$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
Chemotaxis in Escherichia coli is a sensory-driven motility mechanism that enables bacteria to navigate chemical gradients, moving toward beneficial environments while avoiding harmful conditions. This process relies on a signal transduction system integrating external chemical cues with flagellar motor control.
Chemoreceptors and Signal Detection
E. coli detects chemical gradients through methyl-accepting chemotaxis proteins (MCPs), which are membrane-bound chemoreceptors that sense attractants (e.g., sugars, amino acids) and repellents (e.g., toxic compounds). MCPs do not respond to absolute concentrations but instead detect temporal concentration changes, allowing the bacterium to compare current conditions with past stimuli as it moves.
CheA and CheY: Signal Transduction Pathway
When an attractant or repellent binds to an MCP, the receptor undergoes a conformational change that modulates the activity of CheA, a sensor kinase. CheA regulates bacterial motility through phosphorylation of the response regulator CheY:
- Attractant Detection: The binding of an attractant decreases CheA autophosphorylation, leading to reduced levels of phosphorylated CheY (CheY-P).
- Repellent Detection: The binding of a repellent increases CheA autophosphorylation, generating more CheY-P molecules.
Once phosphorylated, CheY diffuses through the cytoplasm and interacts with the flagellar motor, altering the direction of rotation.
Flagellar Motor Control and Bacterial Movement
E. coli moves by rotating its flagella, which can switch between counterclockwise (CCW) and clockwise (CW) rotation:
- CCW Rotation (Smooth Swimming): When CheY-P levels are low (due to attractant binding and reduced CheA phosphorylation), the flagella rotate counterclockwise, allowing the bacterium to swim in a directed, straight-line motion toward the attractant.
- CW Rotation (Tumbling): When CheY-P levels are high (due to repellent binding and increased CheA phosphorylation), the flagella rotates clockwise, causing the bacterium to tumble randomly. Tumbling reorients the cell, helping it change direction and move away from repellents.
Adaptive Methylation and Memory
To maintain sensitivity to prolonged stimuli, MCPs undergo methylation via CheR, an enzyme that adds methyl groups, adjusting receptor activity. CheB, activated by CheA phosphorylation, removes these methyl groups. This methylation-based adaptation ensures that E. coli remains responsive to changes in chemical gradients over time, preventing desensitization to persistent attractants.
Biological Significance
Chemotaxis enhances bacterial survival by directing movement toward nutrient-rich environments and away from toxic conditions. This efficient and dynamic navigation system enables E. coli to optimize its metabolic resources and adapt to fluctuating environmental conditions, illustrating the sophisticated regulatory mechanisms in prokaryotic motility.
