Abstract
Background
In 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.
Methodology/Principal Findings
Azorhizobium caulinodans, the nodule endosymbiont of Sesbania rostrata stems and roots, possesses both orthodox respiratory (exo-)hydrogenase and novel (endo-)hydrogenase activities. These two respiratory hydrogenases are structurally quite distinct and encoded by disparate, unlinked gene-sets. As shown here, in S. rostrata symbiotic nodules, haploid A. caulinodans bacteroids carrying single knockout alleles in either exo- or-endo-hydrogenase structural genes, like the wild-type parent, evolve no detectable H2 and thus are fully competent for endogenous H2 recycling. Whereas, nodules formed with A. caulinodans exo-, endo-hydrogenase double-mutants evolve endogenous H2 quantitatively and thus suffer complete loss of H2 recycling capability. More generally, from bioinformatic analyses, diazotrophic microaerophiles, including rhizobia, which respire H2 may carry both exo- and endo-hydrogenase gene-sets.
Conclusions/Significance
In symbiotic S. rostrata nodules, A. caulinodans bacteroids can use either respiratory hydrogenase to recycle endogenous H2 produced by N2 fixation. Thus, H2 recycling by symbiotic legume nodules may involve multiple respiratory hydrogenases.
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