Bacteria Undergo Chemotaxis Toward Amino Acids, Which Usually Indicates the Presence

Bacteria undergo chemotaxis toward amino acids, which usually indicates the presence of a food source.Chemotaxis receptors bind a particular amino acid and cause changes in the bacterial cell that induce the cell to move toward the source of the amino acid.Four types of chemotaxis receptor that mediate responses to different amino acids have been identified in a bacterium.The receptors are called ChrA, ChrB, ChrC, and ChrD.Each receptor specifically senses serine, aspartate, glutamate, or glycine, although you do not know which receptor senses which amino acid.You have been given a wild-type bacterial strain that contains all four receptors, as well as various mutant bacterial strains that are lacking one or more of the receptors.To figure out which receptor senses which amino acid, you conduct experiments in which you fill a capillary tube with an amino acid to attract the bacteria, dip the capillary tube into a solution containing bacteria, remove the capillary tube after 5 minutes, and count the number of bacteria in the capillary tube.Your results are shown in Table 16-26. $$\begin{array}{l}\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\underline{\text { number of cells in capillary tube} }\\\begin{array} { l l c c c c c } \text { strain } & \text { intact receptors } & \text { serine } & \text { aspartate } & \text { glutamate } & \text { glycine } & \text { none } \\\hline 1 & \text { ChrA, ChrB, ChrC, ChrD } & 590 & 1050 & 950 & 66 & 5 \\2 & \text { ChrA, ChrB, ChrC } & 7 & 840 & 770 & 130 & 8 \\3 & \text { ChrC, ChrD } & 340 & 7 & 590 & 6 & 6 \\4 & \text { ChrA, ChrC, ChrD } & 550 & 6 & 650 & 41 & 5 \\5 & \text { ChrB, ChrC, ChrD } & 700 & 590 & 850 & 9 & 8\end{array}\end{array}$$
Table 16-26
From these results, indicate which receptor is used for which amino acid.

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ChrA senses glycine, ChrB senses asparta...

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