PROPERTIES OF NAD(-DEPENDENT AND NADP(+)-DEPENDENT MALIC ENZYMES OF RHIZOBIUM (SINORHIZOBIUM) MELILOTI AND DIFFERENTIAL EXPRESSION OF THEIRGENES IN NITROGEN-FIXING BACTEROIDS())

The wild-type NAD(+)-dependent malic enzyme (dme) gene of Rhizobium (n ow Sinorhizobium) meliloti was 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 dme mutants. The d me gene was mapped by conjugation to between the cys-11 and leu-53 mar kers on the R. meliloti chromosome. beta-Galactosidase activities meas ured in bacterial strains carrying either dme-lacZ or tme-lacZ gene fu sions (the tme gene encodes NADP(+)-dependent malic enzyme) indicated that the dme gene was expressed constitutively in free-living cells an d in N-2-fixing bacteroids whereas expression of the tme gene was repr essed in bacteroids. The R. meliloti dme gene product (DME) was overex pressed in and partially purified from Escherichia coil. The propertie s of this enzyme, together with those of the NADP(+)-dependent malic e nzyme (TME) partially purified from R. meliloti dme mutants, were dete rmined. Acetyl-CoA inhibited DME but not TME activity. This result sup ports the hypothesis that DME, together with pyruvate dehydrogenase, f orms a pathway in which malate is converted to acetyl-CoA.