P-396 - Beyond transport: The Dct system as a global regulator of carbon and nitrogen metabolism in rhizobia

Adequate carbon supply is crucial for rhizobia to colonize the rhizosphere and establish a nitrogen-fixing symbiosis with legumes. During both free-living and symbiotic stages, rhizobia preferentially utilize C4-dicarboxylates (e.g. succinate, malate), which they sense and transport via the dicarboxylate transport system (DctA, DctB, DctD).

This study demonstrates that Rhizobium leguminosarum employs carbon catabolite repression (CCR) to selectively assimilate dicarboxylates by repressing uptake of alternative carbon sources in their presence. Transport assays confirmed that for instance succinate represses uptake of the less preferred carbon source myo-inositol. Using a high-throughput luminescence-based screen, we identified CCR-defective mutants. Most mutations occurred in Dct genes, indicating that a functional Dct system is required for CCR. This repression is driven by inducer exclusion via post-transcriptional downregulation of ABC transporters and is dependent on active dicarboxylate transport by DctA. However, DctBD are essential for additional Dct system functions, including cross-regulation of ammonia assimilation, which occurs when DctA expression is decoupled from their control.

Our work suggests thus a dual role of the Dct system: Beyond dicarboxylate transport, it acts as a global regulator of carbon uptake and global metabolic balance between carbon and nitrogen. These findings highlight the central role of the Dct system in rhizobial metabolism and symbiosis.