THE C-4-DICARBOXYLATE TRANSPORT-SYSTEM OF RHIZOBIUM-MELILOTI AND ITS ROLE IN NITROGEN-FIXATION DURING SYMBIOSIS WITH ALFALFA (MEDICAGO-SATIVA)

The Rhizobium meliloti C-4-dicarboxylate transport (Dct) system is ess ential for an effective symbiosis with alfalfa plants. C-4-dicarboxyla tes are the major carbon source taken up by bacteroids. Genetic analys is of Dct-mutant strains led to the isolation of the dct carrier gene dctA and the regulatory genes dctB and dctD. The carrier gene dctA is regulated in free-living cells by the alternative sigma factor RpoN an d the two-component regulatory system DctB/D. In addition, DctA is inv olved in its own regulation, possibly by interacting with DctB. In bac teroids, besides the DctB/DctD system an additional symbiotic activato r is thought to be involved in dctA expression. Further regulation of dctA in the free-living state is reflected by diauxic growth of rhizob ia, with succinate being the preferred carbon source. The tight coupli ng of C-4-dicarboxylate transport and nitrogen fixation is revealed by a reduced level of C-4-dicarboxylate transport in nitrogenase negativ e bacteroids.