictions even when ammonia is supplemented during growth, suggesting that the respiratory protection mechanism might be a core principle of metabolism and not just used for nitrogenase protection. We have also shown that rearrangement of flux through the electron transport system allows A. vinelandii to adapt to different oxygen concentrations, metal availability, and genetic disruption, which cause an ammonia excretion phenotype. Accurately determining the energy balance in an aerobic nitrogen-fixing metabolic model is required for future engineering approaches.

Fluxomic Analysis Reveals Central Carbon Metabolism Adaptation for Diazotroph Azotobacter vinelandii Ammonium Excretion

Diazotrophic bacteria are an attractive biological alternative to synthetic nitrogen fertilizers due to their remarkable capacity to fix atmospheric nitrogen gas to ammonium via nitrogenase enzymes. However, how diazotrophic bacteria tailor central carbon catabolism to accommodate the energy requirement for nitrogenase activity is largely unknown. In this study, we used Azotobacter vinelandii DJ and an ammonium excreting mutant, AV3 (ΔNifL), to investigate central carbon metabolism fluxes and central cell bioenergetics in response to ammonium availability and nitrogenase activity. Enabled by the powerful and reliable methodology of ¹³C-metabolic flux analysis, we show that the respiratory TCA cycle is upregulated in association with increased nitrogenase activity and causes a monotonic decrease in specific growth rate. Whereas the activity of the glycolytic Entner-Doudoroff pathway is positively correlated with the cell growth rate. These new observations are formulated into a ¹³C-metabolic flux model which further improves the understanding and interpretation of intracellular bioenergetics. This analysis leads to the conclusion that, under aerobic conditions, respiratory TCA metabolism is responsible for the supply of additional ATP and reducing equivalents required for elevated nitrogenase activity. This study provides a quantitative relationship between central carbon and nitrogen metabolism in an aerobic diazotroph for the first time.

Azotobacter vinelandii Nitrogenase Activity, Hydrogen Production, and Response to Oxygen Exposure

Azotobacter vinelandii selectively utilizes three types of nitrogenase (molybdenum, vanadium, and iron only) to fix N₂, with their expression regulated by the presence or absence of different metal cofactors in its environment. Each alternative nitrogenase isoenzyme is predicted to have different electron flux requirements based on in vitro measurements, with the molybdenum nitrogenase requiring the lowest flux and the iron-only nitrogenase requiring the highest. Here, prior characterized strains, derepressed in nitrogenase synthesis and also deficient in uptake hydrogenase, were further modified to generate new mutants lacking the ability to produce poly-β-hydroxybutyrate (PHB). PHB is a storage polymer generated under oxygen-limiting conditions and can represent up to 70% of the cells' dry weight. The absence of such granules facilitated the study of relationships between catalytic biomass and product molar yields across different adaptive respiration conditions. The released hydrogen gas observed during growth, due to the inability of the mutants to recapture hydrogen, allowed for direct monitoring of in vivo nitrogenase activity for each isoenzyme. The data presented here show that increasing oxygen exposure limits equally the in vivo activities of all nitrogenase isoenzymes, while under comparative conditions, the Mo nitrogenase enzyme evolves more hydrogen per unit of biomass than the alternative isoenzymes.IMPORTANCEA. vinelandii has been a focus of intense research for over 100 years. It has been investigated for a variety of functions, including agricultural fertilization and hydrogen production. All of these endeavors are centered around A. vinelandii's ability to fix nitrogen aerobically using three nitrogenase isoenzymes. The majority of research up to this point has targeted in vitro measurements of the molybdenum nitrogenase, and robust data contrasting how oxygen impacts the in vivo activity of each nitrogenase isoenzyme are lacking. This article aims to provide in vivo nitrogenase activity data using a real-time evaluation of hydrogen gas released by derepressed nitrogenase mutants lacking an uptake hydrogenase and PHB accumulation.

Azotobacter vinelandii: the source of 100 years of discoveries and many more to come

Azotobacter vinelandii has been studied for over 100 years since its discovery as an aerobic nitrogen-fixing organism. This species has proved useful for the study of many different biological systems, including enzyme kinetics and the genetic code. It has been especially useful in working out the structures and mechanisms of different nitrogenase enzymes, how they can function in oxic environments and the interactions of nitrogen fixation with other aspects of metabolism. Interest in studying A. vinelandii has waned in recent decades, but this bacterium still possesses great potential for new discoveries in many fields and commercial applications. The species is of interest for research because of its genetic pliability and natural competence. Its features of particular interest to industry are its ability to produce multiple valuable polymers - bioplastic and alginate in particular; its nitrogen-fixing prowess, which could reduce the need for synthetic fertilizer in agriculture and industrial fermentations, via coculture; its production of potentially useful enzymes and metabolic pathways; and even its biofuel production abilities. This review summarizes the history and potential for future research using this versatile microbe.

