Chemotaxis and Receptor Localization

Abstract

Most motile bacteria are able to migrate towards higher concentrations of certain chemicals (attractants), which are usually nutrients or signaling molecules. At the same time, bacteria can avoid higher concentrations of repellents, potentially harmful chemicals. As the small size of a bacterial cell makes direct spatial detection of chemoeffector gradients inefficient, bacteria have evolved a chemotaxis strategy that differs substantially from that commonly employed by larger eukaryotic cells. Bacterial chemotaxis relies on temporal -rather than spatial -comparisons of chemoeffector concentrations, which are performed while moving in a gradient. Bacteria therefore have to swim first and only then can decide whether the chosen direction is favorable or not. Escherichia coli -the best-studied model for bacterial chemotaxis -has two types of swimming. A smooth swimming run, which propels the cell forward, results from the counterclockwise rotation of the flagellar motors and bundling of the flagellar filaments. Reorienting tumbles are produced by the clockwise motor rotation and dissociation of the flagellar bundle. In the adapted state with no gradients present, runs (around 2 s duration) are constantly interrupted by short tumbles (around 0.1 s) and as a result the cell performs a random walk that allows it to efficiently explore its environment [1]. In the presence of a gradient cells bias their random walk (Figure 9.1A). Although the swimming direction after each tumble is chosen nearly randomly, swimming in a favorable direction in a gradient suppresses tumbles and thus results in longer runs.The chemotactic response is mediated by a signaling system that relies on protein phosphorylation and is a member of a large class of bacterial two-component sensors. The chemotaxis pathway is well conserved across bacteria and Archaea, with the E. coli pathway (Figure 9.1B) being the best-studied example [2, 3]. The two central signaling proteins are the histidine kinase CheA and the response regulator CheY. In contrast to a typical two-component sensory kinase, CheA does not have a sensory domain, but instead -together with the adaptor protein CheW -associates with the dedicated chemosensory receptors at the membrane. Ternary complex Bacterial Signaling. Edited by

Myxococcus xanthus Swarms Are Driven by Growth and Regulated by a Pacemaker

“…FrzE in addition includes a CheY domain, unlike the corresponding E. coli proteins (25). A recent review of chemotaxis and receptors in E. coli is available for comparison with M. xanthus data (35). The absence of any receptor saddles interested parties with the hope that an "unidentified receptor protein in the cell membrane recognizes external stimuli and conveys this information to FrzCD" (25).…”