Cross-regulation between the response regulators PhoB and TctD allows for the integration of diverse environmental signals in Pseudomonas aeruginosa

Two-component systems (TCS) serve as basic stimulus-response coupling mechanisms to allow organisms to sense and respond to a large variety of environmental conditions. In the opportunistic pathogen Pseudomonas aeruginosa, more than 100 genes encode TCS components. To avoid unwanted cross-talk between TCSs, the signaling cascades are usually very specific, with one sensor talking to its cognate response regulator. However, cross-regulation may provide effective means to integrate different environmental stimuli into a harmonized output response and thus to coordinate gene expression in response to complex and challenging habitats. Here, we applied a protein-fragment complementation assay (PCA) and identified a direct interaction of two response regulators of the OmpR/PhoB subfamily, namely PhoB and TctD in P. aeruginosa in vivo. Transcriptional profiling and ChIP-seq analysis uncovered adjacent bipartite binding motifs of the two response regulators in 8 promoter regions, of which 6 showed a common PhoB-TctD binding motif with a fixed spacing between the two motifs. We further demonstrate that phosphate limitation exhibits a PhoB-dependent activating activity on the expression of those genes, whereas the presence of a preferred carbon source inhibits gene expression in a TctD-dependent manner. Thus, the perception and the integration of two important environmental signals are achieved by a titration of the relative amounts of two phosphorylated response regulators that inversely regulate a common subset of genes. In conclusion, our results shed light on the general principles underlying two-component signal transduction pathways and the exploitation of cross-regulation to adapt bacterial behavior to complex environments. Overall design: Transcriptome profiling of the UCBP-PA14 phoB and tctD mutants complemented by phoB ChIP-seq experiments.