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James L O McKellar, Jordan J Minnell, and Monica L Gerth., Mol Microbiol, 2015
Anyone who has swatted away a hungry mosquito - or followed their own nose towards fresh coffee - will appreciate the usefulness of chemosensory cues for insects and mammals. Microorganisms also display surprisingly sophisticated sensory behaviours. By monitoring changes in the chemical composition of their environment, microbes move towards chemo-attractants (e.g. food sources) and flee from chemo-repellents (e.g. noxious chemicals and toxins). This process, termed chemotaxis, allows microbes to navigate towards more favourable environments. For many microbial pathogens, cheomotaxis is a critical part of host invasion and colonisation.
We have begun to explore the structural and functional diversity of the chemoreceptors from Pseudomonas syringae pv. actinidiae (Psa). Psa is best known as the pathogen responsible for recent kiwifruit losses in New Zealand. It also has an unusually diverse chemosensory system, with 43 putative chemoreceptors encoded in its genome, none of which have been previously characterized. In this paper, we developed a high-throughput fluorescence-based thermal shift assay for identifying the ligands recognized by a given chemoreceptor. Using this assay, we screened 3 Psa chemoreceptors against 95 potential ligands. We found that each chemoreceptor recognized a distinct subset of amino acids. We further characterised one of these receptors, named pscA, using isothermal titration calorimetry, site-directed mutagenesis and chemotaxis assays.