Disparate response to microoxia and nitrogen oxides of the Bradyrhizobium japonicum napEDABC, nirK and norCBQD denitrification genes

Enrichment and characterization of a nitric oxide-reducing microbial community in a continuous bioreactor

Nitric oxide (NO) is a highly reactive and climate-active molecule and a key intermediate in the microbial nitrogen cycle. Despite its role in the evolution of denitrification and aerobic respiration, high redox potential and capacity to sustain microbial growth, our understanding of NO-reducing microorganisms remains limited due to the absence of NO-reducing microbial cultures obtained directly from the environment using NO as a substrate. Here, using a continuous bioreactor and a constant supply of NO as the sole electron acceptor, we enriched and characterized a microbial community dominated by two previously unknown microorganisms that grow at nanomolar NO concentrations and survive high amounts (>6 µM) of this toxic gas, reducing it to N2 with little to non-detectable production of the greenhouse gas nitrous oxide. These results provide insight into the physiology of NO-reducing microorganisms, which have pivotal roles in the control of climate-active gases, waste removal, and evolution of nitrate and oxygen respiration.

The copper-responsive regulator CsoR is indirectly involved in Bradyrhizobium diazoefficiens denitrification

The soybean endosymbiont Bradyrhizobium diazoefficiens harbours the complete denitrification pathway that is catalysed by a periplasmic nitrate reductase (Nap), a copper (Cu)-containing nitrite reductase (NirK), a c-type nitric oxide reductase (cNor), and a nitrous oxide reductase (Nos), encoded by the napEDABC, nirK, norCBQD, and nosRZDFYLX genes, respectively. Induction of denitrification genes requires low oxygen and nitric oxide, both signals integrated into a complex regulatory network comprised by two interconnected cascades, FixLJ-FixK2-NnrR and RegSR-NifA. Copper is a cofactor of NirK and Nos, but it has also a role in denitrification gene expression and protein synthesis. In fact, Cu limitation triggers a substantial down-regulation of nirK, norCBQD, and nosRZDFYLX gene expression under denitrifying conditions. Bradyrhizobium diazoefficiens genome possesses a gene predicted to encode a Cu-responsive repressor of the CsoR family, which is located adjacent to copA, a gene encoding a putative Cu+-ATPase transporter. To investigate the role of CsoR in the control of denitrification gene expression in response to Cu, a csoR deletion mutant was constructed in this work. Mutation of csoR did not affect the capacity of B. diazoefficiens to grow under denitrifying conditions. However, by using qRT-PCR analyses, we showed that nirK and norCBQD expression was much lower in the csoR mutant compared to wild-type levels under Cu-limiting denitrifying conditions. On the contrary, copA expression was significantly increased in the csoR mutant. The results obtained suggest that CsoR acts as a repressor of copA. Under Cu limitation, CsoR has also an indirect role in the expression of nirK and norCBQD genes.

Integrated treatment of suburb diffuse pollution using large-scale multistage constructed wetlands based on novel solid carbon: Nutrients removal and microbial interactions

In this study, an integrated treatment system was proposed and applied in situ, including detention tank, multistage constructed wetlands (CWs) and wastewater treatment plants (WWTPs), preventing nutrients flowing into Dianchi Lake, in which the treatment performance of multistage CWs were evaluated principally. Results skillfully realized the bypass purification of upstream river at dry reasons, as well as the effective management and treatment of the collected diffuse pollution at rainy reasons. The purified water flowing into water bodies could satisfy the Grade III of environmental quality standards for surface water in China with the average effluent concentrations of COD, NH₄⁺-N, TN and TP decreased to 10 (51.2-72.7%), 0.5 (67.2-83.0%), 1.0 (71.2-79.6%) and 0.15 (72.3-89.4%) mg L^-1, respectively. High-throughput sequencing results indicated that the application of poly-3-hydroxybutyrate-cohyroxyvelate-sawdust (PS) blends could enrich norank_f_Anaerolineaceae (7.95%) and Bradyrhizobium (10.2%), which were distinct from the dominant genera of Pleurocapsa (13.0%) in gravel-based CWs. Functional genes and metabolism analysis uncovered that the heterotrophic denitrification was the main pathway of nitrogen removal with the abundance of genes encoding TCA cycle, glycolysis and denitrification process up-regulated. In addition, molecular ecological network (MEN) analysis suggested the denitrification genes were positively correlated with the predominant microbes in PS-based CWs, favorable for denitrifiers to transfer and utilize electron donors during denitrification process. This study proved that the developed PS blends as carbon supplies in CWs and the proposed integrated treatment system are effective methods for watershed management, providing valuable reference to low-pollution wastewater treatment in practical engineering projects.

