Leghaemoglobin and the Supply of O2 to Nitrogen-fixing Root Nodule Bacteroids: Presence of Two Oxidase Systems and ATP Production at Low Free O2 Concentration

Abstract

Studies of rates of consumption of dissolved O2 by suspensions of bacteroids (Rhizobium japonicum, strain CB1809) 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 O2 was between 0-01 and 0-1 muM. At 1 muM-O2 or higher, this oxidase system had little activity and O2 was consumed largely by a low-affinity system insensitive to these inhibitors. At low concentrations of dissolved O2, 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 O2, where nitrogenase activity was greatest. Analysis of reactions which were terminated at various stages during the depletion of O2 from oxyleghaemoglobin showed that at low free O2 concentration, the high-affinity pathway produced up to five times greater bacteroid ATP concentrations than the low-affinity oxidase pathway operating about 1 muM free O2 in the absence of leghaemoglobin. At intermediate free O2 concentrations, occurring during the later stages of deoxygenation of oxymyoglobin, intermediate concentrations of ATP were found in the bacteroids.