Silver Nanoparticles Induced Cell Apoptosis, Membrane Damage of Azotobacter vinelandii and Nitrosomonas europaea via Generation of Reactive Oxygen Species

Silver nanoparticles (AgNPs) is widely used as an antibacterial agent, but the specific antibacterial mechanism is still conflicting. This study aimed to investigate the size dependent inhibition of AgNPs and the relationship between inhibition and reactive oxygen species (ROS). Azotobactervinelandii and Nitrosomonaseuropaea were exposed to AgNPs with different particles size (10 nm and 50 nm). The ROS production was measured and the results showed that the generation of ROS related to the particle size and concentrations of AgNPs. At 10 mg/L of 10 nm Ag particles, the apoptosis rate of A. vinelandii and N. europaea were 20.23% and 1.87% respectively. Additionally, the necrosis rate of A. vinelandii and N. europaea reached to 15.20% and 42.20% respectively. Furthermore, transmission electron microscopy images also indicated that AgNPs caused severely bacterial cell membrane damage. Together these data suggested that the toxicity of AgNPs depends on its particle size and overproduction of ROS.

In vitro investigation to explore the toxicity of different groups of pesticides for an agronomically important rhizosphere isolate Azotobacter vinelandii

In this work, an attempt was made to evaluate the effect of pesticides on growth pattern, surface morphology, cell viability and growth regulators of nitrogen fixing soil bacterium. Pesticide tolerant Azotobacter vinelandii strain AZ6 (Accession no. MG028654) was found to tolerate maximum level of pesticide and displayed multifarious PGP activities. At higher concentrations, pesticides triggered cellular/structural damage and reduced the cell viability as clearly shown under SEM and CLSM. With increase in concentration, pesticides exhibited a significant (p < 0.05) decrease in PGP traits of strain AZ6. Among all three groups of pesticides, herbicides glyphosate and atrazine were most toxic. Kitazin, hexaconazole, metalaxyl, glyphosate, quizalofop, atrazine, fipronil, monocrotophos and imidacloprid at 2400, 1800, 1500, 900, 1200, 900, 1800, 2100 and 2700 μg mL^-1, respectively, decreased the production of IAA by 19.5 ± 1.9 (61%), 18.1 ± 1.2 (64%), 36.4 ± 3.4 (28%), 13.1 ± 0.8 (74%), 15.6 ± 1.0 (69%), 7.6 ± 0.5 (83%), 11.9 ± 0.8 (76%), 24.7 ± 1.7 (51%) and 32 ± 2.3 (37%) μg mL^-1, respectively, over control (50.7 ± 3.6 μg mL^-1). A maximum reduction of 8.4 ± 1.2 (46%), 5.8 ± 0.6 (62%) and 4 ± 0.2 (74%) μg mL^-1 in 2, 3-DHBA at 300 (1×), 600 (2×) and 900 (3×) μg mL^-1 glyphosate, respectively, While, 32.8 ± 2.7 (19%), 27.2 ± 2 (33%) and 21.5 ± 1.3 (47%) μg mL^-1, respectively in the production of SA was observed at 300 (1×), 600 (2×) and 900 (3×) μg mL^-1 atrazine, respectively. Likewise, with increase in concentration of pesticides, decrease in P solubilization ability and change in pH of broth was detected. The order of pesticide toxicity to PSE (percent decline over control) at highest concentration was: atrazine (45) > kitazin (44) > metalaxyl (43) > monocrotophos (43) > glyphosate (41) > hexaconazole (39) > quizalofop (33) > imidacloprid (31) > fipronil (25). The present study undoubtedly suggests that even at higher doses of pesticides, A. vinelandii maintained secreting plant growth regulators and this property makes this strain agronomically important microbe for enhancing the growth of plants.

Size-dependent cytotoxicity of silver nanoparticles to Azotobacter vinelandii: Growth inhibition, cell injury, oxidative stress and internalization

The influence of nanomaterials on the ecological environment is becoming an increasingly hot research field, and many researchers are exploring the mechanisms of nanomaterial toxicity on microorganisms. Herein, we studied the effect of two different sizes of nanosilver (10 nm and 50 nm) on the soil nitrogen fixation by the model bacteria Azotobacter vinelandii. Smaller size AgNPs correlated with higher toxicity, which was evident from reduced cell numbers. Flow cytometry analysis further confirmed this finding, which was carried out with the same concentration of 10 mg/L for 12 h, the apoptotic rates were20.23% and 3.14% for 10 nm and 50 nm AgNPs, respectively. Structural damage to cells were obvious under scanning electron microscopy. Nitrogenase activity and gene expression assays revealed that AgNPs could inhibit the nitrogen fixation of A. vinelandii. The presence of AgNPs caused intracellular reactive oxygen species (ROS) production and electron spin resonance further demonstrated that AgNPs generated hydroxyl radicals, and that AgNPs could cause oxidative damage to bacteria. A combination of Ag content distribution assays and transmission electron microscopy indicated that AgNPs were internalized in A. vinelandii cells. Overall, this study suggested that the toxicity of AgNPs was size and concentration dependent, and the mechanism of antibacterial effects was determined to involve damage to cell membranes and production of reactive oxygen species leading to enzyme inactivation, gene down-regulation and death by apoptosis.