Metabolic flux analysis and the NAD(P)H/NAD(P)+ ratios in chemostat cultures of Azotobacter vinelandii

BACKGROUND

Azotobacter vinelandii is a bacterium that produces alginate and polyhydroxybutyrate (P3HB); however, the role of NAD(P)H/NAD(P)+ ratios on the metabolic fluxes through biosynthesis pathways of these biopolymers remains unknown. The aim of this study was to evaluate the NAD(P)H/NAD(P) + ratios and the metabolic fluxes involved in alginate and P3HB biosynthesis, under oxygen-limiting and non-limiting oxygen conditions.

RESULTS

The results reveal that changes in the oxygen availability have an important effect on the metabolic fluxes and intracellular NADPH/NADP+ ratio, showing that at the lowest OTR (2.4 mmol L-1 h-1), the flux through the tricarboxylic acid (TCA) cycle decreased 27.6-fold, but the flux through the P3HB biosynthesis increased 6.6-fold in contrast to the cultures without oxygen limitation (OTR = 14.6 mmol L-1 h-1). This was consistent with the increase in the level of transcription of phbB and the P3HB biosynthesis. In addition, under conditions without oxygen limitation, there was an increase in the carbon uptake rate (twofold), as well as in the flux through the pentose phosphate (PP) pathway (4.8-fold), compared to the condition of 2.4 mmol L-1 h-1. At the highest OTR condition, a decrease in the NADPH/NADP+ ratio of threefold was observed, probably as a response to the high respiration rate induced by the respiratory protection of the nitrogenase under diazotrophic conditions, correlating with a high expression of the uncoupled respiratory chain genes (ndhII and cydA) and induction of the expression of the genes encoding the nitrogenase complex (nifH).

CONCLUSIONS

We have demonstrated that changes in oxygen availability affect the internal redox state of the cell and carbon metabolic fluxes. This also has a strong impact on the TCA cycle and PP pathway as well as on alginate and P3HB biosynthetic fluxes.

A quantitative analysis of the direct and indirect costs of nitrogen fixation: a model based on Azotobacter vinelandii

Nitrogen fixation is advantageous in microbial competition when bioavailable nitrogen is scarce, but has substantial costs for growth rate and growth efficiency. To quantify these costs, we have developed a model of a nitrogen-fixing bacterium that constrains mass, electron and energy flow at the scale of the individual. When tested and calibrated with laboratory data for the soil bacterium Azotobacter vinelandii, the model reveals that the direct energetic cost of nitrogen fixation is small relative to the cost of managing intracellular oxygen. It quantifies the costs and benefits of several potential oxygen protection mechanisms present in nature including enhanced respiration (respiratory protection) as well as the production of extracellular polymers as a barrier to O₂ diffusion, and increasing cell size. The latter mechanisms lead to higher growth efficiencies relative to respiratory protection alone. This simple, yet mechanistic framework provides a quantitative model of nitrogen fixation, which can be applied in ecological simulations.

Alginate Biosynthesis inAzotobacter vinelandii: Overview of Molecular Mechanisms in Connection with the Oxygen Availability

The Gram-negative bacteriumAzotobacter vinelandiican synthetize the biopolymer alginate that has material properties appropriate for plenty of applications in industry as well as in medicine. In order to settle the foundation for improving alginate production without compromising its quality, a better understanding of the polymer biosynthesis and the mechanism of regulation during fermentation processes is necessary. This knowledge is crucial for the development of novel production strategies. Here, we highlight the key aspects of alginate biosynthesis that can lead to producing an alginate with specific material properties with particular focus on the role of oxygen availability linked with the molecular mechanisms involved in the alginate production.

Oxygen transfer rate during the production of alginate by Azotobacter vinelandii under oxygen-limited and non oxygen-limited conditions

Background

The oxygen transfer rate (OTR) and dissolved oxygen tension (DOT) play an important role in determining alginate production and its composition; however, no systematic study has been reported about the independent influence of the OTR and DOT. In this paper, we report a study about alginate production and the evolution of the molecular mass of the polymer produced by a wild-type A. vinelandii strain ATCC 9046, in terms of the maximum oxygen transfer rate (OTR_max) in cultures where the dissolved oxygen tension (DOT) was kept constant.

