Flagellum rotation

Bacteria such as Eschericia coli and Salmonella typhimurium swim by rotating flagella that lie on their surfaces (Figure 34.28). When the flagella rotate in a counterclockwise direction (viewed from outside the bacterium), the separate flagella form a bundle that very efficiently propels the bacterium through solution. 

The balance of forces decrees that when flagella rotate the cell body counter-rotates. If a single flagellum is held fast, a CCW-spinning flagellum turns the cell body CW, and a CW-spinning flagellum turns the cell body CCW. Thus, if a cell is tethered to the bottom of a coverslip and viewed from above, attractants lead to periods of CW-only rotation and repellents lead to briefer periods of CCW-only rotation. 

In swimming, the flagella, which are helical, rotate and thereby exert thrust that drives the bacteria. While the flagella rotate in one direction, the cell body rotates more slowly in the other direction [78]. Bacteria swim relatively quickly. For example, the swimming speed of a rodshaped cell (usually 1-5 xm in length) of enteric bacteria like E. coli and Salmonella is 10-35 xm/s [74,451, 752], and that of rod-shaped soil bacteria like Pseudomonas aeruginosa is even 2-3-fold higher [275, 752]. Marine bacteria swim much faster, up to 200 xm/s [510]. 

The flagella of bacteria such as . coli and Salmonella can rotate counterclockwise and clockwise (the direction of rotation defined for a flagellum viewed from its distal end towards the bacterial cell), and they can also pause [209,405, 407, 591,658]. A pause seems to result from a futile switching attempt from counterclockwise to clockwise [211]. Under non-stimulated conditions, the flagella rotate mostly counterclockwise with brief intermissions of clockwise rotation and pauses. The flagellar motors of other bacterial species may similarly have three functional states, or they may only have two states rotation in one direction and pausing (e.g., in K sphaeroides), or rotation in both directions without pausing (e.g., in Pseudomonas) (Table 1). 

The switch can be directly modified by at least two intracellular constituents the response regulator CheY and fumarate. (The CheY-switch interaction and its outcome will be discussed in Section 7.5. The effect of fumarate will be discussed in Section 8.2.8.) Other factors that affect switching are the proton-motive force and the temperature. As the proton-motive force is reduced, the motor becomes more and more counterclockwise biased, until at about 70% of the maximal motor speed the flagella rotate exclusively counterclockwise [346]. Conversely, in a process that is independent of the presence of CheY and fumarate, switching increases when the temperature decreases [748]. The reason is that the standard free energy difference (AC°) between the clockwise and counterclockwise states of the switch becomes lower at low temperatures. The molecular mechanism underlying this change is not known. 

Spirochetes Flagella rotate without pausing, resulting in coordinated rotation of the two polar bundles. Consequently, the cell swims in a straight line. Flagella pause frequently and extensively, disrupting the coordinated rotation of the two polar bundles. Consequently, the cell flexes and pauses. [232]... 

Macnab, R.M. (1977). Bacterial flagella rotating in bundles A study in helical geometry. Proc. Natl. Acad. Sci. U.S.A 74, 221—225. 

Figure 34.32. Proton Transport-Coupled Rotation of the Flagellum. (A) MotA-MotB may form a structure having two half-channels. (B) One model for the mechanism of coupling rotation to a proton gradient requires protons to be taken up into the outer half-channel and transferred to the MS ring. The MS ring rotates in a counterclockwise direction, and the protons are released into the inner half-channel. The flagellum is linked to the MS ring and so the flagellum rotates as well.

McClain, J., RoUo, D. R, Rushing, B. G., and Bauer, C. E. (2002) Rhodospirillum centenum utilizes separate motor and switch components to control lateral and polar flagellum rotation./. Bacteriol. 184, 2429-2438. 

FIGURE 12-26 The two-component signaling mechanism in bacterial chemotaxis. When an attractant ligand (A) binds to the receptor domain of the membrane-bound receptor, a protein His kinase in the cytosolic domain (component 1) is activated and autophosphorylates on a His residue. This phosphoryl group is then transferred to an Asp residue on component 2 (in some cases a separate protein in others, another domain of the receptor protein). After phosphorylation on Asp, component 2 moves to the base of the flagellum, where it determines the direction of rotation of the flagellar motor. 

The shaft and rings at the base of the flagellum make up a rotary motor that has been called a "proton turbine." Protons ejected by electron transfer flow back into the cell through the turbine, causing rotation of the shaft of the flagellum. This motion differs fundamentally from the motion of muscle and of eukaryotic flagella and cilia, for which ATP hydrolysis is the energy source. 

Some bacteria boast a marvelous swimming device, the flagellum, which has no counterpart in more complex cells.8 In 1973 it was discovered that some bacteria swim by rotating their flagella. So the bacterial flagellum acts as a rotary propeller—in contrast to the cilium, which acts more like an oar. 

When Walker was a postdoc, people argued whether this was rotation, or whether it was beating in a sinusoidal fashion. The key experiment was that somebody made an antibody that recognized the end of the flagellum and they took a cover slip and coated it with the antibody and added the bacteria to it. This then trapped the bacteria attached by the tip of their tail, and one could look in the fight microscope and see the bacteria turning around in this fashion. This was the first demonstration of rotation in these motile bacteria. The ATP experiment mentioned above was a derivative from this bacteria motility experiment, a macroscopic demonstration of a microscopic chemical event. 

It is an interesting question whether there is any common evolutionary origin between the two phenomena. According to Walker, it may turn out to be that the rotary motor of the flagellum and the rotary motor that drives this rotation in the catalytic part of ATP-synthesizing enzyme have a common evolutionary origin. However, that is only a hypothesis. 

Bacterial flagellar motor H+/Na+ gradient Stator and rotor proteins, flagellum Plasma membrane Rotation of flagellum attached to rotor... 

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