Dicarboxylic acid transport in Bradyrhizobium japonicum: use of Rhizobium meliloti dct gene(s) to enhance nitrogen fixation

Abstract

Arch. Microbiol. 144:142-146, 1986) was used to study the relationship between dicarboxylic acid transport and nitrogen fixation in Bradyrhizobium japonicum. The expression of the dct sequences on plasmid pRK290:4:46 in B. japonicum CJ1 resulted in increased growth rates in media containing dicarboxyiic acids as the sole source of carbon. In addition, strain CJl(pRK290:4:46) exhibited enhanced succinate uptake activity when grown on dicarboxylic acids under aerobic conditions. Under free-living nitrogen-fixing conditions, strain CJt(pRK290:4:46) exhibited higher nitrogenase (acetylene reduction) activity co'mpared with that of the wild-type strain. This increase in nitrogenase activity also correlated with an enhanced dicarboxylic acid uptake rate under these microaerobic conditions. The regulation of dicarboxylic acid transport by factors such as metabolic inhibitors and the presence of additional carbon sources was similar in both the wild-type and the engineered strains. The implications of increasing nitrogenase activity through alterations in the dicarboxylic acid transport system are discussed.Nitrogen fixation by the Rhizobium-legume symbiosis is a high-energy-demanding process. The infected host plant supplies bacteroids with photosynthate, which is converted to ATP, and reducing power for the support of nitrogen fixation and the maintenance of nodule functions. The nitrogen-fixing capacity of the Rhizobium-legume symbiosis is thought to be limited by the amount of photosynthate available to the bacteroid (14,27). Therefore, knowledge of the carbon compounds supplied to bacteroids is a prerequisite step for formulating any strategy aimed at improving the nitrogen fixation process.Results of biochemical studies suggest that dicarboxylic acids (succinate, malate, and fumarate) play an important role in the symbiotic nitrogen fixation process. Dicarboxylic acids are the most effective substrates for respiration and promnoting nitrogen fixation by isolated bacteroids (2). The isolation of mutants defective in dicarboxylic acid transport (dct) in a number of Rhizobium strains has also indicated the vital role played by these compounds in symbiotic nitrogen fixation. Dicarboxylic acid transport mutants of Rhizobium leguminosarum (1, 10, 12), Rhizobium trifolii (25), and Rhizobium meliloti (3) that are unable to transport dicarboxylic acids but capable of utilizing other carbon sources form ineffective nodules on their respective host plants. Furthermore, Rhizobium strains that are defective in succinate dehydrogenase (11) and malate dehydrogenase (31) activity have also been found to be incapable of effective symbiotic nitrogen fixation. Taken together, these results indicate the essential nature of the transport and subsequent rnetabolism of dicarboxylic acids for the nitrogen fixation process.With regard to the regulation of the dicarboxylic acid transport system, evidence suggests that succinate, malate, and fumarate are transported via a common system in both cultured cells and bacteroids of Bradyrhizobi...

Factors Influencing Nodule Occupancy by Inoculant Rhizobia

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Control of gluconeogenic growth by pps and pck in Escherichia coli

“…To our knowledge, Pps and Pck are the only examples of suboptimally produced enzymes in central metabolism, other than transport proteins such as LacY in E. coli (10) and the dicarboxylic acid transport protein (d/ct gene product) in Bradyrhizobilni japonicium (2). Other studies have shown that the wild-type levels of 3-galactosidase, GltA, and enzyme III ' of the phosphotransferase system are optimal for E. coli growth on lactose (8), acetate (28), and on glucose (25), respectively.…”

Improved analysis of metabolite uptake by Bradyrhizobium japonicum bacteroids

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