An alternative nitrogenase is not expressed in molybdenum-deficient legume root nodules.

Three legume root nodule bacteria systems (Medicago polymorpha L. -Rhizobium meliloti, Ornithopus sativus Brot. -Bradyrhizobium lupini and Trifolium subterraneum L.- Rhizobium leguminosarum by. trifolii) were grown in solution culture under conditions likely to lead to the production of alternative nitrogenases (molybdenum-deficient, or molybdenum-deficient but supplemented with vanadium). Addition of 1 μM molybdenum produced significant responses in both nodule and top weights while 2 μM vanadium did not. Ethane, which is produced as well as ethylene when acetylene is reduced by vanadium nitrogenase or nitrogenase-3 from Azotobacter, was not found in significant amounts during assays of acetylene reduction in either molybdenum-deficient or molybdenum-deficient, vanadium-supplemented treatments, suggesting that no non-molybdenum nitrogenase was produced by these root nodule bacteria.