Domain Shuffling in a Sensor Protein Contributed to the Evolution of Insect Pathogenicity in Plant-Beneficial Pseudomonas protegens

Pseudomonas protegens is a biocontrol rhizobacterium with a plant-beneficial and an insect pathogenic lifestyle, but it is not understood how the organism switches between the two states. Here, we focus on understanding the function and possible evolution of a molecular sensor that enables P. protegens to detect the insect environment and produce a potent insecticidal toxin specifically during insect infection but not on roots. By using quantitative single cell microscopy and mutant analysis, we provide evidence that the sensor histidine kinase FitF is a key regulator of insecticidal toxin production. Our experimental data and bioinformatic analyses indicate that FitF shares a sensing domain with DctB, a histidine kinase regulating carbon uptake in Proteobacteria. This suggested that FitF has acquired its specificity through domain shuffling from a common ancestor. We constructed a chimeric DctB-FitF protein and showed that it is indeed functional in regulating toxin expression in P. protegens. The shuffling event and subsequent adaptive modifications of the recruited sensor domain were critical for the microorganism to express its potent insect toxin in the observed host-specific manner. Inhibition of the FitF sensor during root colonization could explain the mechanism by which P. protegens differentiates between the plant and insect host. Our study establishes FitF of P. protegens as a prime model for molecular evolution of sensor proteins and bacterial pathogenicity. Author SummaryPseudomonas bacteria are well-known for their capability of adapting to different environments, which enables them to interact with various host organisms. Pseudomonas protegens is a plant-associated biocontrol bacterium with lifestyles that are of interest for agricultural applications, among them one as a competitive root colonizer protecting plants against pathogenic fungi and the other as an insect pathogen invading and killing insect species of importance as pests in agriculture. We recently discovered that P. protegens produces a potent insecticidal toxin only during infection of insects but not when growing on plant roots. Since sensor proteins enable bacteria to sense and respond to changing environments and are important for pathogen-host interactions, we investigated whether a specific sensory protein could explain our observation. We found that this particular protein tightly controls toxin production and during its evolution has recruited a common sensor domain from a regulatory protein involved in control of nutrient uptake. This so-called domain shuffling event was important for the ability of P. protegens to produce its insecticidal toxin only when it infects insects. Our study provides a prime example of how a sensory system can evolve and contribute to the evolution of bacterial pathogenicity.