Studies of rates of consumption of dissolved 0, by suspensions of bacteroids (Rhizobium japonicum, strain ~~1 8 0 9 ) from soybean root nodules showed the presence of two different terminal oxidase systems. A high-affinity system, sensitive to inhibition by N-phenylimidazole and by carbon monoxide, was most active when the dissolved 0, was between 0.01 and 0-1 ,UM. At I ,uM-O, or higher, this oxidase system had little activity and 0, was consumed largely by a low-affinity system insensitive to

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... hese inhibitors. At low concentrations of dissolved O" bacteroid respiration rates appeared to be diffusion-limited. When purified oxyleghaemoglobin was added to such systems, this restriction was relieved and respiration was maintained to much lower concentrations of free dissolved O" where nitrogenase activity was greatest. Analysis of reactions which were terminated at various stages during the depletion of 0, from oxyleghaemoglobin showed that at low free 0, concentration, the high-affinity pathway produced up to five times greater bacteroid ATP concentrations than the low-affinity oxidase pathway operating about I ,UM free 0, in the absence of leghaemoglobin. At intermediate free 0, concentrations, occurring during the later stages of deoxygenation of oxymyoglobin, intermediate concentrations of ATP were found in the bacteroids.