Symbiotic Legume Nodules Employ Both Rhizobial Exo- and Endo-Hydrogenases to Recycle Hydrogen Produced by Nitrogen Fixation

摘要

BackgroundIn symbiotic legume nodules, endosymbiotic rhizobia (bacteroids) fix atmospheric N2, an ATP-dependent catalytic process yielding stoichiometric ammonium and hydrogen gas (H2). While in most legume nodules this H2 is quantitatively evolved, which loss drains metabolic energy, certain bacteroid strains employ uptake hydrogenase activity and thus evolve little or no H2. Rather, endogenous H2 is efficiently respired at the expense of O2, driving oxidative phosphorylation, recouping ATP used for H2 production, and increasing the efficiency of symbiotic nodule N2 fixation. In many ensuing investigations since its discovery as a physiological process, bacteroid uptake hydrogenase activity has been presumed a single entity.