Effect of organic matter on nitrogenase metal cofactors homeostasis in Azotobacter vinelandii under diazotrophic conditions

Biological nitrogen fixation can be catalysed by three isozymes of nitrogenase: molybdenum (Mo)-nitrogenase, vanadium (V)-nitrogenase and iron-only (Fe)-nitrogenase. The activity of these isozymes strongly depends on their metal cofactors, molybdenum, vanadium and iron, and their bioavailability in ecosystems. Here, we show how metal bioavailability can be affected by the presence of tannic acid (organic matter), and the subsequent consequences on diazotrophic growth of the soil bacterium Azotobacter vinelandii. In the presence of tannic acids, A. vinelandii produces a higher amount of metallophores, which coincides with an active, regulated and concomitant acquisition of molybdenum and vanadium under cellular conditions that are usually considered not molybdenum limiting. The associated nitrogenase genes exhibit decreased nifD expression and increased vnfD expression. Thus, in limiting bioavailable metal conditions, A. vinelandii takes advantage of its nitrogenase diversity to ensure optimal diazotrophic growth.

Planktonic and biofilm-grown nitrogen-cycling bacteria exhibit different susceptibilities to copper nanoparticles

Proper characterization of nanoparticle (NP) interactions with environmentally relevant bacteria under representative conditions is necessary to enable their sustainable manufacture, use, and disposal. Previous nanotoxicology research based on planktonic growth has not adequately explored biofilms, which serve as the predominant mode of bacterial growth in natural and engineered environments. Copper nanoparticle (Cu-NP) impacts on biofilms were compared with respective planktonic cultures of the ammonium-oxidizing Nitrosomonas europaea, nitrogen-fixing Azotobacter vinelandii, and denitrifying Paracoccus denitrificans using a suite of independent toxicity diagnostics. Median inhibitory concentration (IC50) values derived from adenosine triphosphate (ATP) for Cu-NPs were lower in N. europaea biofilms (19.6 ± 15.3 mg/L) than in planktonic cells (49.0 ± 8.0 mg/L). However, in absorbance-based growth assays, compared with unexposed controls, N. europaea growth rates in biofilms were twice as resilient to inhibition than those in planktonic cultures. Similarly, relative to unexposed controls, growth rates and yields of P. denitrificans in biofilms exposed to Cu-NPs were 40-fold to 50-fold less inhibited than those in planktonic cells. Physiological evaluation of ammonium oxidation and nitrate reduction suggested that biofilms were also less inhibited by Cu-NPs than planktonic cells. Furthermore, functional gene expression for ammonium oxidation (amoA) and nitrite reduction (nirK) showed lower inhibition by NPs in biofilms relative to planktonic-grown cells. These results suggest that biofilms mitigate NP impacts, and that nitrogen-cycling bacteria in wastewater, wetlands, and soils might be more resilient to NPs than planktonic-based assessments suggest.

Effects of tungsten and titanium oxide nanoparticles on the diazotrophic growth and metals acquisition by Azotobacter vinelandii under molybdenum limiting condition

The acquisition of essential metals, such as the metal cofactors (molybdenum (Mo) and iron (Fe)) of the nitrogenase, the enzyme responsible for the reduction of dinitrogen (N(2)) to ammonium, is critical to N(2) fixing bacteria in soil. The release of metal nanoparticles (MNPs) to the environment could be detrimental to N(2) fixing bacteria by introducing a new source of toxic metals and by interfering with the acquisition of essential metals such as Mo. Since Mo has been reported to limit nonsymbiotic N(2) fixation in many ecosystems from tropical to cold temperate, this question is particularly acute in the context of Mo limitation. Using a combination of microbiology and analytical chemistry techniques, we have evaluated the effect of titanium (Ti) and tungsten (W) oxide nanoparticles on the diazotrophic growth and metals acquisition in pure culture of the ubiquitous N(2) fixing bacterium Azotobacter vinelandii under Mo replete and Mo limiting conditions. We report that under our conditions (≤10 mg·L(-1)) TiO(2) NPs have no effects on the diazotrophic growth of A. vinelandii while WO(3) NPs are highly detrimental to the growth especially under Mo limiting conditions. Our results show that the toxicity of WO(3) NPs to A. vinelandii is due to an interference with the catechol-metalophores assisted uptake of Mo.

