Two C₄-Dicarboxylate Transport Systems in Rhizobium sp. NGR234: Rhizobial Dicarboxylate Transport Is Essential for Nitrogen Fixation in Tropical Legume Symbioses. J. C. van Slooten. University of Geneva, CH1211 Geneva 4, Switzerland. T. V. Bhuvanasvari(2), S. Bardin(1), and J. Stanley(3). (1)University of Geneva, CH1211 Geneva 4, Switzerland; (2)University of Tromso, Institute for Biology and Geology, Dramsveien 201, N-9000 Tromso, Norway; (3)NCTC Plasmid & Molecular Genetics Unit, Central Public Health Laboratory, 61 Colindale Avenue, NW9 5HT London, U.K.. MPMI 5:179-186. Accepted 23 December 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1992.

To investigate the role of dicarboxylate transport in nitrogen-fixing symbioses between Rhizobium and tropical legumes, we made a molecular genetic analysis of the bacterial transport system in Rhizobium sp. NGR234. This broad host range strain fixes nitrogen in association with evolutionarily divergent legumes. Two dicarboxylate transport systems were cloned from Rhizobium NGR234. One locus was chromosomally located, whereas the other was carried on the symbiotic plasmid (pSym) and contained a dctA carrier protein gene, which was analyzed in detail. Although the DNA and derived amino acid sequences of the structural gene were substantially homologous to that of R. meliloti, its promoter sequence was quite distinct, and the upstream sequence also exhibited no homology to dctB, which is found at this position in R. meliloti. A site-directed internal deletion mutant in dctA of NGR234 exhibited a (unique) exclusively symbiotic phenotype that could grow on dicarboxylates ex planta, but could not fix nitrogen in planta. This phenotype was found for tested host plants of NGR234 with either determinate- or indeterminate-type nodules, confirming for the first time that symbiosis-specific uptake of dicarboxylates is a prerequisite for nitrogen fixation in tropical legume symbioses.