title: Isolation of a Rhizobium phaseoli cytochrome mutant with enhanced respiration and symbiotic nitrogen fixation.
abstract: Cultured cells of a Rhizobium phaseoli wild-type strain (CE2) possess b-type and c-type cytochromes and two terminal oxidases: cytochromes o and aa3. Cytochrome aa3 was partially expressed when CE2 cells were grown on minimal medium, during symbiosis, and in well-aerated liquid cultures in a complex medium (PY2). Two cytochrome mutants of R. phaseoli were obtained and characterized. A Tn5-mob-induced mutant, CFN4201, expressed diminished amounts of b-type and c-type cytochromes, showed an enhanced expression of cytochrome oxidases, and had reduced levels of N,N,N',N'-tetramethyl-p-phenylenediamine, succinate, and NADH oxidase activities. Nodules formed by this strain had no N2 fixation activity. The other mutant, CFN4205, which was isolated by nitrosoguanidine mutagenesis, had reduced levels of cytochrome o and higher succinate oxidase activity but similar NADH and N,N,N',N'-tetramethyl-p-phenylenediamine oxidase activities when compared with the wild-type strain. Strain CFN4205 expressed a fourfold-higher cytochrome aa3 content when cultured on minimal and complex media and had twofold-higher cytochrome aa3 levels during symbiosis when compared with the wild-type strain. Nodules formed by strain CFN4205 fixed 33% more N2 than did nodules formed by the wild-type strain, as judged by the total nitrogen content found in plants nodulated by these strains. Finally, low-temperature photodissociation spectra of whole cells from strains CE2 and CFN4205 reveal cytochromes o and aa3. Both cytochromes react with O2 at -180 degrees C to give a light-insensitive compound. These experiments identify cytochromes o and aa3 as functional terminal oxidases in R. phaseoli.
authors.affiliation: Centro de Investigación sobre Fijación de Nitrógeno, Universidad Nacional Autónoma de México, Cuernayaca, Morelos.
authors.forename: M
authors.initials: M
authors.lastname: Soberón
authors.forename: H D
authors.initials: HD
authors.lastname: Williams
authors.forename: R K
authors.initials: RK
authors.lastname: Poole
authors.forename: E
authors.initials: E
authors.lastname: Escamilla
mesh_terms: D002462:Cell Membrane / Q000378:metabolism; D003580:Cytochromes / Q000235:genetics* / Q000378:metabolism; D004579:Electron Transport; D004926:Escherichia coli / Q000235:genetics; D009154:Mutation; D009586:Nitrogen Fixation; D010101:Oxygen Consumption; D012231:Rhizobium / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism; D013053:Spectrophotometry; D013559:Symbiosis
chemical_list: D003580:Cytochromes