This subproject is one of many research subprojects utilizing theresources provided by a Center grant funded by NIH/NCRR. The subproject andinvestigator (PI) may have received primary funding from another NIH source,and thus could be represented in other CRISP entries. The institution listed isfor the Center, which is not necessarily the institution for the investigator.Many types of bacteria propel themselves through liquid media using whip-like structures known as flagella (www.ks.uiuc.edu/Research/flagellum/). The bacterial flagellum is a huge (several micrometers long, 20 nm wide), multiprotein assembly built of three domains: a basal body, acting as a motor; a hook, acting as a joint; and a filament, which makes up the bulk of the length of the flagellum and interacts with solvent to propel the bacterium. Depending on the direction of the torque applied by the basal body, the filament assumes different helical shapes. Under counter-clockwise rotation (as viewed from the exterior of the cell), several flagella form a single helical bundle which propels the cell along a straight line, referred to as running mode [1]. Under clockwise rotation, the individual flagella dissociate from the bundle and form separate right-handed helices, causing the cell to tumble. Varying the duration of running and tumbling, bacteria can move up or down a gradient of an attractant or repellent by a biased random walk. One of the unresolved questions about the flagellum is how the reversal of torque applied by the motor results in a switching between the helical shapes of the filament. This switching is a result of polymorphic transitions in the filament, when individual protein units slide against each other [2], but its molecular mechanism remains poorly understood despite extensive experimental work [3, 2, 4] and a recent computational study [5].
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