Xue-Xian Zhang, Jonathan C. Gauntlett, Darby G. Oldenburg, Gregory M. Cook, Paul B. Rainey

CbrA is an atypical sensor kinase found in Pseudomonas: the autokinase domain is connected to a putative transporter of the sodium:solute symporter family (SSSF). CbrA functions together with its cognate response regulator CbrB and plays an important role in nutrient acquisition, including regulation of hut genes for the utilization of histidine and its derivative urocanate. Here we report a genetic and biochemical analysis of CbrA with focus on the function of the putative transporter domain. The work was initiated with mutagenesis of histidine uptake proficient strains to identify histidine-specific transport genes located outside of the hut operon. Genes encoding transporters were not identified, but mutations were repeatedly found in cbrA. This, coupled with [(3)H]histidine transport assays and further mutagenesis, implicated CbrA in histidine uptake. In addition, mutations in different regions of the SSSF domain abolished signal transduction. Site-specific mutations were made at four conserved residues: W55 and G172 (SSSF domain), H766 (H-box) and N876 (N-box). Mutations W55G, G172H and N876G compromised histidine transport, but had minimal effects on signal transduction. The H766G mutation abolished both transport and signal transduction, but capacity to transport histidine was restored upon complementation with a transport-defective allele of CbrA, most likely due to interdomain interactions. Our combined data implicate the SSSF domain of CbrA in histidine transport and suggest that transport is coupled to signal transduction.IMPORTANCENutrient acquisition in bacteria typically involves membrane-bound sensors that via cognate response regulators determine the activity of specific transporters. However, nutrient perception and uptake are often-coupled processes. Thus, from a physiological perspective it would make sense for systems to have evolved that couple the process of signaling and transport within a single protein and where transport is itself the stimulus that precipitates signal transduction. The CbrA regulator in Pseudomonas represents a unique type of sensor kinase whose autokinase domain is connected to a transporter domain. We present genetic and biochemical evidence that suggests that CbrA plays a dual role in histidine uptake and sensing, and that transport is dependent on signal transduction.