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Cells acclimate to fluctuating environments by utilizing sensory circuits. One common sensory pathway used by bacteria is two-component signaling (TCS), composed of an environmental sensor [the sensor kinase (SK)] and a cognate, intracellular effector [the response regulator (RR)]. The squid symbiontVibrio fischeriuses an elaborate TCS phosphorelay containing a hybrid SK, RscS, and two RRs, SypE and SypG, to control biofilm formation and host colonization. Here, we found that another hybrid SK, SypF, was essential for biofilms by functioning downstream of RscS to directly control SypE and SypG. Surprisingly, although wild-type SypF functioned as an SKin vitro, this activity was dispensable for colonization. In fact, only a single non-enzymatic domain within SypF, the HPt domain, was criticalin vivo. Remarkably, this domain within SypF interacted with RscS to permit a bypass of RscS's own HPt domain and SypF's enzymatic function. This represents the firstin vivoexample of a functional SK that exploits the enzymatic activity of another SK, an adaptation that demonstrates the elegant plasticity in the arrangement of TCS regulators.The squid symbiont, Vibrio fischeri, uses a unique two-component signaling phosphorelay (Syp) to regulate whether it forms biofilms and colonizes its host. Although this pathway contains two functional sensor kinases (RscS and SypF), only a subset of domains within these enzymes is critical in vivo. Together, these domains control the activity of two downstream response regulators (SypE and SypG) leading to biofilms and colonization.