Fine-Tuning Modulation of Oxidation-Mediated Posttranslational Control of Bradyrhizobium diazoefficiens FixK2 Transcription Factor

FixK2 is a CRP/FNR-type transcription factor that plays a central role in a sophisticated regulatory network for the anoxic, microoxic and symbiotic lifestyles of the soybean endosymbiont Bradyrhizobium diazoefficiens. Aside from the balanced expression of the fixK2 gene under microoxic conditions (induced by the two-component regulatory system FixLJ and negatively auto-repressed), FixK2 activity is posttranslationally controlled by proteolysis, and by the oxidation of a singular cysteine residue (C183) near its DNA-binding domain. To simulate the permanent oxidation of FixK2, we replaced C183 for aspartic acid. Purified C183D FixK2 protein showed both low DNA binding and in vitro transcriptional activation from the promoter of the fixNOQP operon, required for respiration under symbiosis. However, in a B. diazoefficiens strain coding for C183D FixK2, expression of a fixNOQP'-'lacZ fusion was similar to that in the wild type, when both strains were grown microoxically. The C183D FixK2 encoding strain also showed a wild-type phenotype in symbiosis with soybeans, and increased fixK2 gene expression levels and FixK2 protein abundance in cells. These two latter observations, together with the global transcriptional profile of the microoxically cultured C183D FixK2 encoding strain, suggest the existence of a finely tuned regulatory strategy to counterbalance the oxidation-mediated inactivation of FixK2 in vivo.

Effect of Copper on Expression of Functional Genes and Proteins Associated with Bradyrhizobium diazoefficiens Denitrification

Nitrous oxide (N₂O) is a powerful greenhouse gas that contributes to climate change. Denitrification is one of the largest sources of N₂O in soils. The soybean endosymbiont Bradyrhizobium diazoefficiens is a model for rhizobial denitrification studies since, in addition to fixing N₂, it has the ability to grow anaerobically under free-living conditions by reducing nitrate from the medium through the complete denitrification pathway. This bacterium contains a periplasmic nitrate reductase (Nap), a copper (Cu)-containing nitrite reductase (NirK), a c-type nitric oxide reductase (cNor), and a Cu-dependent nitrous oxide reductase (Nos) encoded by the napEDABC, nirK, norCBQD and nosRZDFYLX genes, respectively. In this work, an integrated study of the role of Cu in B. diazoefficiens denitrification has been performed. A notable reduction in nirK, nor, and nos gene expression observed under Cu limitation was correlated with a significant decrease in NirK, NorC and NosZ protein levels and activities. Meanwhile, nap expression was not affected by Cu, but a remarkable depletion in Nap activity was found, presumably due to an inhibitory effect of nitrite accumulated under Cu-limiting conditions. Interestingly, a post-transcriptional regulation by increasing Nap and NirK activities, as well as NorC and NosZ protein levels, was observed in response to high Cu. Our results demonstrate, for the first time, the role of Cu in transcriptional and post-transcriptional control of B. diazoefficiens denitrification. Thus, this study will contribute by proposing useful strategies for reducing N₂O emissions from agricultural soils.