Results

The results revealed that in the two dissolved oxygen conditions evaluated, strictly controlled by gas blending at 0.5 and 5% DOT, an increase in the agitation rate (from 300 to 700 rpm) caused a significant increase in the OTR_max (from 17 to 100 mmol L^-1 h^-1 for DOT of 5% and from 6 to 70 mmol L^-1 h^-1 for DOT of 0.5%). This increase in the OTR_max improved alginate production, as well as the specific alginate production rate (SAPR), reaching a maximal alginate concentration of 3.1 g L^-1 and a SAPR of 0.031 g _alg g _biom^-1 h^-1 in the cultures at OTR_max of 100 mmol L^-1 h^-1. In contrast, the mean molecular mass (MMM) of the alginate isolated from cultures developed under non-oxygen limited conditions increased by decreasing the OTR_max, reaching a maximal of 550 kDa at an OTR_max of 17 mmol L^-1 h^-1 . However, in the cultures developed under oxygen limitation (0.5% DOT), the MMM of the polymer was practically the same (around 200 kDa) at 300 and 700 rpm, and this remained constant throughout the cultivation.

Conclusions

Overall, our results showed that under oxygen-limited and non oxygen-limited conditions, alginate production and its molecular mass are linked to the OTR_max, independently of the DOT of the culture.

Manipulating the molecular weight of alginate produced by Azotobacter vinelandii in continuous cultures

Alginate production by Azotobacter vinelandii in chemostat cultures was evaluated at different dilution rates (D) and inlet sucrose concentrations of 5 and 20 g l(-1). At the low inlet sucrose concentration, the molecular weight of alginate increased from 800 to 1800 kDa when D increased from 0.05 to 0.10 h(-1), whereas the opposite trend was observed with the high inlet sucrose concentration. This behaviour can be explained by changes in specific sucrose uptake rate. Thus, a decrease in alginate molecular weight was dependent on the specific sucrose uptake rate when this rate was higher than 0.42 g g(-1) h(-1). The manipulation of the D can be used to select the molecular weight of alginate in continuous culture.

Defined spatial structure stabilizes a synthetic multispecies bacterial community

This paper shows that for microbial communities, "fences make good neighbors." Communities of soil microorganisms perform critical functions: controlling climate, enhancing crop production, and remediation of environmental contamination. Microbial communities in the oral cavity and the gut are of high biomedical interest. Understanding and harnessing the function of these communities is difficult: artificial microbial communities in the laboratory become unstable because of "winner-takes-all" competition among species. We constructed a community of three different species of wild-type soil bacteria with syntrophic interactions using a microfluidic device to control spatial structure and chemical communication. We found that defined microscale spatial structure is both necessary and sufficient for the stable coexistence of interacting bacterial species in the synthetic community. A mathematical model describes how spatial structure can balance the competition and positive interactions within the community, even when the rates of production and consumption of nutrients by species are mismatched, by exploiting nonlinearities of these processes. These findings provide experimental and modeling evidence for a class of communities that require microscale spatial structure for stability, and these results predict that controlling spatial structure may enable harnessing the function of natural and synthetic multispecies communities in the laboratory.

The oxygen transfer rate influences the molecular mass of the alginate produced by Azotobacter vinelandii

The influence of oxygen transfer rate (OTR) on the molecular mass of alginate was studied. In batch cultures without dissolved oxygen tension (DOT) control and at different agitation rates, the DOT was nearly zero and the OTR was constant during biomass growth, hence the cultures were oxygen-limited. The OTR reached different maximum levels (OTR(max)) and enabled to establish various relative respiration rates. Overall, the findings showed that OTR influences alginate molecular mass. The mean molecular mass (MMM) of the alginate increased as OTR(max) decreased. The molecular mass obtained at 3.0 mmol l(-1) h(-1) was 7.0 times higher (1,560 kDa) than at 9.0 mmol l(-1) h(-1) (220 kDa). An increase in molecular mass can be a bacterial response to adverse nutritional conditions such as oxygen limitation.

Respiratory protection of nitrogenase in Azotobacter species: is a widely held hypothesis unequivocally supported by experimental evidence?

