NAD(P)+-Malic Enzyme Mutants of Sinorhizobium sp. Strain NGR234, but Not Azorhizobium caulinodans ORS571, Maintain Symbiotic N2Fixation Capabilities Academic Article uri icon

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abstract

  • ABSTRACTC4-dicarboxylic acids appear to be metabolized via the tricarboxylic acid (TCA) cycle in N2-fixing bacteria (bacteroids) within legume nodules. InSinorhizobium melilotibacteroids from alfalfa, NAD+-malic enzyme (DME) is required for N2fixation, and this activity is thought to be required for the anaplerotic synthesis of pyruvate. In contrast, in the pea symbiontRhizobium leguminosarum, pyruvate synthesis occurs via either DME or a pathway catalyzed by phosphoenolpyruvate carboxykinase (PCK) and pyruvate kinase (PYK). Here we report thatdmemutants of the broad-host-rangeSinorhizobiumsp. strain NGR234 formed nodules whose level of N2fixation varied from 27 to 83% (plant dry weight) of the wild-type level, depending on the host plant inoculated. NGR234 bacteroids had significant PCK activity, and while singlepckAand singledmemutants fixed N2at reduced rates, apckA dmedouble mutant had no N2-fixing activity (Fix). Thus, NGR234 bacteroids appear to synthesize pyruvate from TCA cycle intermediates via DME or PCK pathways. These NGR234 data, together with other reports, suggested that the completely Fixphenotype ofS. meliloti dmemutants may be specific to the alfalfa-S. melilotisymbiosis. We therefore examined the ME-like genesazc3656andazc0119fromAzorhizobium caulinodans, asazc3656mutants were previously shown to form Fixnodules on the tropical legumeSesbania rostrata. We found that purified AZC3656 protein is an NAD(P)+-malic enzyme whose activity is inhibited by acetyl-coenzyme A (acetyl-CoA) and stimulated by succinate and fumarate. Thus, whereas DME is required for symbiotic N2fixation inA. caulinodansandS. meliloti, in other rhizobia this activity can be bypassed via another pathway(s).

publication date

  • April 15, 2012