This chapter discusses the transmembrane signal transducing proteins. The life of a bacterium is characterized by changing environmental conditions. To compete and survive, these cells have evolved signal transduction pathways that allow them to adapt to different surroundings. Responses range from changes in gene expression leading to altered metabolism and differentiation to behavioral responses, such as chemo-taxis that allow cells to move to environments that are more favorable. Adaptive responses are generally mediated by signal transduction pathways initiated by receptors within the cell envelope that communicate with signal transduction proteins in the cytoplasm. This chapter focuses on the structure and function of the cell surface receptors. The sensory inputs that initiate signal transduction pathways in bacteria are diverse and often quite complex. Cells are sensitive to virtually every aspect of their environment, including the concentrations of a large number of different small molecules such as peptides, amino acids, sugars, oxygen, and phosphate, as well as parameters such as osmotic pressure, pH, temperature, and light. Environmental changes are monitored by sensory receptors, and this information is used to regulate appropriate adaptive response mechanisms.
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