Protein Phosphorylation and Bacterial Chemotaxis

Bacteria are able to respond to changes in concentration of a large variety of chemicals and to changes in physical parameters, including viscosity, osmolarity, and temperature, by swimming toward a more favorable location (for review, see Stewart and Dahlquist 1987). Most chemotactic responses are mediated by a series of transmembrane receptor proteins that interact with or bind specific chemicals and thus monitor environmental conditions. Changes in ligand concentration detected by these receptors are transduced into two kinds of intracellular signals, adaptation and excitation signals. The adaptation signal acts as feedback, resulting in the covalent modification of specific glutamic acid residues in the cytoplasmic portion of the transmembrane receptor protein and thus in the modulation of receptor function (Springer et al. 1979). The excitation signal regulates the direction of rotation of bacterial flagella, causing changes in the “tumbling” or “smooth-Swimming” behavior of the cell (Larsen et al. 1974).