Malic Enzymes of Rhizobium meliloti

There is much evidence that C4-dicarboxylic acids, such as malate and succinate, are the principle energy sources utilized by N2-fixing bacteroids within root nodules. These acids are taken into the bacteroids via a C4-dicarboxylate transport (Dct) system which has been extensively studied in a number of Rhizobium species (Ronson et al. 1981; Finan et al. 1983; Dilworth, Glenn 1984; Vance, Heichel 1991). In addition to the C4-dicarboxylates malate and succinate, the possibility that aspartate may also be supplied to bacteroids is supported by the finding that R. meliloti mutants lacking a particular aspartate aminotransferase are unable to fix N2 (Rastogi, Watson 1991). Following transport of the C4-dicarboxylates into the bacteroids, we believe that they are metabolized via the tricarboxylic acid (TCA) cycle. To our knowledge, all results of enzymatic assays are consistent with the operation of an intact TCA cycle in bacteroids. We are studying the pathway by which the C4-dicarboxylates malate and succinate are metabolized in bacteroids. Without an efficient pathway for the synthesis of acetyl-CoA, the metabolism of these and other C4-dicarboxylates via the TCA cycle would lead to a rapid accumulation of oxaloacetate. We have focused on the role of malic enzymes in the generation of acetyl-CoA. Malic enzyme catalyzes the oxidative decarboxylation of malate to pyruvate and CO2 with the simultaneous reduction of NAD(P) to NAD(P)H. The pyruvate thus formed would then be converted to acetyl- CoA by pyruvate dehydrogenase. An additional pathway for the synthesis of acetyl- CoA from oxaloacetate could be mediated by the combined activities of phosphoenolpyruvate carboxykinase (PCK), pyruvate kinase and pyruvate dehydrogenase. The very low PCK activities detected in bacteroids from alfalfa and pea root nodules, together with the symbiotic phenotypes of Pck- mutants, clearly shows that the PCK pathway is not operative in N2-fixing bacteroids (McKay et al. 1985; Finan et al. 1991). We note however that Pck- mutants of R. meliloti have a reduced N2-fixation phenotype. In addition, Pck− mutants of Rhizobium sp. NGR234 have a pronounced plant-host-dependent symbiotic phenotype. We attribute the latter two results to a requirement for the PCK enzyme during the infection process, for example growth of the bacteria in the infection thread, rather than a role for PCK during N2-fixation in the bacteroid (Finan et al. 1991; Østerås et al, 1991).