- The Sinorhizobium meliloti C 4 -dicarboxylate transport (Dct) system is essential for symbiotic nitrogen fixation. The dctA gene, encoding the C 4 -dicarboxylate permease, is expressed in both free living and symbiotic cells. But in free living cells expression of dctD and dctB is absolutely required for the expression of dctA. In this study, in order to investigate the effect of oxygen concentration on the induction of Dct system, E. coli DH5 strain which carries the plasmid-encoded dctABD operon was used in tube assays. It was found that the specific induction of Dct system occurred only at a certain depth under the surface of M63-0.6% agar media, suggesting that Dct system could respond to oxygen concentration during succinate-induced expression. Furthermore, when measured at different oxygen concentrations, the highest expression level was observed at oxygen concentration of 2%. Thus, we predict that in addition to dicarboxylates, the induction of Dct system may also regulated by oxygen concentration.