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C4 -dicarboxylic acids appear to be metabolized via the tricarboxylic acid (TCA) cycle in N2 -fixing bacteria (bacteroids) within legume nodules. InSinorhizobium meliloti bacteroids from alfalfa, NAD+ -malic enzyme (DME) is required for N2 fixation, 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 thatdme mutants of the broad-host-rangeSinorhizobium sp. strain NGR234 formed nodules whose level of N2 fixation 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 singlepckA and singledme mutants fixed N2 at reduced rates, apckA dme double 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 Fix− phenotype ofS. meliloti dme mutants may be specific to the alfalfa-S. meliloti symbiosis. We therefore examined the ME-like genesazc3656 andazc0119 fromAzorhizobium caulinodans , asazc3656 mutants were previously shown to form Fix− nodules 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 N2 fixation inA. caulinodans andS. meliloti , in other rhizobia this activity can be bypassed via another pathway(s).

applied and environmental microbiology

NAD(P) <sup>+</sup> -Malic Enzyme Mutants of Sinorhizobium sp. Strain NGR234, but Not Azorhizobium caulinodans ORS571, Maintain Symbiotic N <sub>2</sub> Fixation Capabilities

NAD(P) + -Malic Enzyme Mutants of Sinorhizobium sp. Strain NGR234, but Not Azorhizobium caulinodans ORS571, Maintain Symbiotic N 2 Fixation Capabilities

NAD(P) ⁺ -Malic Enzyme Mutants of Sinorhizobium sp. Strain NGR234, but Not Azorhizobium caulinodans ORS571, Maintain Symbiotic N ₂ Fixation Capabilities