Bacterial chemotaxis is a motility-based response that biases cell movement toward beneficial molecules, called attractants, and away from harmful molecules, also known as repellents. Since the species of the genus Pseudomonas are characterized by a metabolic versatility, these bacteria have developed chemotactic behaviors towards a wide range of different compounds. The specificity of a chemotactic response is determined by the chemoreceptor, which is at the beginning of the signaling cascade and which receives the signal input. The basic elements of a typical chemoreceptor are the periplasmic ligand binding domain (LBD), responsible for sensing environmental stimuli, and the cytosolic methyl-accepting (MA) domain, that interacts with other components of the cellular signaling cascade. Escherichia coli (E. coli), the traditional model in chemotaxis research, has 5 well-characterized chemoreceptors. However, genome sequence analyses have revealed that many other bacteria possess many more chemoreceptors, some of which with partially overlapping signal profiles. This high number of chemoreceptors complicates their study by the analysis of single chemoreceptor mutants. We have pursued an alternative strategy for chemoreceptor characterization which corresponds to the generation of chimeric receptors composed of the LBD of the chemoreceptor under investigation and the MA domain of an E. coli receptor (Tar). The chimer is then introduced into a chemoreceptor free mutant of E. coli and the chemotaxis of the resulting strain is entirely due to the action of this chimeric receptor. In this publication we describe the use of quantitative capillary and gradient plate assays to study Pseudomonas chemotaxis as well as E. coli strains harboring chimeric receptors.
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