Regulation of the Emissions of the Greenhouse Gas Nitrous Oxide by the Soybean Endosymbiont Bradyrhizobium diazoefficiens

The greenhouse gas nitrous oxide (N2O) has strong potential to drive climate change. Soils are a major source of N2O, with microbial nitrification and denitrification being the primary processes involved in such emissions. The soybean endosymbiont Bradyrhizobium diazoefficiens is a model microorganism to study denitrification, a process that depends on a set of reductases, encoded by the napEDABC, nirK, norCBQD, and nosRZDYFLX genes, which sequentially reduce nitrate (NO3-) to nitrite (NO2-), nitric oxide (NO), N2O, and dinitrogen (N2). In this bacterium, the regulatory network and environmental cues governing the expression of denitrification genes rely on the FixK2 and NnrR transcriptional regulators. To understand the role of FixK2 and NnrR proteins in N2O turnover, we monitored real-time kinetics of NO3-, NO2-, NO, N2O, N2, and oxygen (O2) in a fixK2 and nnrR mutant using a robotized incubation system. We confirmed that FixK2 and NnrR are regulatory determinants essential for NO3- respiration and N2O reduction. Furthermore, we demonstrated that N2O reduction by B. diazoefficiens is independent of canonical inducers of denitrification, such as the nitrogen oxide NO3-, and it is negatively affected by acidic and alkaline conditions. These findings advance the understanding of how specific environmental conditions and two single regulators modulate N2O turnover in B. diazoefficiens.

Dissection of FixK2 protein-DNA interaction unveils new insights into Bradyrhizobium diazoefficiens lifestyles control

The FixK₂ protein plays a pivotal role in a complex regulatory network, which controls genes for microoxic, denitrifying, and symbiotic nitrogen-fixing lifestyles in Bradyrhizobium diazoefficiens. Among the microoxic-responsive FixK₂ -activated genes are the fixNOQP operon, indispensable for respiration in symbiosis, and the nnrR regulatory gene needed for the nitric-oxide dependent induction of the norCBQD genes encoding the denitrifying nitric oxide reductase. FixK₂ is a CRP/FNR-type transcription factor, which recognizes a 14 bp-palindrome (FixK₂ box) at the regulated promoters through three residues (L195, E196, and R200) within a C-terminal helix-turn-helix motif. Here, we mapped the determinants for discriminatory FixK₂ -mediated regulation. While R200 was essential for DNA binding and activity of FixK₂ , L195 was involved in protein-DNA complex stability. Mutation at positions 1, 3, or 11 in the genuine FixK₂ box at the fixNOQP promoter impaired transcription activation by FixK₂ , which was residual when a second mutation affecting the box palindromy was introduced. The substitution of nucleotide 11 within the NnrR box at the norCBQD promoter allowed FixK₂ -mediated activation in response to microoxia. Thus, position 11 within the FixK₂ /NnrR boxes constitutes a key element that changes FixK₂ targets specificity, and consequently, it might modulate B. diazoefficiens lifestyle as nitrogen fixer or as denitrifier.

Bacterial nitric oxide metabolism: Recent insights in rhizobia

Nitric oxide (NO) is a reactive gaseous molecule that has several functions in biological systems depending on its concentration. At low concentrations, NO acts as a signaling molecule, while at high concentrations, it becomes very toxic due to its ability to react with multiple cellular targets. Soil bacteria, commonly known as rhizobia, have the capacity to establish a N₂-fixing symbiosis with legumes inducing the formation of nodules in their roots. Several reports have shown NO production in the nodules where this gas acts either as a signaling molecule which regulates gene expression, or as a potent inhibitor of nitrogenase and other plant and bacteria enzymes. A better understanding of the sinks and sources of NO in rhizobia is essential to protect symbiotic nitrogen fixation from nitrosative stress. In nodules, both the plant and the microsymbiont contribute to the production of NO. From the bacterial perspective, the main source of NO reported in rhizobia is the denitrification pathway that varies significantly depending on the species. In addition to denitrification, nitrate assimilation is emerging as a new source of NO in rhizobia. To control NO accumulation in the nodules, in addition to plant haemoglobins, bacteroids also contribute to NO detoxification through the expression of a NorBC-type nitric oxide reductase as well as rhizobial haemoglobins. In the present review, updated knowledge about the NO metabolism in legume-associated endosymbiotic bacteria is summarized.

