Team:Technion Israel/Chemotaxis
Chemotaxis explained
Bacteria are able to sense a vast range of environmental signals, from nutrient and toxin concentrations
to oxygen levels. In such a dynamic setting, 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.
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).
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, 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 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. This mechanism of signal transduction is called two component system, and is common form of signal transduction in bacteria (1).
Fig. 1: Signaling components and circuit logic (2).
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.
Fig. 2: Random and run movement of E.coli. (2).
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).
References:
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.
