Chapter: Genetics and Molecular Biology: Chemotaxis

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Chemotaxis

Higher organisms have the ability to sense their environment and to process and store information.

Chemotaxis

Higher organisms have the ability to sense their environment and to process and store information. Additionally, higher organisms fre-quently show an excitatory response to a stimulus, but they adapt to continued application of the stimulus and ultimately produce a dimin-ished response. We shall see in this chapter that bacteria perform these functions using a relatively small number of proteins. Some of these are similar in structure to proteins that appear to perform similar function in higher cells. Therefore the principles learned about these much simpler systems should be most helpful in understanding similar sys-tems found in higher organisms.

Genetics, physiology, and biochemistry, the tools of molecular biol-ogy that have been so useful in the study of relatively simple questions, can also be applied to the study of biological phenomena as complicated as behavior. Bacteria have been used in these studies because they respond to their environment and are amenable to genetic and bio-chemical studies.

Bacteria respond to their environment by swimming toward some chemicals like sugars, amino acids, and oxygen and away from others like phenol. This response is called chemotaxis. It involves a number of steps. The bacteria must be able to sense the chemical in question, choose the appropriate direction to swim, and be able to swim. That is, they must contain elementary sensors, computers, and motors. How such elements can be assembled from simple components is a major reason for interest in such studies. A second interest is, as mentioned above, that analogs of some of the individual components of a bacterial system are used in other more complicated systems. It will be much easier to learn how the bacterial systems work since genetic, physiologi-cal, and biochemical experiments are all much quicker and less expen-sive for these systems than for systems from higher cells.

In the last century, microbiologists observed chemotactic behavior of bacteria in the microscope. Motile bacteria swim rapidly, in contrast to nonmotile bacteria strains which vibrate slightly due to Brownian motion. Chemotactic bacteria are motile and direct their motility to move themselves toward attractants and away from repellents. The swimming is accomplished by moving long, slender flagella that are attached to the cell wall.

To swim in the correct direction, bacteria must be capable of deter-mining in which direction the concentration of an attractant or repellent increases or decreases. This means being able to measure concentration differences and relating them to direction in space. It might appear that one way a bacterium could do this would be to compare the concentra-tion of an attractant at its two ends. This simple method has a major flaw, however. If the swimming bacterium is catabolizing the attractant, then the cell surface at the front end encounters a higher average concentration than the back end. Thus the bacterium loses the ability to sense the external concentration of the attractant. Whatever direction it is currently swimming seems to be the direction of increasing concen-tration. To obtain a better determination of attractant or repellent gradient, the bacterium can measure the average concentration of attractant over its entire surface and then move to a different point and measure the concentration again. If the bacterium can retain memory of the direction it moved between the measurements, then it can tell whether attractant or repellent has a concentration gradient in this direction. Complicating the situation is random rotation of the cell. Brownian motion will significantly jostle the cell and reorient it on a time scale of 3-10 seconds. Thus, in order for the “measure-swim-meas-ure” process to work, both the “current” as well as the “less current” concentration measurements must be completed in several seconds.


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