We are interested in how cells respond to changes in their external chemical andphysical environments. Bacterial chemotaxis continues to provide us withinsights into the mechanisms that a simple, single cell uses to modify its behavior.These processes are analogous to those that are found in eukaryotic cells. Theyinvolve specific cell surface receptors, the activation of protein kinases, andprotein phosphatases. In the first step in the process, ligands, e.g., amino acidsor sugars, bind to cell surface receptors. The receptor can initiate intracellularprotein phosphorylation or receptor modification. The interaction between thecontrol of protein phosphorylation (the excitation process) and the regulation ofreceptor responsiveness (desensitization) accounts for many of the behavioralfeatures of the organism. We are trying to understand how changes in ligandconcentration relate to these receptor-mediated processes.