Dual Control of Flagellar Synthesis and Exopolysaccharide Production by FlbD-FliX Class II Regulatory Proteins in Bradyrhizobium diazoefficiens

Bradyrhizobium diazoefficiens, the N₂-fixing symbiont of soybean, has two independent flagellar systems: a single subpolar flagellum and several lateral flagella. Each flagellum is a very complex organelle composed of 30 to 40 different proteins located inside and outside the cell whereby flagellar gene expression must be tightly controlled. Such control is achieved by a hierarchy of regulators that ensure the timing of synthesis and the allocation of the different flagellar substructures. Previously, we analyzed the gene organization, expression, and function of the lateral flagellar system. Here, we studied the role of the response regulator FlbD and its trans-acting regulator FliX in the regulation of subpolar flagellar genes. We found that the LP-ring, distal rod, and hook of the subpolar flagellum were tightly controlled by FlbD and FliX. Furthermore, we obtained evidence for the existence of cross-regulation between these gene products and the expression of LafR, the master regulator of lateral flagella. In addition, we observed that extracellular polysaccharide production and biofilm formation also responded to these flagellar regulators. In this regard, FlbD might contribute to the switch between the planktonic and sessile states.IMPORTANCE Most environmental bacteria switch between two free-living states: planktonic, in which individual cells swim propelled by flagella, and sessile, in which bacteria form biofilms. Apart from being essential for locomotion, the flagellum has accessory functions during biofilm formation. The synthesis of flagella is a highly regulated process, and coordination with accessory functions requires the interconnection of various regulatory networks. Here, we show the role of class II regulators involved in the synthesis of the B. diazoefficiens subpolar flagellum and their possible participation in cross-regulation with the lateral flagellar system and exopolysaccharide production. These findings highlight the coordination of the synthetic processes of external structures, such as subpolar and lateral flagella, with exopolysaccharides, which are the main component of the biofilm matrix.

Rhizobia: highways to NO

The interaction between rhizobia and their legume host plants conduces to the formation of specialized root organs called nodules where rhizobia differentiate into bacteroids which fix atmospheric nitrogen to the benefit of the plant. This beneficial symbiosis is of importance in the context of sustainable agriculture as legumes do not require the addition of nitrogen fertilizer to grow. Interestingly, nitric oxide (NO) has been detected at various steps of the rhizobium-legume symbiosis where it has been shown to play multifaceted roles. Both bacterial and plant partners are involved in NO synthesis in nodules. To better understand the role of NO, and in particular the role of bacterial NO, at all steps of rhizobia-legumes interaction, the enzymatic sources of NO have to be elucidated. In this review, we discuss different enzymatic reactions by which rhizobia may potentially produce NO. We argue that there is most probably no NO synthase activity in rhizobia, and that instead the NO2- reductase nirK, which is part of the denitrification pathway, is the main bacterial source of NO. The nitrate assimilation pathway might contribute to NO production but only when denitrification is active. The different approaches to measure NO in rhizobia are also addressed.

Competition for electrons favours N2 O reduction in denitrifying Bradyrhizobium isolates

Bradyrhizobia are common members of soil microbiomes and known as N₂ -fixing symbionts of economically important legumes. Many are also denitrifiers, which can act as sinks or sources for N₂ O. Inoculation with compatible rhizobia is often needed for optimal N₂ -fixation, but the choice of inoculant may have consequences for N₂ O emission. Here, we determined the phylogeny and denitrification capacity of Bradyrhizobium strains, most of them isolated from peanut-nodules. Analyses of genomes and denitrification end-points showed that all were denitrifiers, but only ~1/3 could reduce N₂ O. The N₂ O-reducing isolates had strong preference for N₂ O- over NO₃ ^- -reduction. Such preference was also observed in a study of other bradyrhizobia and tentatively ascribed to competition between the electron pathways to Nap (periplasmic NO₃ ^- reductase) and Nos (N₂ O reductase). Another possible explanation is lower abundance of Nap than Nos. Here, proteomics revealed that Nap was instead more abundant than Nos, supporting the hypothesis that the electron pathway to Nos outcompetes that to Nap. In contrast, Paracoccus denitrificans, which has membrane-bond NO₃ ^- reductase (Nar), reduced N₂ O and NO₃ ^- simultaneously. We propose that the control at the metabolic level, favouring N₂ O reduction over NO₃ ^- reduction, applies also to other denitrifiers carrying Nos and Nap but lacking Nar.