Oxytetracycline interactions at the soil-water interface: effects of environmental surfaces on natural transformation and growth inhibition of Azotobacter vinelandii

The mechanism of oxytetracycline (OTC) adsorption to a silty clay loam soil was investigated using sorption isotherm experiments, Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction spectroscopy (XRD). Sorption data fit well to a cation-exchange capacity sorption model. Spectroscopic data indicate that the interactions between oxytetracycline and silty clay loam soil were primarily through electrostatic interactions between the protonated dimethylamino group of OTC and the negatively charged moieties on the surface of the soil. Based on XRD results, OTC adsorption appeared to inhibit the ethylene glycol solvation of the expandable clay minerals, suggesting that OTC had diffused into the clay interlayer space. The presence of adsorbed OTC did not significantly affect the transformation frequency of the soil bacterium Azotobacter vinelandii with plasmid DNA (soil alone 3 × 10(6)  ± 4 × 10(6) and soil with adsorbed OTC 4 × 10(6)  ± 0.5 × 10(6) ). Growth was inhibited by adsorbed OTC, although a greater mass of adsorbed OTC was required to achieve the same degree of inhibition as the system of dissolved OTC alone. These results suggest that the interactions of tetracyclines at the soil-water interface will affect the growth of sensitive microorganisms in soil microbial communities.

Cofactor binding protects flavodoxin against oxidative stress

In organisms, various protective mechanisms against oxidative damaging of proteins exist. Here, we show that cofactor binding is among these mechanisms, because flavin mononucleotide (FMN) protects Azotobacter vinelandii flavodoxin against hydrogen peroxide-induced oxidation. We identify an oxidation sensitive cysteine residue in a functionally important loop close to the cofactor, i.e., Cys69. Oxidative stress causes dimerization of apoflavodoxin (i.e., flavodoxin without cofactor), and leads to consecutive formation of sulfinate and sulfonate states of Cys69. Use of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) reveals that Cys69 modification to a sulfenic acid is a transient intermediate during oxidation. Dithiothreitol converts sulfenic acid and disulfide into thiols, whereas the sulfinate and sulfonate forms of Cys69 are irreversible with respect to this reagent. A variable fraction of Cys69 in freshly isolated flavodoxin is in the sulfenic acid state, but neither oxidation to sulfinic and sulfonic acid nor formation of intermolecular disulfides is observed under oxidising conditions. Furthermore, flavodoxin does not react appreciably with NBD-Cl. Besides its primary role as redox-active moiety, binding of flavin leads to considerably improved stability against protein unfolding and to strong protection against irreversible oxidation and other covalent thiol modifications. Thus, cofactors can protect proteins against oxidation and modification.

[Influence of natural minerals on growth of Azotobacter vinelandii IMV B-7076]

It was shown that the cultivation of Azotobacter vinelandii IMV B-7076 in the presence of natural minerals significantly increased the growth of bacteria activity. Adding in the medium glauconite and saponite in optimal concentration (10 g/l) conditioned the increase of cell growth six and four times compared with control, accordingly. The maximum stimulating effect was observed when the concentration of phosphates was 5.0 g/l, herewith the number of viable cells increased 5 times. The presence of contact interaction of bacterial cells with investigated particles minerals was shown.

[Influence of titanium dioxide on growth of Azotobacter vinelandii IMV V-7076]

Adding highly dispersed titanium dioxide particles of the culture medium has a stimulatory effect on growth of Azotobacter vinelandii. The maximum effect was observed when using 5 g/l and 10 g/l concentrations of this component. Under these conditions the number of grown bacteria is several times more than in the control variants. It was shown that the stimulatory effect could not be the consequence of saccharose sorption on the surface of particles of the dispersed compound. Possibly, the contact interaction of bacterial cell with titanium dioxide causes an increase of cell wall permeability which leads to an increase of substrates transport to the cell.

Tungsten effects on microbial community structure and activity in a soil

Tungsten, once deposited onto a soil as a result of private, industrial, and military activities, may persist as tungstate anion or, via polymerization, as a variety of poly-tungstate species, each with varying solubility and soil sorption characteristics. In this study, the impact of weathered tungsten on a soil microbial community was measured. Fatty acid analyses indicated that weathered tungsten at < or =2500 mg kg(-1) was associated with a significant increase in microbial biomass and that concentrations up to 6500 mg kg(-1) did not result in a significant decrease in measured biomass, relative to the control. Analysis of cellular fatty acids also identified significant microbial community shifts between 0 and 325, 1300 and 2600, and 3900 and 6500 mg W kg(-1) soil. In general, the positive effect of tungsten on microbial biomass coincided with an increase in Gram-negative bacterial fatty acids, whereas fatty acids indicative of actinomycetes and Gram-positive bacteria were more abundant at the highest soil tungsten concentrations. The weathered tungsten also inhibited N2 fixing activity of a free living diazotroph at > or =1300 mg W kg(-1) soil. These results indicate that tungsten in soil can alter both the structure and the function of an indigenous soil microbial community.