The hypothesis of respiratory protection, originally formulated on the basis of results obtained with Azotobacter species, postulates that consumption of O(2) at the surface of diazotrophic prokaryotes protects nitrogenase from inactivation by O(2). Accordingly, it is assumed that, at increased ambient O(2) concentrations, nitrogenase activity depends on increased activities of a largely uncoupled respiratory electron transport system. The present review compiles evidence indicating that cellular O(2) consumption as well as both the activity and the formation of the respiratory system of Azotobacter vinelandii are controlled by the C/N ratio, that is to say the ratio at which the organism consumes the substrate (i.e. the source of carbon, reducing equivalents and ATP) per source of compound nitrogen. The maximal respiratory capacity which can be attained at increased C/N ratios, however, is controlled, within limits, by the ambient O(2) concentration. When growth becomes N-limited at increased C/N ratios, cells synthesize nitrogenase and fix N(2). Under these diazotrophic conditions, cellular O(2) consumption remains constant at a level controlled by the O(2) concentration. Control by O(2) has been studied on the basis of both whole cell respiration and defined segments of the respiratory electron transport chain. The results demonstrate that the effect of O(2) on the respiratory system is restricted to the lower range of O(2) concentrations up to about 70 microM. Nevertheless, azotobacters are able to grow diazotrophically at dissolved O(2) concentrations of up to about 230 microM indicating that respiratory protection is not warranted at increased ambient O(2) concentrations. This conclusion is supported and extended by a number of results largely excluding an obvious relationship between nitrogenase activity and the actual rate of cellular O(2) consumption. On the basis of theoretical calculations, it is assumed that the rate of O(2) diffusion into the cells is not significantly affected by respiration. All of these results lead to the conclusion that, in the protection of nitrogenase from O(2) damage, O(2) consumption at the cell surface is less effective than generally assumed. It is proposed that alternative factors like the supply of ATP and reducing equivalents are more important.

Effect of oxygen on formation and structure of Azotobacter vinelandii alginate and its role in protecting nitrogenase

The activity of nitrogenase in the nitrogen-fixing bacterium Azotobacter vinelandii grown diazotrophically under aerobic conditions is generally considered to be protected against O(2) by a high respiration rate. In this work, we have shown that a high rate of respiration is not the prevailing mechanism for nitrogenase protection in A. vinelandii grown in phosphate-limited nitrogen-free chemostat culture. Instead, the formation of alginate appeared to play a decisive role in protecting the nitrogenase that is required for cell growth in this culture. Depending on the O(2) tension and cell growth rate, the formation rate and composition of alginate released into the culture broth varied significantly. Furthermore, transmission electron microscopic analysis of cell morphology and the cell surface revealed the existence of an alginate capsule on the surface of A. vinelandii. The composition, thickness, and compactness of this alginate capsule also varied significantly. In general, increasing O(2) tension led to the formation of alginate with a higher molecular weight and a greater L-guluronic acid content. The alginate capsule was accordingly thicker and more compact. In addition, the formation of the alginate capsule was found to be strongly affected by the shear rate in a bioreactor. Based on these experimental results, it is suggested that the production of alginate, especially the formation of an alginate capsule on the cell surface, forms an effective barrier for O(2) transfer into the cell. It is obviously the quality, not the quantity, of alginate that is decisive for the protection of nitrogenase.

Two NADH:ubiquinone oxidoreductases of Azotobacter vinelandii and their role in the respiratory protection

Initial steps of the Azotobacter vinelandii respiratory chain have been studied on the inside-out subcellular vesicles. Two NADH:ubiquinone oxidoreductases were revealed: (i) proton-motive, capsaicin-sensitive and oxidizing dNADH as well as NADH enzyme and (ii) enzyme non-coupled to the energy conservation, capsaicin-resistant and oxidizing only NADH. The level of the oxidoreductases strongly depends upon [O2] and [NH3] in the growth medium. Increase in [O2] results in lowering of the coupled-enzyme level and in rise of the non-coupled one. Exclusion of NH3 from the growth medium increases the level of the non-coupled enzyme whereas that of the coupled enzyme remains constant. The O2-linked control of NADH:ubiquinone oxidoreductases requires CydR, a Fnr-like regulatory protein. Summarizing the above observations with those made in this group on the terminal steps of the A. vinelandii respiratory chains, one can assume that the respiratory protection of nitrogenase could be carried out by co-operation of the non-coupled NADH:ubiquinone oxidoreductase and the "partially coupled" quinoloxidase of the bd-type. Efficiency of this chain seems to be five-fold lower than that of the usual proton-motive chain (the coupled NADH:ubiquinone oxidoreductase, the Q-cycle and cytochrome oxidase of the o-type) which is also present in A. vinelandii and operates at low [O2].

