The Occurrence and Role of Ubiquinone in Electron Transport to Oxygen and Nitrate in Aerobically, Anaerobically and Symbiotically Grown Rhizobium japonicum

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Abstract
Ubiquinone was extracted from free-living R. japonicum, grown aerobically or anaerobically, and from the symbiotic bacteroid form; it was tentatively identified as the Q-10 homolog. The ubiquinone concentration was highest in symbiotically grown R. japonicum but the ratio ubiquinone:total cytochrome was about 1.5:1 in membrane particles from organisms grown under all 3 conditions. The ubiquinone was reduced 75% by NADH, completely oxidized by O2 but not oxidized by nitrate. NADH oxidase activity and nitrate reductase activity in membrane particles from organisms grown under the different conditions were similar except that nitrate reductase activity was low in aerobically grown organisms. Ubiquinone functions in electron transport to O2 but not to nitrate.