235The respiration of bacteroids from root nodules of soybean (inoculated with Rhizobium japonicum strain CBlSO9) and cowpea (inoculated with Rhizobium sp. strain CB756) was studied at low concentrations of dissolved 0, in a stirred electrode chamber (range 0.1 to 10 ,uM-OJ and during deoxygenation of solutions containing oxyleghaemoglobin (range 0.003 to 0-3 PM-O,) or oxymyoglobin (0.1 to 10 pM-0,). The 0, affinity of terminal oxidase systems of these bacteria (measured as apparent Ks) depended on the range of concentration of free, dissolved 0, in which the measurements were made. This was due to the capacity of the oxidases to express increasing affinity as the 0, concentration declined below certain transition concentrations, thus tending to maintain the respiration rate. In the lowest concentration range (0-003 to 0.01 p M -0 , ) an oxidase of very high affinity for 0, (apparent Ks 0.005 ,UM) appeared to be under allosteric control. Three additional oxidases or oxidase affinity states could be recognized. In strain CB1809, the one with lowest affinity was insensitive to CO and less sensitive to N-phenylimidazole and azide, whilst the oxidases of higher affinity were very sensitive to these inhibitors. Terminal oxidase systems of the same strains from 0,-limited continuous cultures resembled those of bacteroids, when assayed in the same way.The properties of terminal oxidases of Azotobacter vinelandii strain AVO, Azospirillum brasilense strain Sp7 and Klebsiella pneumoniae strain 5023 1, when grown in 0,-limited continuous cultures, were examined in the same experimental systems. In all culture states examined there was no evidence for multiple oxidases as seen with the Rhizobium spp. In two of these bacteria, Azospirillum brasilense and K. pneumoniae, the oxidases appeared to be allosteric but their affinities were very different (apparent Ks 0.006 and 0.1 1 ,UM, respectively). The terminal oxidase of Azotobacter vinelandii obeyed Michaelis-Menten kinetics, but had a lower affinity for O2 (apparent Ks 0.48 ,LAM).
