Genes involved in the formation and assembly of rhizobial cytochromes and their role in symbiotic nitrogen fixation.
Rhizobia fix nitrogen in a symbiotic association with leguminous plants and this occurs in nodules. A low-oxygen environment is needed for nitrogen fixation, which paradoxically has a requirement for rapid respiration to produce ATP. These conflicting demands are met by control of oxygen flux and production of leghaemoglobin (an oxygen carrier) by the plant, coupled with the expression of a high-affinity oxidase by the nodule bacteria (bacteroids). Many of the bacterial genes encoding cytochrome synthesis and assembly have been identified in a variety of rhizobial strains. Nitrogen-fixing bacteroids use a cytochrome cbb3-type oxidase encoded by the fixNOQP operon; electron transfer to this high-affinity oxidase is via the cytochrome bc1 complex. During free-living growth, electron transport from the cytochrome bc1 complex to cytochrome aa3 occurs via a transmembrane cytochrome c (CycM). In some rhizobia (such as Bradyrhizobium japonicum) there is a second cytochrome oxidase that also requires electron transport via the cytochrome bc1 complex. In parallel with these cytochrome c oxidases there are quinol oxidases that are expressed during free-living growth. A cytochrome bb3 quinol oxidase is thought to be present in B. japonicum; in Rhizobium leguminosarum, Rhizobium etli and Azorhizobium caulinodans cytochrome d-type oxidases have been identified. Spectroscopic data suggest the presence of a cytochrome o-type oxidase in several rhizobia, although the absence of haem O in B. japonicum may indicate that the absorption attributed to cytochrome o could be due to a high-spin cytochrome b in a cytochrome bb3-type oxidase. In some rhizobia, mutation of genes involved in cytochrome c assembly does not strongly affect growth, presumably because the bacteria utilize the cytochrome c-independent quinol oxidases. In this review, we outline the work on various rhizobial mutants affected in different components of the electron transport pathways, and the effects of these mutations on symbiotic nitrogen fixation and free-living growth.
Delgado
M J
MJ
Departamento de Microbiología del Suelo y Sistemas Simbióticos, Estación Experimental del Zaidin, CSIC, Granada, Spain.
Bedmar
E J
EJ
Downie
J A
JA
D003300:Copper / Q000378:metabolism; D003580:Cytochromes / Q000096:biosynthesis / Q000235:genetics*; D004579:Electron Transport; D007887:Fabaceae / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D006418:Heme / Q000378:metabolism; D009586:Nitrogen Fixation / Q000235:genetics* / Q000502:physiology; D010088:Oxidoreductases / Q000378:metabolism; D010946:Plants, Medicinal; D012230:Rhizobiaceae / Q000235:genetics* / Q000378:metabolism / Q000502:physiology; D013559:Symbiosis / Q000235:genetics*
D003580:Cytochromes; D006418:Heme; D003300:Copper; D010088:Oxidoreductases