Functional Differences of Two Distinct Catalases in Mesorhizobium loti MAFF303099 under Free-Living and Symbiotic Conditions

Protection against reactive oxygen species (ROS) is important for legume-nodulating rhizobia during the establishment and maintenance of symbiosis, as well as under free-living conditions, because legume hosts might assail incoming microbes with ROS and because nitrogenase is extremely sensitive to ROS. We generated mutants of two potential catalase genes inMesorhizobium loti MAFF303099 to investigate their physiological significance. Biochemical results indicated that genes with the locus tags mlr2101 and mlr6940 encoded a monofunctional catalase and a bifunctional catalase-peroxidase, respectively, that were namedkatE andkatG . Under free-living conditions, thekatG mutant demonstrated an extended generation time and elevated sensitivity to exogenous H2 O2 , whereas thekatE mutant exhibited no generation time extension and only a slight increase in sensitivity to exogenous H2 O2 . However, thekatE mutant showed a marked decrease in its survival rate during the stationary phase. With regard to symbiotic capacities withLotus japonicus , thekatG mutant was indistinguishable from the wild type; nevertheless, the mutants with disruptedkatE formed nodules with decreased nitrogen fixation capacities (about 50 to 60%) compared to those formed by the wild type. These mutant phenotypes agreed with the expression profiles showing that transcription ofkatG , but notkatE , was high during the exponential growth phase and that transcription levels ofkatE versussigA were elevated during stationary phase and were approximately fourfold higher in bacteroids than mid-exponential-phase cells. Our results revealed functional separation of the two catalases, as well as the importance of KatE under conditions of strong growth limitation.