Generation of protonic potential by the bd-type quinol oxidase of Azotobacter vinelandii

Inside-out subcellular vesicles of Azotobacter vinelandii are found to produce delta pH and delta psi (interior acidic and positive) when oxidising malate or menadiol. These effects are inherent in both Cyd+ Cyo- (lacking the o-type oxidase) and Cyd- Cyo+ (lacking the bd-type oxidase) strains. They appear to be myxothiazol-sensitive in the Cyd- Cyo+ strain but not in the Cyd+ Cyo- strain. The H+/e- ratio for the terminal part of respiratory chain of a bd-type oxidase overproducing strain is established as being close to 1. It is also shown that NADH oxidation by the vesicles from the Cyd- Cyo+ strain is sensitive to low concentrations of myxothiazol and antimycin A whereas that of the Cyd+ Cyo- strain is resistant to these Q-cycle inhibitors. It is concluded that (i) the bd-type oxidase of A. vinelandii is competent in generating a protonic potential but its efficiency is lower than that of the o-type oxidase and (ii) Q-cycle does operate in the o-type cytochrome oxidase terminated branch of the A. vinelandii respiratory chain and does not in the bd-type quinol oxidase terminated branch. These relationships are discussed in the context of the respiratory protection function of the bd-type oxidase in A. vinelandii.

The Klebsiella pneumoniae cytochrome bd' terminal oxidase complex and its role in microaerobic nitrogen fixation

Cytochrome bd' has been implicated in having an important role in microaerobic nitrogen fixation in the enteric bacterium Klebsiella pneumoniae, where it is expressed under all conditions that permit diazotrophy. In this paper the sequence of the genes encoding this terminal oxidase (cydAB) of Klebsiella pneumoniae and the characterization of a cyd mutant are reported. The deduced amino acid sequences support the proposal that His 19, His 186 and Met 393 provide three of the four axial ligands to the Fe of the three haems in the oxidase complex. The nitrogen-fixing ability of the mutant was severely impaired in the presence of low concentrations of oxygen compared with the wild-type bacterium. Only the wild-type organism was capable of microaerobic nitrogenase activity supported by fermentation products. It is proposed that formate dehydrogenase-O may be involved in supplying electrons to a respiratory chain terminated by the bd-type oxidase, which would remove inhibitory oxygen and supply ATP for nitrogenase activity.

Nitrogenase activity and regeneration of the cellular ATP pool in Azotobacter vinelandii adapted to different oxygen concentrations

The in vivo activity of nitrogenase under aerobiosis was studied with diazotrophic chemostat cultures of Azotobacter vinelandii grown under glucose- or phosphate-limited conditions at different dilution rates (Ds, representing the growth rate mu) and different dissolved oxygen concentrations. Under steady-state conditions, the concentration as well as the cellular level of ATP increased in glucose-limited cultures when D was increased. Irrespective of the type of growth limitation or the dissolved oxygen concentration, the steady-state concentrations of ATP and of dinitrogen fixed by nitrogenase increased in direct proportion to each other. Specific rates of dinitrogen fixation as well as of the regeneration of the cellular ATP pool were compared with specific rates of cellular respiration. With glucose-limited cultures, the rate of regeneration of the ATP pool and the rate of respiration varied in direct proportion to each other. This relationship, however, was dependent on the dissolved oxygen concentration. As compared to the phosphate-sufficient control, phosphate-limited cultures exhibited the same nitrogenase activity but significantly increased respiratory activities. Rates of ATP regeneration and of cellular respiration of phosphate-limited cultures did not fit into the relationship characteristic of glucose-limited cultures. However, a linear relationship between the rates of dinitrogen fixation and ATP regeneration was identified irrespective of the type of growth limitation and the dissolved oxygen concentration. The results suggest that the ATP supply rather than cellular oxygen consumption is of primary importance in keeping nitrogenase activity in aerobic cultures of A. vinelandii.

Expression and Content of Terminal Oxidases in Azotobacter Vinelandii Grown with Excess NH4+ are Modulated by O2 Supply

The influence of the rate of O2 supply to batch cultures on the contents of cytochromes bd and ‘o’ in NH4 +-grown Azotobacter vinelandii has been investigated. Difference spectra at room temperature (reduced + CO minus reduced) were recorded for whole cells of a wild-type strain and mutants which either lacked or over-produced the cytochrome bd-type terminal oxidase encoded by cydAB. A Tn5-B20 insertion in cydB in the former mutant also provided a means of monitoring cydAB gene expression from measurements of β-galactosidase activity. The content of cytochrome d in the wild-type, and the expression of cydAB-lacZ, in the mutant, increased as the O2 supply was raised, suggesting that O2 regulates cydAB expression even in the absence of diazotrophy. In a strain carrying a mutation in cydR, a regulatory gene upstream of cydAB, and which over-produces cytochrome bd, the responses to O2 supply during growth at different O2 supply rates were reversed. Changes in the content of a haemoprotein detectable in low temperature photodissociation spectra, and attributed to cytochrome b 595 -the high-spin cytochrome b component of the cytochrome bd complex - followed the changes in cytochrome d levels. CO difference spectra of both the wild-type strain and the cytochrome bd-deficient mutant revealed a haemoprotein with spectral characteristics similar to cytochrome o, the levels of which increased as the O2 supply was raised. These results are discussed with reference to previous reports of cytochrome changes in cells grown under N2-fixing conditions.

