Fundamental Properties of Chemotaxis
The capillary tube assay permits several simple measurements delimiting methods by which cells accomplish chemotaxis. The first is that metabolism of attractant is not required for chemotaxis. An analog of galactose, fucose, is not metabolized by Escherichia coli, and yet this serves as an attractant in the capillary tube assay. Another line of evidence leading to this same conclusion is that some mutants that are unable to metabolize galactose are still able to swim toward galactose.
Since cells can swim toward or away from a wide variety of chemi - cals, it is likely that they possess a number of receptors, each with different specificity. Therefore the question arises as to how many different types of receptors a cell possesses. This question can be answered by a cross-inhibition test. Consider the case of fucose and galactose. Since their structures are similar, it seems likely that cells detect both chemicals with the same receptor protein. This conjecture
can be tested by using a high concentration of fucose in both the tube and the drop so as to saturate the galactose receptor and blind cells to the galactose that is placed only in the tube. Fucose does blind cells to a galactose gradient, but it does not blind them to a serine gradient. These findings prove that galactose and fucose share the same receptor, but that serine uses a different receptor.
By blinding experiments and the use of mutants, nine receptors have been found for sugars and three for amino acids (Table 22.1). The three amino acid receptors are not highly selective, and they allow chemotaxis of E. coli toward 10 different amino acids. Except for the glucose receptor, synthesis of the sugar receptors is inducible, as is the proline receptor. The glucose, serine, and aspartate receptors are synthesized constitutively.
The capillary assay also allows a convenient determination of the sensitivity of the receptors. By varying the concentration of attractant within the capillary, the ranges over which the receptor will respond can be easily determined. Typically, the lowest concentration is about 10-7 M, and the highest concentration is about 10-1 or 10-2 M (Fig. 22.3). It is not at all surprising that a detection system is no more sensitive than 10-7 M. First, at about 10-6 M the rate of diffusion of a sugar to a bacterium is just adequate to support a 30-minute doubling time if every sugar molecule reaching the cell is utilized. The ability to chemotact would not change this lower limit by much.
Figure 22.3 A typical re-sponse curve showing the number of cells that have en-tered the capillary tube as a function of the concentration of attractant placed in the tube.
The second limitation on detection sensitivity is the lowest concentration at which a sufficiently accurate measurement may be made in the one second measurement window set by the rate of random rotations. At a concentration of 10-7 M, the number of attractant molecules reaching the cell surface is so low that statistical fluctuations in their number necessitate averaging for a full second to obtain sufficient accuracy. Thus 10- 7 M is about the lowest practical concentration to which bacteria can be expected to chemotact.
The actual receptor proteins for the different sugars have been sought, and several have been identified biochemically. The receptors for galactose, maltose, and ribose are found in the periplasm outside the inner membrane but inside the peptidoglycan layer. These and other periplasmic proteins can be removed from cells by osmotic shock.
No periplasmic binding proteins have been found for the glucose, mannitol, and trehalose systems. Instead, these systems use receptors that are tightly bound to or located in the inner membrane. These receptors serve a double purpose as they also function in the group translocation of their substrates into the cell via the phosphotransferase transport system.
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