Catechol siderophores control tungsten uptake and toxicity in the nitrogen-fixing bacterium Azotobacter vinelandii

Molybdenum (Mo) and tungsten (W), which have similar chemistry, are present at roughly the same concentration in the earth's continental crust, and both are present in oxic systems as oxoanions, molybdate and tungstate. Molybdenum is a cofactor in the molybdenum-nitrogenase enzyme and is thus an important micronutrient for N2-fixing bacteria such as Azotobacter vinelandii (A. vinelandii). Tungsten is known to be toxic to N2-fixing bacteria, partly by substituting for Mo in nitrogenase. We showthatthe catechol siderophores produced by A. vinelandii, in addition to being essential for iron acquisition, modulate the relative uptake of Mo and W. These catechol siderophores (particularly protochelin), whose concentrations in the growth medium increase sharply at high W, complex all the tungstate along with molybdate and some of the iron. The molybdenum-catechol complex is taken up much more rapidly than the W complex, allowing A. vinelandii to satisfy its Mo requirement and avoid W toxicity. Mutants deficient in the production of catechol siderophores are more sensitive to tungstate and have higher cellular W quotas than the wild type. The binding of metals by excreted catechol siderophores allows A. vinelandii to discriminate in its uptake of essential metals, such as Fe and Mo, over that of toxic metals, such as W, and to sustain high growth rates under adverse environmental conditions.

Utilization of the metal-cyano complex tetracyanonickelate (II) by Azotobacter vinelandii

AIMS

The ability of Azotobacter vinelandii, a N(2)-fixing bacterium, to biodegrade tetracyanonickelate (TCN) was evaluated.

METHODS AND RESULTS

The amounts of TCN were measured spectrophotometrically. Ammonia was determined colorimetrically by the indophenol method. The produced methane from TCN conversion by A. vinelandii was detected by gas chromatography. Results showed that A. vinelandii was able to biodegrade 1 mmol l(-1) of TCN. Ammonia and methane were detected during the process of TCN degradation. Effects of exogenous nitrogen sources on TCN degradation were addressed in this study. Results revealed that the addition of ammonia (1, 5 and 10 mmol l(-1)) into the reaction mixtures caused decrease of TCN degradation rate during a 24-h incubation period. This inhibition was also observed when nitrite (5 and 10 mmol l(-1)) was added, whereas TCN degradation still proceeded after the addition of nitrate at the same concentrations. Furthermore, the rate of TCN utilization was strikingly enhanced when 0.8% of glucose was added.

CONCLUSIONS

Azotobacter vinelandii can degrade 1 mmol l(-1) of TCN into ammonia and methane. However, the inhibitory effects of exogenous ammonia and nitrite on TCN degradation by this bacterium were found in this study.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report defining the capability of A. vinelandii to degrade TCN. This bacterium might have potential value in applied strategies for removing metal-cyano wastes. Furthermore, these findings would be helpful in designing a practical system inoculated with A. vinelandii for the treatment of TCN.

Properties of polysaccharide produced by Azotobacter vinelandii cultured on 4-hydroxybenzoic acid

AIMS

Characterization of the exopolysaccharide produced by Azotobacter vinelandii grown on 4-hydroxybenzoic acid (EPS I), and the comparison between this exopolysaccharide and commercial alginate, constituted the main objective of this work.

METHODS AND RESULTS

Total carbohydrates, uronic acids, acetyl and pyruvyl groups and proteins were determined by colorimetric methods and composition was confirmed by Nuclear Magnetic Resonance studies. Rheological properties were analysed under different physical and chemical conditions. Results showed differences between EPS I and commercial alginate, in relation to both composition and viscosity. Higher amount of guluronnosyl residues were found in EPS I, whereas commercial alginate contained the same proportion of mannuronosyl and guluronnosyl residues. In accordance with this result, EPS I gave rise to solutions of higher viscosity than commercial alginate, although solutions of this polysaccharide showed greater stability when conditions were altered.

CONCLUSIONS

The exopolysaccharide produced by A. vinelandii grown on 4-hydroxybenzoic acid showed a different composition in comparison with commercial alginate, which leads to higher viscosity values for the aqueous solutions of EPS I.

SIGNIFICANCE AND IMPACT OF STUDY

This work describes for the first time the characteristics of an exopolysaccharide produced by A. vinelandii from 4-hydroxybenzoic acid, a substrate rarely used as sole carbon source.

Signal transduction to the Azotobacter vinelandii NIFL-NIFA regulatory system is influenced directly by interaction with 2-oxoglutarate and the PII regulatory protein

PII-like signal transduction proteins, which respond to the nitrogen status via covalent modification and signal the carbon status through the binding of 2-oxoglutarate, have been implicated in the regulation of nitrogen fixation in several diazotrophs. The NIFL-NIFA two-component regulatory system, which integrates metabolic signals to fine-tune regulation of nitrogenase synthesis in Azotobacter vinelandii, is a potential target for PII-mediated signal transduction. Here we demonstrate that the inhibitory activity of the A.vinelandii NIFL protein is stimulated by interaction with the non-uridylylated form of PII-like regulatory proteins. We also observe that the NIFL-NIFA system is directly responsive to 2-oxoglutarate. We propose that the PII protein signals the nitrogen status by interaction with the NIFL-NIFA system under conditions of nitrogen excess, and that the inhibitory activity of NIFL is relieved by elevated levels of 2-oxoglutarate when PII is uridylylated under conditions of nitrogen limitation. Our observations suggest a model for signal transduction to the NIFL-NIFA system in response to carbon and nitrogen status which is clearly distinct from that suggested from studies on other diazotrophs.

