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DctBD-Dependent and -Independent Expression of the Sinorhizobium (Rhizobium) meliloti C₄-Dicarboxylate Transport Gene (dctA) During Symbiosis

The Sinorhizobium meliloti C₄-dicarboxylate transport gene (dctA) is essential for symbiotic nitrogen fixation. Under free-living conditions, the expression of dctA is fully dependent on the cognate regulatory genes dctBD. However, during symbiosis with the Medicago sativa host plant, the dctA gene is efficiently expressed even in the absence of the dctBD genes. The spatial expression of the dctA gene has been monitored in situ in mature nitrogen-fixing nodules formed by wild-type and dctD mutant strains. In nodules induced by a wild-type strain, expression was observed in both the infection zone and the nitrogen-fixing zone of the nodule. DctD-independent expression of dctA was observed with a previously described dctA∷lacZ fusion (pCU700) and was found to be confined to the fixation zone (zone III) of mature nodules. Therefore, the operation of the alternative system of symbiotic dctA activation (ASA) is concomitant with the onset of nitrogen fixation, which could be consistent with an increased need for transport of (C)₄(-dicarboxylic acids by the nitrogen-fixing) bacteroids. Sequences in the 5′ part of the dctA coding region were found to be essential for the activation of the dctA∷lacZ gene fusions by the ASA. Deletion of these sequences resulted in gene fusions that were found to be strictly dependent on dctBD for expression, under all conditions tested including symbiosis. Such gene fusions allowed us to establish that the DctBD-dependent dctA expression was occurring throughout the whole nodule.