In order to ensure their survival, bacteria must sense and adapt to a variety of environmental signals. Motile bacteria are able to orient their movement in a chemical gradient by chemotaxis. During chemotaxis, environmental signals are detected by chemotaxis receptors and are propagated via a signal transduction cascade to affect bacterial motility. In a model organism Escherichia coli, chemotaxis receptors, also called MCPs (for methyl-accepting chemotaxis proteins) sense changes in concentration gradients by making temporal comparisons about the chemical composition of their surroundings. Decreased attractant concentration or increased repellant concentration results in conformational changes in the MCPs that culminate in autophosphorylation of histidine kinase CheA that in turn phosphorylates response regulator CheY. Phosphorylated CheY interacts with flagellar rotor switch protein FliM and causes it to switch direction of rotation.
In E. coli, MCPs form mixed trimers-of-receptor dimers. Together with CheA and CheW proteins they further organize into large patches at the cell poles called arrays. This architecture is important for signal amplification and propagation and is universally conserved among many bacterial species. In contrast to E. coli, nitrogen-fixing soil bacteria, Azospirillum brasilense, encode four chemotaxis pathways and 41 MCPs. Previous work shows both Che1 and Che4 contribute to chemotaxis and aerotaxis implying that signals detected by chemotactic receptors must be integrated to generate a coordinated motility response. In this work, fluorescent microscopy imaging studies of some A. brasilense MCPs (Tlp1, Tlp2, Tlp4a, and AerC) in various mutant backgrounds demonstrate their localization in respect to each other and to CheA1 and CheA4 proteins
