Escherichia coli propels itself through liquid by rotating its flagella. Normally the flagella are left-handed helices and their rotation generates a thrust that moves the cell. In this section we consider the structure of flagella, how it’s known they rotate, how the rotation is created, and how the several flagella present on a single cell can function together.

Flagella are too thin to be easily seen by ordinary light microscopes, but they can be visualized with light microscopes operating in the dark-field mode, interference microscopes, video processing, or electron microscopes. Careful isolation of flagella shows that they are attached to a hook-shaped structure connected to a set of rings that is embedded in the cell’s membranes. The rings have the appearance of a motor that rotates the flagella (Fig. 1). The hook is a flexible connector between the basal structure and the flagella. Such a universal joint is necessary because in E. coli the flagella sprout from random points on the cell’s surface and point in several different directions. A cell typically contains about six flagella. These must join together in a bundle in a way that permits each to rotate in response to its motor. The hook acts as a universal joint that permits the torque to be transmitted around a bend.

Fig1. The appearance of a flagellum at low magnification in the electron microscope and the structure of the basal body, the motor, at high magnification showing the rings and the membranes of the cell wall.

Because of the size of the flagella, indirect means must be used to demonstrate that they rotate. One simple experiment uses the observation that antibody against flagellin can block motility. More precisely, bivalent antibody blocks motility, but monovalent antibody does not (Fig. 2). This result can be understood if the flagella form a bundle and each flagellum rotates within this bundle. A bivalent antibody molecule can link different flagella and prevent their rotation, but if flagella waved or rotated as a group, bivalent antibodies would have an effect no different from monovalent antibodies.

Fig2. A bacterium with flagella sprouting from various locations bending via the hook portion and coming together in a bundle. The section taken at point A shows flagella rotating individually and their movement if the bundle waves as a whole.

The most graphic demonstration that flagella rotate is also the basis of many other important experiments on chemotaxis. Simon used one mutation to block flagellin synthesis and another to permit greater than usual growth of the hook. As the synthesis of flagella is sensitive to catabolite repression, growth of cells in glucose reduced the number of the resulting polyhooks from about six per cell to about one. These cells could be bound to a microscope slide by means of antihook antibody that had bound to the hook and nonspecifically bound to the glass as well (Fig. 3). Chemotactic cells immobilized in this way rotated at two to nine revolutions per second. This leads to the conclusion that the hook normally rotates, but when it was fastened to the microscope slide instead, the cell rotated. Nonmotile cells did not rotate. Of course, motile but nonchemotactic mutants did rotate because they can swim but do so in a nondirected fashion and are incapable of swimming up a gradient.

Fig3. Demonstration of the hook’s rotation. After attachment of the hook or a short flagellum to a cover slip, the cell rotates, and this can easily be observed.

The immobilization experiment shows that a single flagellum rotates and that the rotation is associated with chemotaxis. Dark-field micros copy has shown that the bundle of flagella on a cell is stable as long as the flagella rotate counterclockwise. If the flagella reverse their rotation, their left-helical structure compels the bundle to fly apart temporarily. Furthermore, if the reversal is sufficiently vigorous, the flagella snap into a right-helical conformation. This further ensures that the bundle of flagella disperses.

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مواضيع ذات صلة

Fundamental Properties of Chemotaxis

Assaying Chemotaxis

Lambda Phage Assembly: Formation of the Tail

Lambda Phage Assembly: Packaging the DNA and Formation of the cos Ends

Lambda Phage Assembly: The Head Assembly Sequence and Host Proteins

Lambda Phage Assembly: The Geometry of Capsids

Determining Details of Local Ribosomal Structure

Experiments with in vitro Ribosome Assembly

The Global Structure of Ribosomes

RNAse and Ribosomes

Assaying for Sequence Requirements of Gene Rearrangements

The D Regions in H Chains