Levels of Periplasmic Nitrate Reductase during Denitrification are Lower in Bradyrhizobium japonicum than in Bradyrhizobium diazoefficiens

Soybean plants host endosymbiotic dinitrogen (N₂)-fixing bacteria from the genus Bradyrhizobium. Under oxygen-limiting conditions, Bradyrhizobium diazoefficiens and Bradyrhizobium japonicum perform denitrification by sequentially reducing nitrate (NO₃^–) to nitrous oxide (N₂O) or N₂. The anaerobic reduction of NO₃^– to N₂O was previously shown to be lower in B. japonicum than in B. diazoefficiens due to impaired periplasmic nitrate reductase (Nap) activity in B. japonicum. We herein demonstrated that impaired Nap activity in B. japonicum was due to low Nap protein levels, which may be related to a decline in the production of FixP and FixO proteins by the cbb₃-type oxidase.

The Hemoglobin Bjgb From Bradyrhizobium diazoefficiens Controls NO Homeostasis in Soybean Nodules to Protect Symbiotic Nitrogen Fixation

Legume-rhizobia symbiotic associations have beneficial effects on food security and nutrition, health and climate change. Hypoxia induced by flooding produces nitric oxide (NO) in nodules from soybean plants cultivated in nitrate-containing soils. As NO is a strong inhibitor of nitrogenase expression and activity, this negatively impacts symbiotic nitrogen fixation in soybean and limits crop production. In Bradyrhizobium diazoefficiens, denitrification is the main process involved in NO formation by soybean flooded nodules. In addition to denitrification, nitrate assimilation is another source of NO in free-living B. diazoefficiens cells and a single domain hemoglobin (Bjgb) has been shown to have a role in NO detoxification during nitrate-dependent growth. However, the involvement of Bjgb in protecting nitrogenase against NO in soybean nodules remains unclear. In this work, we have investigated the effect of inoculation of soybean plants with a bjgb mutant on biological nitrogen fixation. By analyzing the proportion of N in shoots derived from N₂-fixation using the ¹⁵N isotope dilution technique, we found that plants inoculated with the bjgb mutant strain had higher tolerance to flooding than those inoculated with the parental strain. Similarly, reduction of nitrogenase activity and nifH expression by flooding was less pronounced in bjgb than in WT nodules. These beneficial effects are probably due to the reduction of NO accumulation in bjgb flooded nodules compared to the wild-type nodules. This decrease is caused by an induction of expression and activity of the denitrifying NO reductase enzyme in bjgb bacteroids. As bjgb deficiency promotes NO-tolerance, the negative effect of NO on nitrogenase is partially prevented and thus demonstrates that inoculation of soybean plants with the B. diazoefficiens bjgb mutant confers protection of symbiotic nitrogen fixation during flooding.

Transcriptional and environmental control of bacterial denitrification and N2O emissions

In oxygen-limited environments, denitrifying bacteria can switch from oxygen-dependent respiration to nitrate (NO3-) respiration in which the NO3- is sequentially reduced via nitrite (NO2-), nitric oxide (NO) and nitrous oxide (N2O) to dinitrogen (N2). However, atmospheric N2O continues to rise, a significant proportion of which is microbial in origin. This implies that the enzyme responsible for N2O reduction, nitrous oxide reductase (NosZ), does not always carry out the final step of denitrification either efficiently or in synchrony with the rest of the pathway. Despite a solid understanding of the biochemistry underpinning denitrification, there is a relatively poor understanding of how environmental signals and respective transcriptional regulators control expression of the denitrification apparatus. This minireview describes the current picture for transcriptional regulation of denitrification in the model bacterium, Paracoccus denitrificans, highlighting differences in other denitrifying bacteria where appropriate, as well as gaps in our understanding. Alongside this, the emerging role of small regulatory RNAs in regulation of denitrification is discussed. We conclude by speculating how this information, aside from providing a better understanding of the denitrification process, can be translated into development of novel greenhouse gas mitigation strategies.

