Characterization of bacterial chemotaxis receptors sensing and signaling (614.1)

Abstract

Motile bacteria navigate in gradients of diverse chemical signals (toward attractants or away from repellents) by chemotaxis. Chemotaxis is essential for the adaptation of motile bacteria to changes in their environment. Bacterial chemotaxis depends on a functioning signal transduction pathway comprising several cytoplasmic proteins that form a phosphorylation cascade. Chemotaxis signal transduction is initiated via sensing cues by dedicated chemoreceptors that are localized in the membrane. The sensing activity of a bacterial chemotaxis receptor, named Tlp1 from Azospirillum brasilense, was recently shown to be regulated by binding to the second messenger molecule, c‐di‐GMP and a novel mechanism of sensory adaptation was proposed. To further characterize this mechanism, a polyclonal antibody against the C‐terminal region of Tlp1 was purified. This antibody was then used to test whether it was c‐di‐GMP bound or c‐di‐GMP free. Tlp1 receptors could be distinguished, using the wild type strain and a mutant derivative over‐expressing c‐di‐GMP. Biophysical methods such as X‐ray crystallography are currently being used to characterize how c‐di‐GMP binding to the C‐terminal region of Tlp1 affects conformational change and signaling activity of this receptor. Although chemotaxis receptors are essential for bacteria to monitor their environments, the sensory specificity of bacterial chemoreceptors is largely unknown (88% unknown function) despite the public databases harboring over 22,000 such chemoreceptors. Tlp2 is a chemoreceptor from A. brasilense with a periplasmic domain of unknown function. Genetic and behavioral evidence indicate that Tlp2 senses nitrogenous compounds (ammonium and nitrate). Further characterization of the sensory specificity of Tlp2 for these compounds is being conducted using Isothermal Titration Calorimetry to determine the affinity of the isolated and purified recombinant sensory domain of Tlp2.