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The bacterial flagellar switch that controls the direction of flagellar rotation during chemotaxis has a highly cooperative response. This has previously been understood in terms of the classic two-state, concerted model of allosteric regulation. Here, we used high-resolution optical microscopy to observe switching of single motors and uncover the stochastic multistate nature of the switch. Our observations are in detailed quantitative agreement with a recent general model of allosteric cooperativity that exhibits conformational spread--the stochastic growth and shrinkage of domains of adjacent subunits sharing a particular conformational state. We expect that conformational spread will be important in explaining cooperativity in other large signaling complexes.

Original publication




Journal article



Publication Date





685 - 689


Allosteric Regulation, Bacterial Proteins, Binding Sites, Escherichia coli, Escherichia coli Proteins, Flagella, Membrane Proteins, Methyl-Accepting Chemotaxis Proteins, Models, Biological, Models, Molecular, Molecular Motor Proteins, Monte Carlo Method, Protein Binding, Protein Conformation, Protein Subunits, Signal Transduction, Thermodynamics