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Mutations that mediated the ability of Pck-deficient Rhizobium meliloti mutants to utilize succinate and tricarboxylic acid (TCA) intermediates were analyzed to characterize the biosynthesis of phosphoenolpyruvate (PEP). The activity of pyruvate orthophosphate dikinase (PPDK) in Rhizobium (Sinorhizobium) meliloti was enhanced by pckA suppressor mutations. Gluconeogenesis from malate in the pckA suppressor strain was also mediated by the combined activities of malic enzyme and PPDK.

Diphosphopyridine-nucleotide-dependent malic enzyme (DME) of Rhizobium meliloti is needed for symbiotic nitrogen fixation since it forms a pathway in which malate is converted to acetyl-CoA in bacteroids. This enzyme shows maximal activity with NAD+ as cofactor but has reduced activity when NADP+ is used. Acetyl-CoA strongly inhibited DME activity but not TME activity. The dme gene was expressed in free-living cells and in nitrogen-fixing bacteroids while tme gene was repressed in bacteroids.