Abstract: The invention relates to DNA fragments encoding genes which complement dicarboxylic acid transport mutants of Rhizobium species to plasmids encoding such DNA sequences and to bacterial hosts carrying the said plasmids. The invention further relates to a method of increasing dicarboxylic acid transport in Rhizobium species and to a method of increasing the nitrogen fixing ability of plants by infecting or inoculating the plants with bacterial hosts containing the above described DNA fragments.