Expanding the Regulon of the Bradyrhizobium diazoefficiens NnrR Transcription Factor: New Insights Into the Denitrification Pathway

Denitrification in the soybean endosymbiont Bradyrhizobium diazoefficiens is controlled by a complex regulatory network composed of two hierarchical cascades, FixLJ-FixK₂-NnrR and RegSR-NifA. In the former cascade, the CRP/FNR-type transcription factors FixK₂ and NnrR exert disparate control on expression of core denitrifying systems encoded by napEDABC, nirK, norCBQD, and nosRZDFYLX genes in response to microoxia and nitrogen oxides, respectively. To identify additional genes controlled by NnrR and involved in the denitrification process in B. diazoefficiens, we compared the transcriptional profile of an nnrR mutant with that of the wild type, both grown under anoxic denitrifying conditions. This approach revealed more than 170 genes were simultaneously induced in the wild type and under the positive control of NnrR. Among them, we found the cycA gene which codes for the c₅₅₀ soluble cytochrome (CycA), previously identified as an intermediate electron donor between the bc₁ complex and the denitrifying nitrite reductase NirK. Here, we demonstrated that CycA is also required for nitrous oxide reductase activity. However, mutation in cycA neither affected nosZ gene expression nor NosZ protein steady-state levels. Furthermore, cycA, nnrR and its proximal divergently oriented nnrS gene, are direct targets for FixK₂ as determined by in vitro transcription activation assays. The dependence of cycA expression on FixK₂ and NnrR in anoxic denitrifying conditions was validated at transcriptional level, determined by quantitative reverse transcription PCR, and at the level of protein by performing heme c-staining of soluble cytochromes. Thus, this study expands the regulon of NnrR and demonstrates the role of CycA in the activity of the nitrous oxide reductase, the key enzyme for nitrous oxide mitigation.

Rhizobium etli Produces Nitrous Oxide by Coupling the Assimilatory and Denitrification Pathways

More than two-thirds of the powerful greenhouse gas nitrous oxide (N₂O) emissions from soils can be attributed to microbial denitrification and nitrification processes. Bacterial denitrification reactions are catalyzed by the periplasmic (Nap) or membrane-bound (Nar) nitrate reductases, nitrite reductases (NirK/cd ₁Nir), nitric oxide reductases (cNor, qNor/ Cu_ANor), and nitrous oxide reductase (Nos) encoded by nap/nar, nir, nor and nos genes, respectively. Rhizobium etli CFN42, the microsymbiont of common bean, is unable to respire nitrate under anoxic conditions and to perform a complete denitrification pathway. This bacterium lacks the nap, nar and nos genes but contains genes encoding NirK and cNor. In this work, we demonstrated that R. etli is able to grow with nitrate as the sole nitrogen source under aerobic and microoxic conditions. Genetic and functional characterization of a gene located in the R. etli chromosome and annotated as narB demonstrated that growth under aerobic or microoxic conditions with nitrate as nitrogen source as well as nitrate reductase activity requires NarB. In addition to be involved in nitrate assimilation, NarB is also required for NO and N₂O production by NirK and cNor, respectively, in cells grown microoxically with nitrate as the only N source. Furthermore, β-glucuronidase activity from nirK::uidA and norC::uidA fusions, as well as NorC expression and Nir and Nor activities revealed that expression of nor genes under microoxic conditions also depends on nitrate reduction by NarB. Our results suggest that nitrite produced by NarB from assimilatory nitrate reduction is detoxified by NirK and cNor denitrifying enzymes that convert nitrite into NO which in turn is reduced to N₂O, respectively.

