Functional Characterization of Integrated Chemotaxis Pathways and Proteins in the Soil Bacterium, <i>Azospirillum brasilense</i>

Chemotaxis is the biased movement of cells in a chemical gradient. Most of what we know about chemotaxis comes from work in Escherichia coli, which encodes one chemotaxis signaling pathway. However, recent genome sequencing provided evidence that most chemotactic bacteria encode for more than one chemotaxis signaling pathway, but little is known about how these multiple pathways function. In the soil bacterium, Azospirillum brasilense, two chemotaxis pathways are used to control motility in response to chemicals. We aim to characterize these pathways and determine whether or not interaction exists between proteins of the two pathways. We find that paralogs of the two chemotaxis pathways can interact to form mixed arrays of signaling clusters. Furthermore, we characterize a specific protein in one of these pathways, CheA1, and find that it is present as two isoforms, unlike canonical CheA proteins. This is made possible by the presence of an additional domain on the N-terminus which is not involved in chemotaxis but instead responsible for a cell length defect in cheA1 mutants. Characterization of this domain, TMX, reveals that it is present as a single domain protein in Bacteria and Eukarya and predicted to span the membrane seven times. TMX affects the localization of membrane-associated protein, alters membrane properties, and directly controls membrane fluidity in response to temperature shifts. Here, we characterize TMX’s role as a universal fluidity sensor in bacteria and point to a possible mechanism of action.