Genes for a microaerobically induced oxidase complex in Bradyrhizobium japonicum are essential for a nitrogen-fixing endosymbiosis.
We report the discovery of a Bradyrhizobium japonicum gene cluster (fixNOQP) in which mutations resulted in defective soybean root-nodule bacteroid development and symbiotic nitrogen fixation. The predicted, DNA-derived protein sequences suggested that FixN is a heme b and copper-binding oxidase subunit, FixO a monoheme cytochrome c, FixQ a polypeptide of 54 amino acids, and FixP a diheme cytochrome c and that they are all membrane-bound. The isolation and analysis of membrane proteins from B. japonicum wild-type and mutant cells revealed two c-type cytochromes of 28 and 32 kDa as the likely products of the fixO and fixP genes and showed that both were synthesized only under oxygen-limited growth conditions. Furthermore, fixN insertion and fixNO deletion mutants grown microaerobically or anaerobically (with nitrate) exhibited a strong decrease in whole-cell oxidase activity as compared with the wild type. The data suggest that the fixNOQP gene products are induced at low oxygen concentrations and constitute a member of the bacterial heme/copper cytochrome oxidase superfamily. The described features are compatible with the postulate that this oxidase complex is specifically required to support bacterial respiration in endosymbiosis.
Preisig
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Mikrobiologisches Institut, Eidgenössische Technische Hochschule, Zurich, Switzerland.
Anthamatten
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Hennecke
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D000332:Aerobiosis; D000595:Amino Acid Sequence; D000693:Anaerobiosis; D001426:Bacterial Proteins / Q000235:genetics*; D003574:Cytochrome c Group / Q000235:genetics*; D003576:Electron Transport Complex IV / Q000235:genetics; D005798:Genes, Bacterial; D008565:Membrane Proteins / Q000235:genetics*; D008969:Molecular Sequence Data; D005810:Multigene Family; D016254:Mutagenesis, Insertional; D009586:Nitrogen Fixation / Q000235:genetics*; D010088:Oxidoreductases / Q000235:genetics*; D015183:Restriction Mapping; D012230:Rhizobiaceae / Q000201:enzymology* / Q000235:genetics* / Q000502:physiology; D017384:Sequence Deletion; D017386:Sequence Homology, Amino Acid; D013025:Glycine max / Q000382:microbiology; D013559:Symbiosis / Q000235:genetics*
D001426:Bacterial Proteins; D003574:Cytochrome c Group; C080744:FixQ protein, Bacteria; D008565:Membrane Proteins; C080743:cytochrome FixO; C080742:cytochrome FixP; D010088:Oxidoreductases; D003576:Electron Transport Complex IV