The wild-type NAD+-dependent malic enzyme (
dme) gene of Rhizobium(now Sinorhizobium) melilotiwas cloned and localized to a 3.1 kb region isolated on the cosmid pTH69. This cosmid complemented the symbiotic nitrogen fixation (Fix-) phenotype of R. meliloti dmemutants. The dmegene was mapped by conjugation to between the cys-11and leu-53markers on the R. melilotichromosome. β-Galactosidase activities measured in bacterial strains carrying either dme-lacZor tme-lacZgene fusions (the tmegene encodes NADP+-dependent malic enzyme) indicated that the dmegene was expressed constitutively in free-living cells and in N2-fixing bacteroids whereas expression of the tmegene was repressed in bacteroids. The R. meliloti dmegene product (DME) was overexpressed in and partially purified from Escherichia coli.The properties of this enzyme, together with those of the NADP+-dependent malic enzyme (TME) partially purified from R. meliloti dmemutants, were determined. Acetyl-CoA inhibited DME but not TME activity. This result supports the hypothesis that DME, together with pyruvate dehydrogenase, forms a pathway in which malate is converted to acetyl-CoA.