Chemotaxis in Escherichia coli


Chemotaxis is regulated by chemoreceptors located in the cytoplasmic membrane or periplasm of the bacterium bind chemical attractants or repellents. This leads to either the methylation or demethylation of methyl-accepting chemotaxis proteins (MCPs) that in turn, eventually trigger either a counterclockwise or clockwise rotation of the flagellum.

For example, in Escherichia coli, which posesses peritrichous flagella, if the concentration of an attractant (def) stays the same or decreases, the MCPs become demethylated and this eventually leads to a clockwise rotation of the flagellum. When an attractant molecule is not bound to an MCP, ATP can donate a phosphate to a protein called CheA that, in turn donates the phoshate to another protein called CheY. The phosphorylated CheY then interacts with a switch protein called FliM at the base of the flagellum promoting clockwise rotation and tumbling. When attractant are bound to the MCPs, CheA molecules become dephosphorylated and the CheY molecules are unable to interact with the switch proteins and throw the switch. This leads to counterclockwise flagellar rotation and swimming in a run.

Once a bacterium has been successful in responding to its environment, it needs to reset its chemosensing system. This is accomplished by CheB molecules. CheB is a methylase; it removes methyl groups from the MCPs. When fully methylated, MCPs can no longer respond to attractants. As CheB molecules demethylate the MCPs, they become "reset" and can again respond to attractants. Fully methylated MCPs, on the other hand, respond best to increasing gradients of repellents.

In summary, an increasing concentration of attractant or decreasing concentration of repellent (def) (both conditions beneficial) causes less tumbling and longer runs; a decreasing concentration of attractant or increasing concentration of repellent (both conditions harmful) causes normal tumbling and a greater chance of reorienting in a "better" direction. As a result, the organism's net movement is toward the optimum environment..

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Updated: March, 2014

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