More Than One Way To Sense Chemicals

Every way makes my gain . . ." Othello, Act V, Scene I Sensing various chemicals in the environment and responding to changes in their concentrations is a fundamental property of a living cell.It is especially important for unicellular organisms that constantly interact with the environment.Microorganisms possess simple yet effective systems that allow them to regulate numerous cellular functions in response to changes in their surroundings (for recent reviews, see references (34 and 54).Active motility of microbial cells along chemical gradients, usually referred to as chemotaxis, is controlled by probably the best-studied signal transduction system.Taxis responses allow motile microorganisms to rapidly move toward a microenvironment optimal for their growth and survival.The mechanism of flagellar motility and its control via chemotaxis have been studied in great detail in Escherichia coli and Salmonella enterica serovar Typhimurium (for reviews, see references (8, 15, 55, and 56).Enteric bacteria measure concentrations of chemicals outside the cell using transmembrane receptors (chemotaxis transducers) that transmit information into the cell interior.Transmembrane chemoreceptors are arranged into arrays, resulting in the sensor system, which is extremely sensitive to subtle conformational changes (14, 55).The transmembrane signaling in E. coli is a current paradigm for chemical sensing (for a recent review, see reference (21).However, there is a growing evidence that transmembrane signaling is not the only way to sense chemicals.This minireview focuses on alternative strategies used by microorganisms in order to monitor the chemical composition of the environment. METABOLISM-INDEPENDENT CHEMICAL SENSINGTransmembrane chemoreceptors.In 1969, Julius Adler provided for the first time extensive biochemical and genetic evidence that E. coli chemotaxis is independent of uptake or metabolism of the chemical stimulus (1).For example, chemotaxis to galactose was normal in mutants deficient in galactose transport and metabolism.From that time on, research focused on the metabolism-independent information flow from membrane receptors to flagellar motors.E. coli possess five chemoreceptors, and four of them are involved in transmembrane signaling: Tar for aspartate, Tsr for serine, Trg for ribose