Chemotaxis enables bacteria to sense their immediate environment and quickly adapt to changes in it. The ability to
sense changes in the environment and quickly respond to them is essential to the cell's life (3).
The bacterial chemotaxis system has evolved to answer this need.
Chemotaxis is the movement of an organism towards or away from a chemical stimulus, as is presented in figure 1. The most common sensory pathways in prokaryotes use a chemotaxis system that contains at least two components - a dimeric histidine protein kinase (HPK) and a response regulator (RR).
Fig. 1: Chemotaxis concept, adapted from iGEM gottingen 2012.
The E.coli chemotaxis system is considered a model system that demonstrates some of the core principles of chemotaxis. Through use of its flagella, E.coli has the ability to move rapidly towards attractants and away from repellents, by means of a random movement, with “runs” and “tumbles” by rotating its flagellum counter-clockwise and clockwise, respectively.
When E.coli moves in a medium that lacks a concentration gradient – the cell travels,
stops or tumbles and then continues moving in a new random direction.
When the flagella rotate counter-clockwise it forms a tight bundle and directs the cell forward in a straight running motion. After a brief period, the direction of rotation is reversed, causing a tumble. As the cell moves up the chemical gradient the runs become longer in comparison to moving down the gradient.
The overall result is random movement in the absence of a chemical gradient, and movement towards or away from a chemical when a gradient exists (3).
The response of E.coli to environmental signals is achieved by intercellular reactions.
Chemo-sensing is carried out by a broad repertoire of chemoreceptors. Most have a N-terminal
region that spans the membrane twice, which results in an intertwined periplasmic domain that
can sense an extracellular signal. The C-terminal cytoplasmic region comprises a
HAMP region (that connects the extracellular sensory domain with intracellular signaling domains), a dimerization domain and a kinase domain
that interacts with its RR (3)
Ligands bind to the periplasmic domain of the chemoreceptor, at the interface between the two monomers of the dimer, with residues from both monomers being involved in the binding process. Ligand binding alters the interactions between the periplasmic domains, and between the transmembrane dimer (3).
The cytoplasmic region of the chemoreceptors interacts with two additional proteins - CheA and CheW, to form stable ternary complexes that generate stimulus signals. The activation of the receptor by an external stimulus causes auto-phosphorylation in the histidine kinase, CheA. CheA, in turn, transfers phosphoryl groups to residues in the response regulators CheB and CheY, which are cytoplasmic proteins. CheW is an adaptor protein, physically bridges CheA to the chemoreceptor to allow regulated phosphor-transfer to CheY and CheB. CheB (methylesterase) regulates the chemoreceptor methylation state. It works oppositely with CheR, a methyltransferase. When an attractant is bound to the receptors binding domain – the chemoreceptor is methylated by CheR. Then, CheY transmits the signals to the flagellar motors. The phosphate in CheY is continuously removed by CheZ. The short life time of phosphorylated CheY means that the bacteria are very responsive to changes in attractant concentration. This mechanism of signal transduction is called two component system, and is common form of signal transduction in bacteria (1).
1. GREBE, Thorsten W.; STOCK, Jeff. Bacterial chemotaxis: the five sensors of a bacterium. Current Biology, 1998, 8.5: R154-R157.
2. John S. Parkinson, An overview of E. coli chemotaxis, Biology Department, University of Utah
3. WADHAMS, George H.; ARMITAGE, Judith P. Making sense of it all: bacterial chemotaxis. Nature Reviews Molecular Cell Biology, 2004, 5.12: 1024-1037.
4. WANG, Qingfeng, et al. Sensing wetness: a new role for the bacterial flagellum. The EMBO journal, 2005, 24.11: 2034-2042.