Redundancy in Citrate and -Aconitate Transport in .

Tricarboxylates such as citrate are the preferred carbon sources for Pseudomonas aeruginosa, an opportunistic pathogen that causes chronic human infections. However, the membrane transport process for the tricarboxylic acid cycle intermediates citrate and -aconitate is poorly characterized. Transport is thought to be controlled by the TctDE two-component system, which mediates transcription of the putative major transporter OpdH. Here, we search for previously unidentified transporters of citrate and -aconitate using both protein homology and RNA sequencing approaches. We uncover new transporters and show that OpdH is not the major citrate importer; instead, citrate transport primarily relies on the tripartite TctCBA system, which is encoded in the operon. Deletion of causes a growth lag on citrate and loss of growth on -aconitate. Combinatorial deletion of newly discovered transporters can fully block citrate utilization. We then characterize transcriptional control of the operon in mutants and show that loss of blocks citrate utilization due to an inability to express . However, and mutants evolve heritable adaptations that restore growth on citrate as the sole carbon source. Pseudomonas aeruginosa is a bacterium that infects hospitalized patients and is often highly resistant to antibiotic treatment. It preferentially uses small organic acids called tricarboxylates rather than sugars as a source of carbon for growth. The transport of many of these molecules from outside the cell to the interior occurs through unknown channels. Here, we examined how the tricarboxylates citrate and -aconitate are transported in P. aeruginosa. We then sought to understand how production of proteins that permit citrate and -aconitate transport is regulated by a signaling system called TctDE. We identified new transporters for these molecules, clarified the function of a known transport system, and directly tied transporter expression to the presence of an intact TctDE system.