Hydrogenase does not confer significant benefits to Azotobacter vinelandii growing diazotrophically under conditions of glucose limitation

The presumed beneficial effect of hydrogenase on growth of diazotrophic bacteria was reinvestigated with carbon-limited chemostat cultures of the hydrogenase-deficient mutant hoxKG of Azotobacter vinelandii and its parent. The results revealed that hydrogen recycling was too low to benefit the cellular energy metabolism or activities of nitrogenase and respiration.

Cellular ATP levels and nitrogenase switchoff upon oxygen stress in chemostat cultures of Azotobacter vinelandii

When Azotobacter vinelandii, growing diazotrophically in chemostat culture, was subjected to sudden increases in the ambient oxygen concentration (oxygen stress), nitrogenase activity was switched off and cellular ATP pools decreased at rates depending on the stress level. Following a fast decrease, the ATP pool approached a lower level. When the stress was released, these effects were reversed. The reversible decrease of the ATP pool upon oxygen stress could also be observed with cultures assimilating ammonium and, at the same time, fixing dinitrogen because of growth at a high C/N ratio but not with cultures growing only at the expense of ammonium. When strains OP and UW136 of A. vinelandii were subjected to long-term increases in ambient oxygen, the sizes of cellular ATP pools eventually started to increase to the level before stress and diazotrophic growth resumed. The cytochrome d-deficient mutant MK5 of A. vinelandii, however, impaired in aerotolerant diazotrophic growth, was unable to recover from stress on the basis of its ATP pool. The results suggest that adaptation to higher ambient oxygen depends on increased ATP synthesis requiring increased electron flow through the entire respiratory chain, which is possible only in combination with the more active, yet possibly uncoupled, branch terminated by cytochrome d. It is proposed that the decrease of the cellular ATP level under oxygen stress resulted from the increased energy and electron donor requirement of nitrogenase in reacting with oxygen.

Redundancy of the conserved His residue in Azotobacter vinelandii NifL, a histidine autokinase homologue which regulates transcription of nitrogen fixation genes

The NifL protein of Azotobacter vinelandii inhibits NifA, the activator of nif (nitrogen fixation) transcription, in response to oxygen and fixed nitrogen. NifL shows strong homology in its C-terminal domain to the histidine autokinase domains of the canonical two-component sensor proteins, including the region around His-304, which corresponds to the residue known to be phosphorylated in other systems. To examine the mechanism of sensory transduction by NifL, mutations encoding 10 substitutions for His-304 were introduced into the A. vinelandii chromosome. Regulation of nif transcription was measured using acetylene reduction and RNA blots. The substitutions His-304-->Arg and His-304-->Pro impaired regulation by both fixed nitrogen and oxygen, but substitution of Ala, Phe, Ile, Lys, Asn, Ser, Thr, Val had no effect. None of the mutants, including His-304-->Arg and His-304-->Pro, excreted ammonium during diazotrophy, a phenotype of nifL deletion mutants, suggesting that the molecular basis of this effect differs from that responsible for the inhibition of nif transcription. The data show conclusively that phosphorylation of His-304 is not essential for any of the known functions of A. vinelandii NifL. Homology to the family of histidine autokinases is therefore inadequate evidence for a mechanism of sensory transduction involving phosphorylation of the conserved histidine residue.

Augmented rates of respiration and efficient nitrogen fixation at nanomolar concentrations of dissolved O2 in hyperinduced Azoarcus sp. strain BH72

Azoarcus sp. strain BH72 is an aerobic diazotrophic bacterium that was originally found as an endophyte in Kallar grass. Anticipating that these bacteria are exposed to dissolved O2 concentrations (DOCs) in the nanomolar range during their life cycle, we studied the impact of increasing O2 deprivation on N2 fixation and respiration. Bacteria were grown in batch cultures, where they shifted into conditions of low pO2 upon depletion of O2 by respiration. During incubation, specific rates of respiration (qO2) and efficiencies of carbon source utilization for N2 reduction increased greatly, while the growth rate did not change significantly, a phenomenon that we called "hyperinduction." To evaluate this transition from high- to low-cost N2 fixation in terms of respiratory kinetics and nitrogenase activities at nanomolar DOC, bacteria which had shifted to different gas-phase pO2s in batch cultures were subjected to assays using leghemoglobin as the O2 carrier. As O2 deprivation in batch cultures proceeded, respiratory Km (O2) decreased and Vmax increased. Nitrogenase activity at nanomolar DOC increased to a specific rate of 180 nmol of C2H4 min-1 mg of protein-1 at 32 nM O2. Nitrogenase activity was proportional to respiration but not to DOC in the range of 12 to 86 nM O2. Respiration supported N2 fixation more efficiently at high than at low respiratory rates, the respiratory efficiency increasing from 0.14 to 0.47 mol of C2H4 mol of O2 consumed-1. We conclude that (i) during hyperinduction, strain BH72 used an increasing amount of energy generated by respiration for N2 fixation, and (ii) these bacteria have a high respiratory capacity, enabling them to develop ecological niches at very low pO2, in which they may respire actively and fix nitrogen efficiently at comparatively high rates.