Effect of redox mediators on nitrogenase and hydrogenase activities in Azotobacter vinelandii

In bioelectrochemical studies, redox mediators such as methylene blue, natural red, and thionine are used to studying the redox characteristics of enzymes in the living cell. Here we show that nitrogenase activity in Azotobacter vinelandii is completely inhibited by oxidized methylene blue (MBo) when the concentration of this mediator in the medium is increased up to 72 microM. This activity in A. vinelandii is somewhat inhibited by a coenzyme, ascorbic acid (AA). However, the nitrogenase activity within the A. vinelandii cell is unchanged even for a high concentration of oxidized natural red (NRo) alone. Interestingly, these mediators and AA do not have the capacity to inhibit the H2 uptake activity of the hydrogenase in A. vinelandii. Average active rates of 66 nM H2 evolved/mg cell protein/min from the nitrogenase and 160 nM H2-uptake/mg cell protein/min from the hydrogenase in A. vinelandii are found in aid of the activities of the enzymes for H2 evolution and for H2 uptake are compared. The activities of both enzymes in A. vinelandii are strongly inhibited by thionine having high oxidative potential. Mechanisms of various mediators acting in vivo for both enzymes in A. vinelandii are discussed.

Dual regulation of catecholate siderophore biosynthesis in Azotobacter vinelandii by iron and oxidative stress

Azotobacter vinelandii forms both catecholate and azotobactin siderophores during iron-limited growth. Azotobactin is repressed by about 3 microM iron, but catecholate siderophore synthesis continues up to a maximum of 10 microM iron. This suggests that catecholate siderophore synthesis is regulated by other factors in addition to the ferric uptake repressor (Fur). In this study the first gene required for catecholate siderophore biosynthesis, which encodes an isochorismate synthase (csbC), was isolated. The region upstream of csbC contained a typical sigma(70) promoter, with an iron-box overlapping the -35 sequence and a Sox-box (Box 1) overlapping the -10 sequence. Another Sox-box was found further upstream of the -35 sequence (Box 2). Also upstream, an unidentified gene (orfA) was detected which would be transcribed from a divergent promoter, also controlled by an iron-box. The activity of csbC and a csbC::luxAB fusion was negatively regulated by iron availability and upregulated by increased aeration and by superoxide stress. The iron-box in the csbC promoter was 74% identical to the Fur-binding consensus sequence and bound the Fur protein of Escherichia coli with relatively high affinity. Both Box 1 and Box 2 were in good agreement with the consensus sequence for binding the SoxS protein of E. coli and Box 1 was in very good agreement with the Sox-box found in the fpr promoter of A. vinelandii, which is also regulated by superoxide stress. Both Sox-boxes bound a protein found in A. vinelandii cell extracts, with Box 1 exhibiting the higher binding affinity. The Sox protein identified in this assay appeared to be constitutive, rather than inducible by superoxide stress. This indicates that the Sox response in A. vinelandii is different from that in E. coli. These data support the hypothesis that catecholate siderophore biosynthesis is under dual control, repressed by a Fur-iron complex and activated by another DNA-binding protein in response to superoxide stress. The interaction between these regulators is likely to account for the delay in ferric repression of catecholate siderophore production, since these siderophores have an additional role to play in the protection of iron-limited cells against oxidative damage.

Regulated expression of the nifM of Azotobacter vinelandii in response to molybdenum and vanadium supplements in Burk's nitrogen-free growth medium

Azotobacter is a diazotrophic bacterium that harbors three genetically distinct nitrogenases referred to as nif, vnf, and anf systems. The nifM is an accessory gene located in the nif gene cluster and is transcriptionally regulated by the NifA. However, Azotobacter mutants that lack NifA are known to synthesize functional NifM and this accessory protein is known to be needed for the activity of nitrogenase-2 and nitrogenase-3. To determine how the transcription of nifM is regulated when Azotobacter is grown under conditions in which nitrogenase-2 or nitrogenase-3 is expressed, we generated an Azotobacter vinelandii strain that carries a nifM:lacZ-kanamycin resistance gene cassette in its chromosome. In this strain the nifM open reading frame was disrupted by the presence of a lacZ-kanamycin resistance gene cassette so that it could not produce active NifM. Moreover, the lacZ gene was placed under the transcriptional control elements of the nifM gene so that the lacZ expression could be used as a marker to determine the extent of expression of the nifM gene under different growth conditions. Our results show that this strain was unable to grow in Burk's nitrogen-free medium supplemented with either molybdenum or vanadium or lacking both metals suggesting that in the absence of functional NifM none of the nitrogenases were active. It was also found that the nifM expression was differentially regulated when the A. vinelandii cells were grown under conditions that activate nitrogenase-2 and nitrogenase-3, as determined by liquid beta-galactosidase activity measurements. These results suggest that the transcriptional activators, VnfA and AnfA, may regulate the nifM expression.

