The complete denitrification pathway of the symbiotic, nitrogen-fixing bacterium <i>Bradyrhizobium japonicum</i>

Bedmar, Eulogio J.; Robles, Eloy F.; Delgado, Manuel Lorenzo

doi:10.1042/bst0330141

Cited by 147 publications

(129 citation statements)

References 32 publications

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“…Expression of denitrification genes is dependent upon FixJ and FixK (Fig. 4) as in B. japonicum (8,36), although we did not find that nnrR was induced by FixJ in S. meliloti or induced under low oxygen. There is indeed increasing evidence for a role of denitrification in the successful interaction of pathogenic or symbiotic bacteria with their hosts (40).…”

Section: Discussionmentioning

confidence: 51%

FixJ: a Major Regulator of the Oxygen Limitation Response and Late Symbiotic Functions of Sinorhizobium meliloti

2006

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Sinorhizobium meliloti exists either in a free-living state in the soil or in symbiosis within legume nodules, where the bacteria differentiate into nitrogen-fixing bacteroids. Expression of genes involved in nitrogen fixation and associated respiration is governed by two intermediate regulators, NifA and FixK, respectively, which are controlled by a two-component regulatory system FixLJ in response to low-oxygen conditions. In order to identify the FixLJ regulon, gene expression profiles were determined in microaerobic free-living cells as well as during the symbiotic life of the bacterium for the wild type and a fixJ null-mutant strain. We identified 122 genes activated by FixJ in either state, including 87 novel targets. FixJ controls 74% of the genes induced in microaerobiosis (2% oxygen) and the majority of genes expressed in mature bacteroids. Ninetyseven percent of FixJ-activated genes are located on the symbiotic plasmid pSymA. Transcriptome profiles of a nifA and a fixK mutant showed that NifA activates a limited number of genes, all specific to the symbiotic state, whereas FixK controls more than 90 genes, involved in free-living and/or symbiotic life. This study also revealed that FixJ has no other direct targets besides those already known. FixJ is involved in the regulation of functions such as denitrification or amino acid/polyamine metabolism and transport. Mutations in selected novel FixJ targets did not affect the ability of the bacteria to form nitrogen-fixing nodules on Medicago sativa roots. From these results, we propose an updated model of the FixJ regulon.

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Section: Discussionmentioning

confidence: 51%

FixJ: a Major Regulator of the Oxygen Limitation Response and Late Symbiotic Functions of Sinorhizobium meliloti

2006

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“…Aerobic stationary-phase P. aeruginosa cells were inoculated to a final optical density at 600 nm (OD 600 ) of 0.001 in anaerobic BHI broth supplemented with various amounts of KNO 3 . Growth was measured by OD 600 , and samples were removed for nitrate analysis at 24 h postinoculation.…”

Section: Methodsmentioning

confidence: 99%

Membrane-Bound Nitrate Reductase Is Required for Anaerobic Growth in Cystic Fibrosis Sputum

