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.
Mikrobiologisches Institut, Eidgenössische Technische Hochschule, Zurich, Switzerland.
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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