Mutagenesis studies of the FeSII protein of Azotobacter vinelandii: roles of histidine and lysine residues in the protection of nitrogenase from oxygen damage

The Azotobacter FeSII protein, also known as the Shethna protein, forms a protective complex with nitrogenase during periods when nitrogenase is exposed to oxygen. One possible mechanism for its action is an oxidation state-dependent conformational interaction with nitrogenase whereby the FeSII protein dissociates from the MoFe and Fe proteins of nitrogenase under reducing conditions. Herein we report the construction and characterization of five site-directed mutants of the FeSII protein (H12Q, H55Q, K14A, K15A, and the double mutant K14A/K15A) which were individually purified after being individually overexpressed in Escherichia coli. These mutant FeSII proteins maintain native-like assembly and orientation of the 2Fe-2S center on the basis of EPR and NMR spectroscopic characterization and their redox midpoint potentials, which are within 25 mV of that of the wild type protein. The abilities of the individual mutant proteins to protect nitrogenase were assessed by determining the remaining nitrogenase activities after adding each pure version back to extracts from an FeSII deletion strain, and then exposing the mixture to oxygen. In these assays, the H12Q mutant functioned as well as the wild type protein. However, mutation of His55, a few residues away from a cluster-liganding cysteine, results in much less efficient protection of nitrogenase. These results are consistent with pH titrations in both oxidation states, which show that His12 is insensitive to 2Fe-2S cluster oxidation state. His55's pK is weakly responsive to oxidation state, and the pK increase of 0. 16 pH unit upon 2Fe-2S cluster oxidation is indicative of ionization of another group between His55 and the 2Fe-2S cluster, which could modulate the FeSII protein's affinity for nitrogenase in a redox state-dependent manner. Both K14A and K15A mutant FeSII proteins partially lost their ability to protect nitrogenase, but the lysine double mutant lost almost all its protective ability. The nitrogenase component proteins in an Azotobacter strain bearing the double lysine mutation (in the chromosome) were degraded much more rapidly in vivo than those in the wild type strain under carbon substrate-limited conditions. These results indicate that the two lysines may have an important role in FeSII function, perhaps in the initial steps of recognizing the nitrogenase component proteins.

Regulation of the transcriptional activators AnfA and VnfA by metals and ammonium in Azotobacter vinelandii

Transcription of the genes encoding molybdenum (Mo)-independent nitrogenases 2 and 3 of Azotobacter vinelandii requires the activators VnfA and AnfA, respectively. The effect of NH4+, Mo, or V (vanadium) was tested on the expression of vnfA-lacZ and anfA-lacZ transcriptional fusions. Mo repressed expression of both fusions whereas NH4+ and V repressed the anfA-lacZ fusion, but not the vnfA-lacZ fusion. Thus the repressive effect on transcription of the anfHDGKOR operon by NH4+, Mo, or V is mediated through their effect on transcription of anfA and the repressive effect of Mo on the vnfHFd and vnfDGK operons is mediated through Mo repression of vnfA transcription. Mo-dependent repression of anfA transcription is influenced but not entirely mediated by the Mo-responsive regulator ModE.

Ammonium repression of the nitrite-nitrate (nasAB) assimilatory operon of Azotobacter vinelandii is enhanced in mutants expressing the nifO gene at high levels

A number of Tn5 mutants were isolated which were unable to fix nitrogen and showed enhanced ammonium repression of the nitrate/nitrite assimilation genes. They also had reduced nitrate reductase activity under fully inducing conditions. Insertions were localized within the nifB gene, and inability to fix nitrogen was shown to be due to disruption of the nifB gene. However, enhanced ammonium repression proved to be the result of constitutive expression of the downstream nifO gene from an 'out' promoter present in Tn5. Our results suggest that molybdenum metabolism might function as a regulatory factor that acts through the nitrate reductase.

Phenotypic characterization of a tungsten-tolerant mutant of Azotobacter vinelandii

