Soluble and membrane-bound nitrate reductase from Bradyrhizobium japonicum bacteroids

Nitrate reductase (NR) activity was detected in the cytosol and membranes from bacteroids of Bradyrhizobium japonicum strain L-236 isolated from nodules of soybean (Glycine max, Merr., cv. Williams) plants that were only N2-dependent. The molecular mass (M) of the NR from each source was estimated by gel filtration to be about 76 kDa. Treatment of plants with 20 mM KNO3 induced NR activity in the bacteroid cytosol, but did not affect rates of activity in the membranes. Furthermore, the M of the cytosolic enzyme was 160 kDa, while that of the NR from the membranes was 76 kDa. When bacteroids that had been isolated from nodules of plants not receiving nitrate were incubated microaerobically (2 % O2 v/v) in both the absence and presence of nitrate, the M of the membrane-bound NR enzyme was calculated to be 230 kDa; in bacteroids incubated in the absence of nitrate, the M of the soluble enzyme was 76 kDa, whereas that of bacteroids incubated with nitrate was about 160 kDa. Incubation of bacteroids under aerobic conditions with nitrate induced NR activity in the soluble fraction, and the M of the enzyme was 160 kDa; in the absence of nitrate the M of the soluble NR was 76 kDa. No NR activity was found in membranes from aerobically incubated bacteroids.