Organization of Proteins from Multiple Chemotaxis Systems in Rhizobium Leguminosarum and Impact on Short-Term Root Colonization

Bacteria must sense and adapt to a variety of environmental signals in order to survive. Motile bacteria utilize chemotaxis to orient their movement within a chemical gradient. Chemotaxis receptors (chemoreceptors) located in the membrane detect environmental signals which are then propagated through a signal transduction cascade to alter the motility pattern. In the model organism Escherichia coli, chemoreceptors sense changes in concentration gradients and relay these changes to chemotaxis cytoplasmic signaling proteins, including the chemotaxis histidine kinase CheA. In E.coli, five chemoreceptors signals to a single set of chemotaxis signaling proteins. Chemoreceptors assemble with CheA and other chemotaxis proteins into large patches called receptor arrays at the cell poles. This architecture is important for signal amplification and propagation; therefore, it is universally conserved across many bacterial species. In contrast to E. coli, Rhizobium leguminosarum biovar viciae, a nitrogen-fixing soil bacterium, encodes two chemotaxis operons (Che1 and Che2) and 27 chemoreceptors. Previous work shows that Che1 is the major controller of chemotaxis while Che2 plays a minor role under laboratory conditions. However, it is not known how paralogs of chemotaxis proteins from both operons are organized to integrate signals into motility output. In this study, fluorescent microscopy to determine subcellular localization and protein-protein interactions were assessed between the histidine kinases from each operon (CheA1 and CheA2) as well as four chemoreceptors (HemAT, Mcp2A, McpC, McpD). We show that chemoreceptors segregate into distinct clusters based on length class, as expected. In addition, CheA1 could interact with both receptor clusters while CheA2 could only interact with one cluster. We also provide evidence for potential competition between CheA1 and CheA2 for integration into the arrays. Additionally, a preliminary study explores the role of different chemoreceptors in plant root colonization and suggests differential contributions of chemoreceptors to the competitive colonization of distinct rhizospheres.