A tungsten-tolerant mutant strain (CA6) of Azotobacter vinelandii first described in 1980 (P. E. Bishop, D. M. L. Jarlenski, and D. R. Hetherington, Proc. Natl. Acad. Sci. USA 77:7342-7346, 1980) has been further characterized. Results from growth experiments suggest that both nitrogenases 1 and 3 are utilized when CA6 grows in N-free medium containing Na2MoO4. Strain CA6.1.71, which lacks both nitrogenases 2 and 3, grew as well as strain CA in N-free medium containing Na2MoO4 after an initial lag. This indicates that nitrogenase 1 is fully functional in strain CA6. nifH-lacZ and anfH-lacZ transcriptional fusions were expressed in CA6 in the presence of Na2MoO4. Thus, in contrast to wild-type strain CA, transcription of the anfHDGK gene cluster in strain CA6 is not repressed by Mo. Expression of the vnfD-lacZ fusion was the same in both strains CA and CA6. In agreement with the results obtained with lac fusions, subunits of both nitrogenases 1 and 3 were found in protein extracts of CA6 cells grown in N-free medium containing Na2MoO4. However, CA6 cells, cultured in the presence of Na2WO4, accumulated nitrogenase 3 proteins without detectable amounts of nitrogenase 1 proteins. This indicates that expression of Mo-independent nitrogenase 3 is the basis for the tungsten tolerance phenotype of strain CA6. A measure of Mo accumulation as a function of time showed that accumulation by strain CA6 was slower than that for strain CA. When Mo accumulation was studied as a function of Na2MoO4 concentration, the two strains accumulated similar amounts of Mo in the concentration range of 0 to 1 microM Na2MoO4 during a 2-h period. Within the range of 1 to 5 microM Na2MoO4, Mo accumulation by strain CA increased linearly with increasing concentration whereas no further increases were observed for strain CA6. These results are consistent with the possibility that the tungsten tolerance mutation carried by CA6 is in a Mo transport system.

Mo-independent nitrogenase 3 is advantageous for diazotrophic growth of Azotobacter vinelandii on solid medium containing molybdenum

Competition experiments between wild-type Azotobacter vinelandii and a mutant lacking Mo-independent nitrogenase 3 indicate that nitrogenase 3 provides an advantage during diazotrophic growth on agar media containing 100 to 500 nM Na2MoO4 but not in liquid media under the same conditions. Expression of nitrogenase 3 in wild-type cells growing on agar surfaces was verified with an anfH-lacZ fusion and by detection of nitrogenase 3 subunits. These results show that nitrogenase 3 is important for diazotrophic growth on agar medium at molybdenum concentrations that are not limiting for Mo-dependent diazotrophic growth in liquid medium.

Changes of ploidy during the Azotobacter vinelandii growth cycle

The size of the Azotobacter vinelandii chromosome is approximately 4,700 kb, as calculated by pulsed-field electrophoretic separation of fragments digested with the rarely cutting endonucleases SpeI and SwaI. Surveys of DNA content per cell by flow cytometry indicated the existence of ploidy changes during the A. vinelandii growth cycle in rich medium. Early-exponential-phase cells have a ploidy level similar to that of Escherichia coli or Salmonella typhimurium (probably ca. four chromosomes per cell), but a continuous increase of DNA content per cell is observed during growth. Late-exponential-phase cells may contain > 40 chromosomes per cell, while cells in the early stationary stage may contain > 80 chromosomes per cell. In late-stationary-phase cultures, the DNA content per cell is even higher, probably over 100 chromosome equivalents per cell. A dramatic change is observed in old stationary-phase cultures, when the population of highly polyploid bacteria segregates cells with low ploidy. The DNA content of the latter cells resembles that of cysts, suggesting that the process may reflect the onset of cyst differentiation. Cells with low ploidy are also formed when old stationary-phase cultures are diluted into fresh medium. Addition of rifampin to exponential-phase cultures causes a rapid increase in DNA content, indicating that A. vinelandii initiates multiple rounds of chromosome replication per cell division. Growth in minimal medium does not result in the spectacular changes of ploidy observed during rapid growth; this observation suggests that the polyploidy of A. vinelandii may not exist outside the laboratory.

Mutagenesis of a gene encoding a cytochrome o-like terminal oxidase of Azotobacter vinelandii: a cytochrome o mutant is aero-tolerant during nitrogen fixation

The amino acid sequence obtained by translating the nucleotide sequence of a 0.55 kb fragment, amplified from Azotobacter vinelandii chromosomal DNA by PCR, was 57% identical to part of the Escherichia coli cyoB gene, encoding subunit I of the cytochrome bo-type quinol oxidase. This fragment was mutated in vitro by insertion of a kanamycin-resistance cassette and introduced into the chromosome of A. vinelandii by homologous recombination. The mutant contained no spectrally detectable cytochrome o. However, in the stationary phase of growth, the level of the alternative oxidase (cytochrome bd) was 11-fold higher than in the wild-type strain. Respiration of the mutant was insensitive to chlorpromazine, an inhibitor thought to act specifically on cytochrome o. Cytochrome o-deficient mutants fixed nitrogen in air, clearly distinguishing the role of this oxidase from that of cytochrome bd, which is required for respiratory protection of oxygen-labile nitrogenase.

