Agrobacterium fabrum (tumefaciens) Chemosensory System: A Typical Model of One Histidine Kinase for Two Coupling Proteins and Multiple Response Regulators.

Bacteria utilize chemotaxis to sense the surrounding chemical signals to seek a more favorable survival environment. The chemotaxis process includes signal sensing, signal transduction, and signal response (i.e., regulating flagellar rotation to control motility). Agrobacterium fabrum, as a soil-born facultative phytopathogen, can survive in diverse environments from bulk soil, the rhizosphere, to the plant-associated niches, and needs to cope with diverse challenges from various survival environments. It must recognize a variety of environmental signals and thus has evolved a chemosensory signaling system more complicated than the prototypical chemotaxis system. The chemosensory system of A. fabrum possesses one histidine kinase, but more chemoreceptors, coupling proteins (2 CheWs), and response regulators (2 CheYs and 1 CheB) than the well-studied prototypical system in the model bacterium Escherichia coli, which has only one CheW, one CheY, and fewer chemoreceptors. More response regulators imply that the chemosensory system may involve other physiological functions beyond chemotaxis. In this review, we outline the recent advances in the prototypical chemosensory signaling system and discuss the functions of protein components in A. fabrum's chemosensory system by comparing those proteins with the homologous proteins observed in the paradigm and other closely related species. Meanwhile, we place particular emphasis on reviewing the data about the chemosensory system of A. fabrum, propose a "one-system two-pathways" model depicting that A. fabrum possibly utilizes one histidine kinase to assemble two chemosensory signaling pathways, and envision future directions for studying this system. The insights provided will aid in understanding the diversity of chemosensory signaling pathways in other organisms and the molecular mechanism mediating the signal crosstalk among chemosensory signaling pathways.