The Mechanism of Chemotaxis
As viewed in the microscope, chemotactic bacteria
appear to swim smoothly for about 20 bacterial lengths in 1 second, to tumble
for about 0.1 second, and then to swim smoothly in another direction. How this
run-tumble-run behavior is converted to overall swimming toward increasing
concentrations of an attractant was one of the major problems in chemotaxis
research.
Berg constructed an elaborate tracking microscope
that quantitated the movement of a chemotactic bacterium in three dimensions.
This showed that, indeed, the impression obtained from simple visual
observation was correct. Following a tumble, a cell’s subsequent run was in a
random direction. More important, however, was the behavior of bacteria under
conditions simulating the presence of a gradient in attractant. It is difficult
to control a gradient in the concentration of an attractant around the cell,
but it is likely that a cell detects not a spatial gradient but the change in
the concentration of attractant from one time to the next. Therefore, Berg
devised a system to vary the concentration of attractant with time.
Alanine aminotransferase catalyzes the
interconversion of alanine and α-ketoglutarate
to pyruvate and L-glutamate. Both alanine and glutamate are attractants, and
therefore the reaction cannot be used to create or destroy total attractant.
This problem can be overcome by using a mutant that is blind to alanine but not
blind to glutamate. Then the enzyme-catalyzed reaction can be used to vary the
concentration with time of attractant surrounding a cell. If alanine, α-ketoglutarate, and alanine aminotransferase are
put in the medium bathing the cell
being observed, then the concentration of
attractant, glutamate, in-creases with time. In this case, the runs of the
bacteria are longer than average. When the glutamate concentration is
decreasing as a result of adding glutamate, pyruvate, and alanine
aminotransferase to the bath-ing medium, the distribution of run lengths is the
same as when attractant concentration remains constant.
The technique of immobilizing a cell via its hook
or flagellum has also been highly useful in characterizing chemotaxis
responses. After such an immobilization, a microprobe containing attractant or
repel-lent can be positioned near a cell, and application of a brief electrical
pulse will drive a known amount of the chemical into the medium in the
immediate area (Fig. 22.7). If the concentration changes are within the linear
response range of the cell, the response to a brief pulse of
Figure
22.7 A method for generating small
impulses of attractant from themicropipette and observing the cell’s responses.
The microscope contains additional electronics that record the rotation of the
bacterium.
attractant contains all the information necessary
to predict the response of a cell to any function of attractant concentration
as a function of time. Any other function can be approximated as a series of
impulses of appropriate magnitude, and the responses to each of these can be
summed. Such a technique of circuit analysis is well known to electrical
engineers, but it has not yet been widely applied to biochemical systems.
Figure 22.8 A random walkand the same walk with the run duration increased when in the direction of increasing attrac-tant concentration.
All the experiments mentioned above show that cells
modulate the duration of their intervals of smooth swimming. Such a modulation
is sufficient for the cells to achieve a net drift up a gradient of attractant.
When a cell is swimming up a gradient in attractant concentration, that is,
experiencing an increase in attractant concentration, it decreases the chances
of its tumbling. When the attractant is decreasing, the average run duration is
the same as when the cell is in constant attractant concentration. These
properties mean that cells drift up gradients in attractant concentration, that
is, they undergo a biased random walk, taking lengthened steps when they are
moving up the gradient (Fig. 22.8).
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