An Integrated Systems Approach Unveils New Aspects of Microoxia-Mediated Regulation in Bradyrhizobium diazoefficiens

The adaptation of rhizobia from the free-living state in soil to the endosymbiotic state comprises several physiological changes in order to cope with the extremely low oxygen availability (microoxia) within nodules. To uncover cellular functions required for bacterial adaptation to microoxia directly at the protein level, we applied a systems biology approach on the key rhizobial model and soybean endosymbiont Bradyrhizobium diazoefficiens USDA 110 (formerly B. japonicum USDA 110). As a first step, the complete genome of B. diazoefficiens 110spc4, the model strain used in most prior functional genomics studies, was sequenced revealing a deletion of a ~202 kb fragment harboring 223 genes and several additional differences, compared to strain USDA 110. Importantly, the deletion strain showed no significantly different phenotype during symbiosis with several host plants, reinforcing the value of previous OMICS studies. We next performed shotgun proteomics and detected 2,900 and 2,826 proteins in oxically and microoxically grown cells, respectively, largely expanding our knowledge about the inventory of rhizobial proteins expressed in microoxia. A set of 62 proteins was significantly induced under microoxic conditions, including the two nitrogenase subunits NifDK, the nitrogenase reductase NifH, and several subunits of the high-affinity terminal cbb₃ oxidase (FixNOQP) required for bacterial respiration inside nodules. Integration with the previously defined microoxia-induced transcriptome uncovered a set of 639 genes or proteins uniquely expressed in microoxia. Finally, besides providing proteogenomic evidence for novelties, we also identified proteins with a regulation similar to that of FixK₂: transcript levels of these protein-coding genes were significantly induced, while the corresponding protein abundance remained unchanged or was down-regulated. This suggested that, apart from fixK₂, additional B. diazoefficiens genes might be under microoxia-specific post-transcriptional control. This hypothesis was indeed confirmed for several targets (HemA, HemB, and ClpA) by immunoblot analysis.

Anaerobic Reduction of Nitrate to Nitrous Oxide Is Lower in Bradyrhizobium japonicum than in Bradyrhizobium diazoefficiens

When soil oxygen levels decrease, some bradyrhizobia use denitrification as an alternative form of respiration. Bradyrhizobium diazoefficiens (nos⁺) completely denitrifies nitrate (NO₃^-) to dinitrogen, whereas B. japonicum (nos^-) is unable to reduce nitrous oxide to dinitrogen. We found that anaerobic growth with NO₃^- as the electron acceptor was significantly lower in B. japonicum than in B. diazoefficiens, and this was not explained by the absence of nos in B. japonicum. Our results indicate that the reason for the limited growth of B. japonicum is weak NO₃^- reduction due to impaired periplasmic nitrate reductase activity, which may rely on posttranscriptional events.

FixK2 Is the Main Transcriptional Activator of Bradyrhizobium diazoefficiens nosRZDYFLX Genes in Response to Low Oxygen

The powerful greenhouse gas, nitrous oxide (N₂O) has a strong potential to drive climate change. Soils are the major source of N₂O and microbial nitrification and denitrification the main processes involved. The soybean endosymbiont Bradyrhizobium diazoefficiens is considered a model to study rhizobial denitrification, which depends on the napEDABC, nirK, norCBQD, and nosRZDYFLX genes. In this bacterium, the role of the regulatory cascade FixLJ-FixK₂-NnrR in the expression of napEDABC, nirK, and norCBQD genes involved in N₂O synthesis has been previously unraveled. However, much remains to be discovered regarding the regulation of the respiratory N₂O reductase (N₂OR), the key enzyme that mitigates N₂O emissions. In this work, we have demonstrated that nosRZDYFLX genes constitute an operon which is transcribed from a major promoter located upstream of the nosR gene. Low oxygen was shown to be the main inducer of expression of nosRZDYFLX genes and N₂OR activity, FixK₂ being the regulatory protein involved in such control. Further, by using an in vitro transcription assay with purified FixK₂ protein and B. diazoefficiens RNA polymerase we were able to show that the nosRZDYFLX genes are direct targets of FixK₂.