大腸菌における走化性

E. coli uses methyl-accepting chemotaxis proteins (MCPs), membrane-bound chemoreceptors that detect temporal changes in attractant and repellent concentrations rather than absolute levels. MCPs sense sugars, amino acids, and toxic compounds, allowing bacteria to compare current conditions with past stimuli as they move through their environment, enabling precise navigation.

When an attractant binds to MCPs, CheA autophosphorylation decreases, producing fewer phosphorylated CheY molecules. With reduced CheY-P levels, the flagellum rotates counterclockwise, enabling smooth, directed swimming toward the attractant. This counterclockwise rotation allows the bacterium to maintain straight-line movement rather than tumbling randomly.

CheA is a sensor kinase activated by MCP conformational changes. CheA autophosphorylates and transfers phosphate to CheY, creating phosphorylated CheY (CheY-P). CheY-P diffuses through the cytoplasm and binds the flagellar motor, switching rotation direction between counterclockwise and clockwise based on chemical signals detected by the coordination of gene expression processes in bacteria.

MCPs undergo methylation by the CheR enzyme and demethylation by CheB to maintain sensitivity to prolonged chemical stimuli. This methylation-based adaptation prevents desensitization, ensuring E. coli remains responsive to changing chemical gradients over time and continues navigating effectively through variable environments.

Repellent binding to MCPs increases CheA autophosphorylation, generating more phosphorylated CheY molecules. Elevated CheY-P levels cause the flagellum to rotate clockwise, inducing random tumbles that reorient the cell, allowing E. coli to change direction and move away from harmful substances effectively.

Chemotaxis enables E. coli to navigate toward nutrient-rich environments and away from toxic conditions, optimizing metabolic resource allocation. This efficient, dynamic navigation system allows bacteria to adapt to fluctuating environmental conditions and enhance survival through directed movement rather than random swimming.

Counterclockwise (CCW) rotation occurs when CheY-P levels are low, producing smooth, straight-line swimming toward attractants. Clockwise (CW) rotation happens when CheY-P levels are high, causing tumbling and random reorientation. This switching mechanism allows E. coli to alternate between directed movement and exploratory tumbling.