REGULATORY ASPECTS OF THE C4-DICARBOXYLATE TRANSPORT IN RHIZOBIUM-MELILOTI - TRANSCRIPTIONAL ACTIVATION AND DEPENDENCE ON EFFECTIVE SYMBIOSIS

The transcriptional regulation of the Rhizobium meliloti gene region coding for C4-dicarboxylate transport was analyzed by constructing transcriptional lacZ and translational phoA fusions. In the case of the regulatory genes dctB and dctD, it was found that both genes were constitutively expressed when grown in minimal medium supplemented with succinate or with glucose. The transcription of the dctB gene was always lower than that of the dctD gene. Since a transposon-induced mutation in dctB did not abolish the transcription of dctD, it was concluded that both genes were expressed from their own promoters. In contrast to the regulatory dct genes, the expression of the dctA gene coding for the C4-dicarboxylate permease, was regulated by C4-dicarboxylates. It was found that a dctA+ background was necessary for the induction of the dctA expression. In a dctA- background, the dctA promoter was no longer regulated by C4-dicarboxylates and was constitutively expressed at its highest level. Bacteroids isolated from alfalfa nodules gave similar results with respect to the regulation of the dct gene region; the dctB gene was transcribed at a lower level than dctD and a dctA+ background was necessary for the transcriptional regulation of the dctA gene. In addition, it was demonstrated that in bacteroids, the NifA protein was not involved in the expression of the dctA gene. In general, the dctA expression rate was approximately equal in effective and ineffective bacteroids. In contrast, the C4-dicarboxylate transport rate was reduced to 50 % in ineffective bacteroids, indicating that the remaining 50 % was necessary for the nitrogen fixation process.