Organisms within all domains of life must acclimate to fluctuating environments to survive. To do this, cells utilize sensory circuits, which function to connect environmental stimuli to an intracellular response. One common sensory pathway utilized 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 marine bacterium Vibrio fischeri uses an elaborate TCS phosphorelay containing a hybrid SK, RscS, and two RRs, SypE and SypG, to colonize its natural squid host, Euprymna scolopes. This TCS pathway regulates V. fischeri\u27s ability to form a biofilm, or a community of cells encased in an extracellular matrix, a process required to initiate colonization. Between the sypE and sypG genes lies sypF, which encodes another putative hybrid SK. Due to its location and predicted function, I hypothesized that sypF might also regulate biofilms. Indeed, I found that SypF was critical for biofilms by functioning downstream of RscS to directly control SypE and SypG. Surprisingly, although a mutant variant of SypF, SypF*, functioned as an SK both in vitro and in vivo, this did not seem to be the case for wild-type SypF. Specifically, wild-type SypF exhibited SK activity in vitro, but this activity was dispensable for colonization. In fact, only a single non-enzymatic domain within SypF, the HPt domain, was critical in vivo. Remarkably, this domain within SypF directly interacted with RscS, permitting a bypass of RscS\u27s own HPt domain and SypF\u27s enzymatic function. These unique findings represent the first in vivo example 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. This flexibility in TCS pathways likely permits bacteria to acutely manage a vast repertoire of different environments thus promoting their survival both inside and outside a hos