Nitrogen Fixation and Hydrogen Metabolism in Relation to the Dissolved Oxygen Tension in Chemostat Cultures of the Wild Type and a Hydrogenase-Negative Mutant of Azorhizobium caulinodans

Both the wild type and an isogenic hydrogenase-negative mutant of Azorhizobium caulinodans growing ex planta on N 2 as the N source were studied in succinate-limited steady-state chemostat cultures under 0.2 to 3.0% dissolved O 2 tension. Production or consumption of O 2 , H 2 , and CO 2 was measured with an on-line-connected mass spectrometer. In the range of 0.2 to 3.0%, growth of both the wild type and the mutant was equally dependent on the dissolved O 2 tension: the growth yield decreased, and the specific O 2 consumption and CO 2 production increased. A similar dependency on the dissolved O 2 tension was found for the mutant with 2.5% H 2 in the influent gas. The H 2 /N 2 ratio (moles of H 2 evolved per mole of N 2 consumed via nitrogenase) of the mutant, growing with or without 2.5% H 2 , increased with increasing dissolved O 2 tensions. This increase in the H 2 /N 2 ratio was small but significant. The dependencies of the ATP/N 2 ratio (moles of ATP consumed per mole of N 2 fixed) and the ATP/2e - ratio [moles of ATP consumed per mole of electron pairs transferred from NAD(P)H to nitrogenase] on the dissolved O 2 tension were estimated. These dependencies were interpreted in terms of the physiological concepts of respiratory protection and autoprotection.

Oxygen relations of nitrogen fixation in cyanobacteria

The enigmatic coexistence of O2-sensitive nitrogenase and O2-evolving photosynthesis in diazotrophic cyanobacteria has fascinated researchers for over two decades. Research efforts in the past 10 years have revealed a range of O2 sensitivity of nitrogenase in different strains of cyanobacteria and a variety of adaptations for the protection of nitrogenase from damage by both atmospheric and photosynthetic sources of O2. The most complex and apparently most efficient mechanisms for the protection of nitrogenase are incorporated in the heterocysts, the N2-fixing cells of cyanobacteria. Genetic studies indicate that the controls of heterocyst development and nitrogenase synthesis are closely interrelated and that the expression of N2 fixation (nif) genes is regulated by pO2.

Dependence of nitrogenase switch-off upon oxygen stress on the nitrogenase activity in Azotobacter vinelandii

Azotobacter vinelandii was grown diazotrophically in chemostat cultures limited by sucrose, citrate, or acetate. Specific activities of cellular oxygen consumption (qO2) and nitrogenase (acetylene reduction) were measured in situ at different dilution rates (D, representing the specific growth rate mu at steady state). Sucrose-limited cultures exhibited linear relationships between qO2 and D, each of which, however, depended on the dissolved oxygen concentration in the range of 12 to 192 microM O2. From these plots, qO2 required for maintenance processes (mO2) were extrapolated. mO2 values did not increase linearly with increasing dissolved oxygen concentrations. With citrate- or acetate-limited cultures qO2 also depended on D. At 108 microM O2, however, qO2 and mO2 of the latter cultures were significantly lower than those of sucrose-limited cultures. Specific rates of acetylene reduction increased linearly with D, irrespective of the type of limitation and of the dissolved oxygen concentration (J. Kuhla and J. Oelze, Arch. Microbiol. 149:509-514, 1988). The reversible switch-off of nitrogenase activity under oxygen stress also depended on D and was independent of qO2, mO2, or the limiting substrate. Increased switch-off effects resulting from increased stress heights could be compensated for by increasing D. Since D represents not only the supply of the carbon source but also the supply of electrons and energy, the results suggest that the flux of electrons to the nitrogenase complex, rather than qO2, stabilizes nitrogenase activity against oxygen inactivation in aerobically growing A. vinelandii.

