An Alternative Succinate (2-Oxoglutarate) Transport System in Rhizobium tropici Is Induced in Nodules of Phaseolus vulgaris

Abstract

ABSTRACT The rhizobial DctA permease is essential for the development of effective nitrogen-fixing bacteroids, which was correlated with its requirement for growth on C 4 -dicarboxylates. A previously described dctA mutant of Rhizobium tropici CIAT899, strain GA1 ( dctA ), however, was unexpectedly still able to grow on succinate as a sole carbon source but less efficiently than CIAT899. Like other rhizobial dctA mutants, GA1 was unable to grow on fumarate or malate as a carbon source and induced the formation of ineffective nodules. We report an alternative succinate uptake system identified by Tn 5 mutagenesis of strain GA1 that was required for the remaining ability to transport and utilize succinate. The alternative uptake system required a three-gene cluster that is highly characteristic of a dctABD locus. The predicted permease-encoding gene had high sequence similarity with open reading frames encoding putative 2-oxoglutarate permeases (KgtP) of Ralstonia solanacearum and Agrobacterium tumefaciens . This analysis was in agreement with the requirement for this gene for optimal growth on and induction by 2-oxoglutarate. The permease-encoding gene of the alternative system was also designated kgtP in R. tropici . The dctBD -like genes in this cluster were found to be required for kgtP expression and were designated kgtSR . Analysis of a kgtP :: lacZ transcriptional fusion indicated that a kgtSR -dependent promoter of kgtP was specifically induced by 2-oxoglutarate. The expression of kgtPp was found in bacteroids of nodules formed with either CIAT899 or GA1 on roots of Phaseolus vulgaris . Results suggested that 2-oxoglutarate might be transported or conceivably exported in nodules induced by R. tropici on roots of P. vulgaris .