The genome of the opportunistic pathogen Pseudomonas aeruginosa encodes over 60 two-component sensor kinases and uses several (including RetS and GacS) to reciprocally regulate the production of virulence factors involved in the development of acute or chronic infections. We demonstrate that RetS modulates the phosphorylation state of GacS by a direct and specific interaction between these two membrane-bound sensors. The RetS-GacS interaction can be observed in vitro, in heterologous systems in vivo, and in P. aeruginosa. This function does not require the predicted RetS phosphorelay residues and provides a mechanism for integrating multiple signals without cross-phosphorylation from sensors to noncognate response regulators. These results suggest that multiple two-component systems found in a single bacterium can form multisensor signaling networks while maintaining specific phosphorelay pathways that remain insulated from detrimental cross-talk. Two-component system (TCS) signaling pathways are a major signaling mechanism in bacteria and archaea, and are also found in simple eukaryota and higher plants (Wolanin et al. 2002). These diverse organisms capitalize on TCS pathways to monitor critical external and internal stimuli (including levels of nutrients, concentration of ions and gases, temperature, redox states, and cell density) and translate these signals into adaptive responses. Classical TCS pathways share a conserved core architecture: a homodimerizing histidine kinase protein domain (the ''sensor'') and a cognate receiver domain (the ''response regulator''), coupled mechanistically through a histidine-aspartic acid phosphorelay (Stock et al. 2000). Most cognate sensor response regulator pairs are also linked genetically, encoded by adjacent loci in the chromosome (Alm et al. 2006). Although a single bacterial species can encode up to hundreds of genes specifying TCS pathways, it appears that these systems are insulated against detrimental cross-phosphorylation between sensors and noncognate response regulators (Bijlsma and Groisman 2003;Baker and Stock 2007;Laub and Goulian 2007). The identification of multistep phosphorelays (with intermediary proteins between sensor and regulator) and branched pathways (phosphotransfer between one sensor and multiple response regulators and vice versa) (Laub and Goulian 2007) suggests that TCS pathways have the capacity to form sensitive and complex signaling networks.In contrast to microorganisms with restricted habitats, the genomes of bacteria capable of occupying a number of diverse environments typically contain a disproportionately large number of genes encoding signal transduction and regulatory systems, including TCSs that allow them to sense and respond to a wide range of environmental signals. For opportunistic bacterial pathogens, a number of these systems regulate the expression of genes necessary for transitioning from the environmental reservoir to the host, overcoming innate defense mechanisms and initiating the disease process. The bacterial pathogen ...