2007

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The autosomal recessive disorder cystic fibrosis (CF) affects approximately 70,000 people worldwide and is characterized by chronic bacterial lung infections with the opportunistic pathogen Pseudomonas aeruginosa. To form a chronic CF lung infection, P. aeruginosa must grow and proliferate within the CF lung, and the highly viscous sputum within the CF lung provides a likely growth substrate. Recent evidence indicates that anaerobic microenvironments may be present in the CF lung sputum layer. Since anaerobic growth significantly enhances P. aeruginosa biofilm formation and antibiotic resistance, it is important to examine P. aeruginosa physiology and metabolism in anaerobic environments. Measurement of nitrate levels revealed that CF sputum contains sufficient nitrate to support significant P. aeruginosa growth anaerobically, and mutational analysis revealed that the membrane-bound nitrate reductase is essential for P. aeruginosa anaerobic growth in an in vitro CF sputum medium. In addition, expression of genes coding for the membrane-bound nitrate reductase complex is responsive to CF sputum nitrate levels. These findings suggest that the membrane-bound nitrate reductase is critical for P. aeruginosa anaerobic growth with nitrate in the CF lung.Cystic fibrosis (CF) is a heritable disease characterized by accumulation of large volumes of mucus (sputum) within the lungs and persistent colonization with opportunistic pathogens. While numerous bacterial species transiently colonize CF sputum or persist at low cell densities, the opportunistic pathogen Pseudomonas aeruginosa establishes a chronic infection and can attain densities of Ͼ10 9 cells/ml of sputum (19). Chronic P. aeruginosa infection is the leading cause of morbidity and mortality in CF patients (20). Once established, P. aeruginosa CF lung infections are highly refractory to antibiotic treatments, and even aggressive therapies fail to fully eradicate the bacterium from the lung (20). Several aspects of P. aeruginosa physiology are believed to contribute to its high antibiotic resistance in vivo, including biofilm formation and overproduction of the viscous exopolysaccharide alginate (23).P. aeruginosa CF lung infections are frequently modeled under aerobic laboratory conditions; however, direct oxygen measurements within CF lung sputum in situ indicate that sputum contains hypoxic, and potentially anaerobic, regions (38). In addition, strict anaerobic bacteria have been detected in sputum samples from multiple CF patients (21, 29), suggesting that environments capable of sustaining these species are present in vivo. There are several factors that could potentially reduce oxygen levels within CF sputum: cultured CF lung epithelial cells consume oxygen at a higher rate than non-CF lung epithelial cells, oxygen diffusion through sputum is restricted, and oxygen is consumed by resident sputum microorganisms, including P. aeruginosa (38). Collectively these factors may effectively reduce oxygen to levels insufficient for aerobic respiration. Interestingly, ...

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“…Within the nodules, bacteria differentiate into bacteroids, which reduce atmospheric dinitrogen (N 2 ) into ammonia (NH z 4 ), a reaction catalysed by the molybdoenzyme nitrogenase (Lawson & Smith, 2002). In addition to assimilating nitrate (NO À 3 ) to NH z 4 (Bergersen, 1977), B. japonicum is also capable of denitrification, that is, the reduction of NO À 3 or nitrite (NO À 2 ) via nitric oxide (NO) and nitrous oxide (N 2 O) to N 2 , when cultured under oxygen-limiting conditions (Bedmar et al, 2005). The first step of denitrification, the anaerobic reduction of nitrate to nitrite, is carried out by the periplasmic Mo-containing nitrate reductase (Delgado et al, 2003).…”

Section: Introductionmentioning

confidence: 99%

Functional characterization of the Bradyrhizobium japonicum modA and modB genes involved in molybdenum transport

et al. 2006

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A modABC gene cluster that encodes an ABC-type, high-affinity molybdate transporter from Bradyrhizobium japonicum has been isolated and characterized. B. japonicum modA and modB mutant strains were unable to grow aerobically or anaerobically with nitrate as nitrogen source or as respiratory substrate, respectively, and lacked nitrate reductase activity. The nitrogen-fixing ability of the mod mutants in symbiotic association with soybean plants grown in a Mo-deficient mineral solution was severely impaired. Addition of molybdate to the bacterial growth medium or to the plant mineral solution fully restored the wild-type phenotype. Because the amount of molybdate required for suppression of the mutant phenotype either under free-living or under symbiotic conditions was dependent on sulphate concentration, it is likely that a sulphate transporter is also involved in Mo uptake in B. japonicum. The promoter region of the modABC genes has been characterized by primer extension. Reverse transcription and expression of a transcriptional fusion, P modA -lacZ, was detected only in a B. japonicum modA mutant grown in a medium without molybdate supplementation. These findings indicate that transcription of the B. japonicum modABC genes is repressed by molybdate.

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The complete denitrification pathway of the symbiotic, nitrogen-fixing bacterium Bradyrhizobium japonicum

Cited by 147 publications

References 32 publications

FixJ: a Major Regulator of the Oxygen Limitation Response and Late Symbiotic Functions of Sinorhizobium meliloti

FixJ: a Major Regulator of the Oxygen Limitation Response and Late Symbiotic Functions of Sinorhizobium meliloti

Membrane-Bound Nitrate Reductase Is Required for Anaerobic Growth in Cystic Fibrosis Sputum

Functional characterization of the Bradyrhizobium japonicum modA and modB genes involved in molybdenum transport

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