In vivo and in vitro nickel-dependent processing of the [NiFe] hydrogenase in Azotobacter vinelandii

H2 oxidation in Azotobacter vinelandii is catalyzed by a membrane-bound, alpha beta dimeric [NiFe] hydrogenase. Maturation of the enzyme involves cleavage of a putative N-terminal signal sequence in the beta subunit and removal of 15 amino acids from the C terminus of the alpha subunit. Cells limited for nickel exhibited low hydrogenase activities and contained an apparently large form of the alpha subunit. Addition of nickel to such cells increased hydrogenase activities fivefold over 2 h. The increase in the first hour did not require transcription and translation and correlated with processing of the large form of the alpha subunit (pre-alpha) to the small form (alpha) resembling the alpha subunit from the purified enzyme. In vivo, pre-alpha appeared soluble whereas the majority of alpha was membrane bound. Processing of pre-alpha to alpha was reproduced in vitro in membrane-depleted extracts of nickel-limited cells. Processing specifically required the addition of Ni2+, whereas Co2+, Cu2+, Ca2+, Fe2+, Mn2+, and Zn2+ were ineffective. However, Zn2+, Co2+, and Cu2+ inhibited nickel-dependent processing. Mg-ATP and Mg-GTP stimulated processing, whereas anaerobic conditions and/or the addition of dithiothreitol and sodium dithionite was unnecessary. Processing was not inhibited by the protease inhibitors phenylmethylsulfonyl fluoride, E64, and pepstatin.

Characterization of transcripts expressed from nitrogenase-3 structural genes of Azotobacter vinelandii

Five major anfH-hybridizing mRNA species accumulated in Azobacter vinelandii cells derepressed for nitrogenase-3 (an alternative nitrogenase, which appears to lack Mo and V). Using anfH-, anfD-, anfG-, anfK-, and orflorf2-specific probes and mutant strains of A. vinelandii these mRNA species have been identified as encoding anfHDGKorflorf2 (6.0 kb), anfHDGK (4.3 kb), anfHD (2.6 kb), vnfHorfFd (1.3 kb), and vnfH and (or) anfH (1.0 kb). A 0.6-kb mRNA species, which hybridized only to the orflorf2-specific probe, and a 3.5-kb mRNA species, which hybridized to anfD or anfK, also accumulated under these conditions. Northern blot analysis and S1 nuclease mapping indicate that transcription of the anf structural gene cluster initiates at a unique nif consensus promoter situated 127 base pairs upstream from the anfH coding region. Observation of anfH-hybridizing mRNA species that accumulate in strains derepressed for nitrogen fixation demonstrates that transcription of the anfHDGKorflorf2 cluster is normally repressed by Mo, V, and NH4+, whereas transcription of the vnfHorfFd cluster does not require the presence of V and is repressed only by Mo, but not NH4+. Analysis of the accumulation of mRNAs in a tungsten-tolerant strain revealed that Mo and V repression of anf transcription must occur by different mechanisms.

Regulation of nitrogenase-2 in Azotobacter vinelandii by ammonium, molybdenum, and vanadium

Under diazotrophic conditions in the absence of molybdenum and in the presence of vanadium, Azotobacter vinelandii reduces N2 to NH4+ by using nitrogenase-2, a V-containing enzyme complex encoded by vnfH (the gene for dinitrogenase reductase-2), and vnfDGK (the genes for dinitrogenase-2 subunits). Accumulation of the vnfHorfFd and vnfDGK transcripts occurred under Mo-deficient conditions in the presence and absence of V; however, in the case of vnfDGK, the protein products only accumulated in the presence of V. This suggests that V is required for translation of the vnfDGK transcripts. In addition, expression of vnfH-lacZ and vnfD-lacZ transcriptional fusions was only partially repressed in the presence of NH4+. Transcripts hybridizing with vnfH (1.4 and 1.0 kb), vnfDG (3.4 and 1.8 kb), and vnfK (3.4 kb) were detected in RNA extracted from wild-type cells cultured with NH4+ in the presence or absence of V. However, nitrogenase-2 subunits were not detected in extracts of cells derepressed for nitrogenase-2 in the presence of NH4+. These results indicate that this nitrogen source acts at the posttranscriptional level as well as at the transcriptional level. vnf transcripts were not detected in the presence of Mo (with or without NH4+).

Acetylene inhibition of Azotobacter vinelandii hydrogenase: acetylene binds tightly to the large subunit

Acetylene is a slow-binding inhibitor of the Ni- and Fe-containing dimeric hydrogenase isolated from Azotobacter vinelandii. Acetylene was released from hydrogenase during the recovery from inhibition. This indicates that no transformation of acetylene to another compound occurred as a result of the interaction with hydrogenase. However, the release of C2H2 proceeds more rapidly than the recovery of activity, which indicates that release of C2H2 is not sufficient for recovery of activity. Acetylene binds tightly to native hydrogenase; hydrogenase and radioactivity coelute from a gel permeation column following inhibition with 14C2H2. Acetylene, or a derivative, remains bound to the large 65,000 MW subunit (and not to the small 35,000 MW subunit) of hydrogenase following denaturation as evidenced by SDS-PAGE and fluorography of 14C2H2-inhibited hydrogenase. This result suggests that C2H2, and by analogy H2, binds to and is activated by the large subunit of this dimeric hydrogenase. Radioactivity is lost from 14C2H2-inhibited protein during recovery. The inhibition is remarkably specific for C2H2: propyne, butyne, and ethylene are not inhibitors.