The Escherichia coli flagellum is a highly complex, self-assembling biological nanomachine, representing one of the most studied examples of bacterial motility. This rotary apparatus allows the bacterium to navigate its environment, a function fundamental to its survival. Flagella enable E. coli to move toward nutrient sources or away from harmful substances, a process known as taxis. The construction of this apparatus is energetically costly, demanding precise and tightly regulated genetic control. This motility mechanism is a primary factor in the bacterium’s ability to disperse and infect host tissues.
The Three-Part Physical Architecture
The E. coli flagellum is structurally separated into three components: the basal body, the hook, and the filament. The basal body serves as the anchor and motor complex, embedded within the Gram-negative cell envelope. It contains four rings: the L and P rings in the outer membrane and peptidoglycan layer, and the S and M rings forming the core rotary motor near the plasma membrane.
Extending outward from the basal body is the hook, a short, curved structure that acts as a universal joint. This flexible connector transmits the motor’s torque to the external propeller.
The filament is the longest part, a hollow, helical tube that can extend up to 15 micrometers from the cell surface. It is composed of thousands of polymerized subunits of the protein flagellin (FliC). This protein self-assembles at the distal tip, forming the propeller that pushes the bacterium through liquid media.
The Motor and Mechanics of Motility
The physical mechanism driving flagellar rotation is the bacterial flagellar motor (BFM), a multi-protein complex located within the basal body. The BFM is powered not by ATP hydrolysis, but by the proton motive force (PMF), which is the electrochemical gradient across the inner cell membrane.
The stator units (MotA and MotB proteins) are embedded in the inner membrane and form proton channels. As protons flow through these channels into the cytoplasm, the released energy generates the torque that drives the rotor’s rotation.
Counter-clockwise (CCW) rotation causes the flagella to twist into a cohesive bundle, resulting in a smooth forward movement called a “run.” Clockwise (CW) rotation forces the bundle apart, leading to a disorganized “tumble.” This tumble randomly reorients the cell, setting a new direction for the next run phase.
Sensing the Environment: Chemotaxis
E. coli uses the alternating run-and-tumble motion for chemotaxis, which is directed movement in response to chemical gradients. Signal recognition begins with Methyl-Accepting Chemotaxis Proteins (MCPs), which are transmembrane receptors clustered at the cell poles. These MCPs bind to specific attractants and repellents.
When an attractant binds, it inhibits the autophosphorylation of the sensor kinase, CheA. CheA normally phosphorylates the response regulator CheY, creating CheY-P. CheY-P is the direct signal that interacts with the motor switch protein, FliM, to induce clockwise rotation and a tumble.
Moving up an attractant gradient keeps CheA inhibited, resulting in less CheY-P and a prolonged CCW rotation that extends the run. The cell also uses an adaptation mechanism involving the methyltransferase CheR and the methylesterase CheB. These enzymes adjust the methylation state of the MCPs, resetting the receptor’s sensitivity to the background chemical concentration.
Hierarchical Genetic Control and Assembly
The construction of the flagellum involves the sequential expression of over 50 genes arranged in a transcriptional cascade. This hierarchy is divided into three classes, ensuring components are manufactured in the correct order. The process begins with the expression of the Class I gene, the flhDC operon.
The FlhD4C2 heterohexamer, the product of this operon, acts as the master regulator, activating Class II gene transcription. Class II genes encode the hook-basal body components and the regulatory proteins FliA and FlgM. Completion of the hook-basal body serves as an architectural checkpoint controlling the transition to Class III gene expression.
Once assembled, the anti-sigma factor FlgM is secreted through the central channel. This releases the sigma factor FliA (σ²⁸), which was previously inhibited by FlgM. Free FliA binds to RNA polymerase, initiating the transcription of Class III genes. These genes encode external components, such as flagellin and the chemotaxis proteins, completing the functional apparatus.