Levels and activities of nitrogenase proteins in Azotobacter vinelandii grown at different dissolved oxygen concentrations

Azotobacter vinelandii was grown diazotrophically at different dissolved oxygen concentrations (in the range of 3 to 216 microM) in sucrose-limited continuous culture. The specific nitrogenase activity, measured on the basis of acetylene reduction in situ, was dependent solely on the growth rate and was largely independent of oxygen and sucrose concentration. FeMo (Av1) and Fe (Av2) nitrogenase proteins were quantified after Western blotting (immunoblotting). When the cultures were grown at a constant dilution rate (D, representing the growth rate, mu) of 0.15.h-1, the cellular levels of both proteins were constant regardless of different dissolved oxygen concentrations. The same was true when the organisms were grown at D values above 0.15.h-1. At a lower growth rate (D = 0.09.h-1), however, and at lower oxygen concentrations cellular levels of both nitrogenase proteins were decreased. This means that catalytic activities of nitrogenase proteins were highest at low oxygen concentrations, but at higher oxygen concentrations they increased with growth rate. Under all conditions tested, however, the Av1:Av2 molar ratio was 1:(1.45 +/- 0.12). Cellular levels of flavodoxin and FeS protein II were largely constant as well. In order to estimate turnover of nitrogenase proteins in the absence of protein synthesis, chloramphenicol was added to cultures adapted to 3 and 216 microM oxygen, respectively. After 2 h of incubation, no significant decrease in the cellular levels of Av1 and Av2 could be observed. This suggests that oxygen has no significant effect on the breakdown of nitrogenase proteins.

How is nitrogenase regulated by oxygen?

Oxygen can be either beneficial or detrimental for diazotrophy in organisms capable of an aerobic catabolism. It supports the production of a substrate for nitrogenase (ATP), but it can also inhibit the activity and repress the synthesis of this enzyme. Here, aspects of the relevant physiology are reviewed with particular emphasis on those relating to the mechanism of O2 regulation of nitrogenase synthesis.

Root-Zone-Specific Oxygen Tolerance of Azospirillum spp. and Diazotrophic Rods Closely Associated with Kallar Grass

The effect of oxygen on N 2 -dependent growth of two Azospirillum strains and two diazotrophic rods closely associated with roots of Kallar grass ( Leptochloa fusca ) was studied. To enable precise comparison, bacteria were grown in dissolved-oxygen-controlled batch and continuous cultures. Steady states were obtained from about 1 to 30 μM O 2 , some of them being carbon limited. All strains needed a minimum amount of oxygen for N 2 -dependent growth. Nitrogen contents between 10 and 13% of cell dry weight were observed. The response of steady-state cultures to increasing O 2 concentrations suggested that carbon limitation shifted to internal nitrogen limitation when N 2 fixation became so low that the bacteria could no longer meet their requirements for fixed nitrogen. For Azospirillum lipoferum Rp5, increase of the dilution rate resulted in decreased N 2 fixation in steady-state cultures with internal nitrogen limitation. Oxygen tolerance was found to be strain specific in A. lipoferum with strain Sp59b as a reference organism. Oxygen tolerance of strains from Kallar grass was found to be root zone specific. A. halopraeferens Au 4 and A. lipoferum Rp5, predominating on the rhizoplane of Kallar grass, and strains H6a2 and BH72, predominating in the endorhizosphere, differed in their oxygen tolerance profiles. Strains H6a2 and BH72 still grew and fixed nitrogen in steady-state cultures at O 2 concentrations exceeding those which absolutely inhibited nitrogen fixation of both Azospirillum strains. It is proposed that root-zone-specific oxygen tolerance reflects an adaptation of the isolates to the microenvironments provided by the host plant.

Studies on the mechanism of electron transport to nitrogenase in Azotobacter vinelandii

The involvement of the cytoplasmic membrane in electron transport to nitrogenase has been studied. Evidence shows that nitrogenase activity in Azotobacter vinelandii is coupled to the flux of electrons through the respiratory chain. To obtain information about proteins involved, the changes occurring in A. vinelandii cells transferred to nitrogen-free medium after growth on NH4Cl (depression of nitrogenase activity) were studied. Synthesis of the nitrogenase polypeptides was detectable 5 min after transfer to nitrogen-free medium. No nitrogenase activity could be detected until t = 20 min, whereupon a linear increase of nitrogenase activity with time was observed. Synthesis of nitrogenase was accompanied by synthesis of flavodoxin II and two membrane-bound polypeptides of Mr 29,000 and 30,000. Analysis with respect to changes in membrane-bound NAD(P)H dehydrogenase activities revealed the induction of an NADPH dehydrogenase activity, which was not detectable in membranes isolated from cells grown in the presence of NH4OAc. This induced activity was associated with the appearance of a polypeptide of Mr 29,000 in the NADPH dehydrogenase complex.