INCREASED PYRUVATE ORTHOPHOSPHATE DIKINASE ACTIVITY RESULTS IN AN ALTERNATIVE GLUCONEOGENIC PATHWAY IN RHIZOBIUM (SINORHZOBIUM) MELILOTI

The formation of phosphoenolpyruvate (PEP) is a major step in the gluc oneogenic pathway in which tricarboxylic acid (TCA) cycle intermediate s are converted to hexose sugars. In Rhizobium (now Sinorhizobium) mel iloti this step is catalysed by the enzyme PEP carboxykinase (PCK) whi ch converts oxaloacetate to PEP. R. meliloti Pck(-) mutants grow very poorly with TCA cycle intermediates as the sole source of carbon. Here , the isolation and mapping of suppressor mutations which allow Pck(-) mutants to grow on succinate and other TCA cycle intermediates is rep orted. Tn5 insertions which abolished the suppressor phenotype and map ped to the suppressor locus were located within the pod gene encoding pyruvate orthophosphate dikinase (PPDK). Strains carrying suppressor m utations had increased PPDK activity compared to the wild-type. The su ppressor phenotype was dependent on the combined activities of malic e nzyme and PPDK, which thus represent an alternative route for the form ation of PEP in R. meliloti. PPDK activity was not required for symbio tic N-2 fixation.