The molecular basis of excitation and adaptation during chemotactic sensory transduction in bacteria

Chemotaxis is the process by which cells sense chemical gradients in their environment and then move towards more favorable conditions. In the case of Escherichia coli, the paradigm organism for chemotaxis, the pathway is now arguably the best characterized in all of biology. If one broadens their perspective to include other species of bacteria, then our knowledge of chemotaxis is far less developed. In particular, the chemotaxis pathways in unrelated species are quite different despite the conservation of many core signaling proteins. Here, we summarize the current state of knowledge regarding the chemotaxis pathways in E. coli and Bacillus subtilis, with a specific focus on the mechanisms for excitation and adaptation. The mechanisms vary widely, and the B. subtilis process, similar to those found in Thermotoga maritima and many archaea, may represent a new paradigm for bacterial chemotaxis. For instance, B. subtilis has three interacting means for restoring prestimulus behavior after stimulation, including one involving CheYp feedback. The one shared with E. coli, the receptor methylation system, is vastly different, as is the mechanism for conveying signals across the membrane.