Properties of Terminal Oxidase Systems of Bacteroids from Root Nodules of Soybean and Cowpea and of N2-fixing Bacteria Grown in Continuous Culture

Abstract

The respiration of bacteroids from root nodules of soybean (inoculated with Rhizobium japonicum strain CB1809) and cowpea (inoculated with Rhizobium sp. strain CB756) was studied at low concentrations of dissolved O2 in a stirred electrode chamber (range 0·1 to 10 µ m-O2) and during deoxygenation of solutions containing oxyleghaemoglobin (range 0·003 to 0·3 µ m-0·2) or oxymyoglobin (0·1 to 10 µ m-O2). The O2 affinity of terminal oxidase systems of these bacteria (measured as apparent K s) depended on the range of concentration of free, dissolved O2 in which the measurements were made. This was due to the capacity of the oxidases to express increasing affinity as the O2 concentration declined below certain transition concentrations, thus tending to maintain the respiration rate. In the lowest concentration range (0.003 to 0·01 µ m-O2) an oxidase of very high affinity for O2 (apparent K s 0·005 µ m) 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 O2-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 50231, when grown in O2-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 K s 0·006 and 0·11 µ m, respectively). The terminal oxidase of Azotobacter vinelandii obeyed Michaelis-Menten kinetics, but had a lower affinity for O2 (apparent K s 0·48 µ m).