the cbrA/cbrB system is a two-component signal transduction system known to participate in the regulation of the cellular carbon/nitrogen balance and to play a central role in carbon catabolite repression in Pseudomonas species. cbrA is composed of a domain with similarity to proteins of the solute/sodium symporter family (SLC5) and domains typically found in bacterial sensor kinases. Here, the functional properties of the sensor kinase cbrA and its domains are analyzed at the molecular level using the system of the soil bacterium P. putida KT2440 as a model. It is demonstrated that CbrA can bind and transport L-histidine. Transport is specific for L-histidine and probably driven by an electrochemical proton gradient. The kinase domain is not required for L-histidine uptake by the SLC5 domain of CbrA, and has no significant impact on transport kinetics. Furthermore, it is shown that the histidine kinase can autophosphorylate and transfer the phosphoryl group to the response regulator CbrB. The SLC5 domain is not essential for these activities but appears to modulate the autokinase activity. A phosphatase activity of CbrA is not detected. None of the activities is significantly affected by L-histidine. the results demonstrate that cbrA functions as a L-histidine transporter and sensor kinase. Transporters integral to cytoplasmic membranes usually catalyze the selective uptake of nutrients or the extrusion of metabolic end products and toxic solutes. However, some of these transporters play a central role also in signal transduction 1,2. In bacteria, so-called trigger transporters (temporarily) interact with membrane components of signal transduction systems and modulate their activity 2. For example, the lysine transporter LysP allows activation of a CadC-dependent acid stress response only when lysine can be taken up from the environment 3. The C 4-dicarboxylite transporter DcuB and the glucose-6-phosphate transporter UhpC interact with histidine kinases of specific two-component systems (TCSs) and stimulate phosphotransfer to the cognate response regulators when the respective substrate is present 1,4. While the interaction of transporters with separate signal transductions systems and the functional consequences are relatively well understood, little is known about the role of transporters that are covalently linked to domains typically found in bacterial signaling cascades. Prominent examples are members of the solute/ sodium symporter family (SLC5) 5,6. Besides sodium-motive force-dependent transporters for proline (PutP of archaea and bacteria 7), monosaccharides (SGLT of bacteria and higher eukaryotes 8) and others 9-11 , the family contains bacterial proteins in which a complete SLC5 domain is connected via a STAC (SLC5 and TCS Associated Component) domain to domains found in histidine kinases or diguanylate cyclase 5,12,13. SLC5 transporters fused via STAC to histidine kinase domains are usually associated with response regulators and resemble TCSs. CbrA/ CbrB represents such a histidine kinase/response r...

Tripartite efflux systems of the ABC-type family transport a variety of substrates and contribute to the antimicrobial resistance of Gram-negative bacteria. PvdRT-OpmQ, a member of this family, is thought to be involved in the secretion of the newly synthesized and recycled siderophore pyoverdine in Pseudomonas species. Here, we purified and characterized the inner membrane component PvdT and the periplasmic adapter protein PvdR of the plant growth-promoting soil bacterium Pseudomonas putida KT2440. We show that PvdT possesses an ATPase activity that is stimulated by the addition of PvdR. In addition, we provide the first biochemical evidence for direct interactions between pyoverdine and PvdRT.

MxtR/ErdR and homologous two-component systems play important roles in the regulatory networks that control cell metabolism and influence bacterial-host interactions. Using the MxtR/ErdR two-component system of the plant growth-promoting soil bacterium Pseudomonas putida KT2440 as a model, this work elucidates the function of previously uncharacterized target genes of MxtR/ErdR and extends the knowledge of the physiological significance of the two-component system.

Histidine is an important carbon and nitrogen source of c-proteobacteria and can affect bacteria-host interactions. The mechanisms of histidine uptake are only partly understood. Here, we analyze functional properties of the putative histidine transporter HutT of the soil bacterium Pseudomonas putida. The hutT gene is part of the histidine utilization operon, and the gene product belongs to the amino acid-polyamine-organocation (APC) family of secondary transporters. Deletion of hutT severely impairs growth of P. putida on histidine, suggesting that the encoded transporter is the major histidine uptake system of P. putida. Transport experiments with cells and purified and reconstituted protein indicate that HutT functions as a high-affinity histidine : proton symporter with high specificity for the amino acid. Substitution analyses identified amino acids crucial for HutT function.