Building bacterial flagella

Most bacterial movement is the result of the action of a subcellular structure, the flagellar organelle. Bacterial flagella propel the cell by rotating, and this rotation is regulated in response to information transmitted by chemoreceptors on the surface of the cell. Rotation is driven by a motor anchored in the cell membrane. To understand such processes as the assembling of flagellar organelles, energy transduction to produce flagellar rotation, and the integration of sensory information necessary for chemotaxis, one prerequisite is the determination of the architecture of flagella. Emphasis is given here to experimental approaches using Escherichia coli and Salmonella in order to identify the flagellar components and to determine how these components are used to construct the functional organelle. The purification of apparently intact organelles has revealed an intricate structure composed to at least 11 polypeptide components. Genetic techniques have been developed which enable the identification of the components of the flagellar system. More than 35 genes which are necessary for flagellar function have been defined in E. coli, and the gene products of 17 of the genes have been identified. From such genetic studies, it is apparent that the flagellar system is complex and includes many elements in addition to those found in the intact flagellar structures purified from the cell membrane. Proteins required for energy and sensory transduction have been identified and located in the cytoplasm and cell membrane. Since these components are not integral parts of the isolated organelle, it is clear that flagellar movement requires the interaction of several functionally distinct and spatially separate systems. The organelle appears to be constructed in discrete stages consisting of the sequential association of individual components. As with bacteriophage assembly, some flagellar proteins function not as structural components, but to regulate the assembly of the organelle. A structural variation is found in Salmonella, which have the ability to alternately express one of two components of the flagellar filament. The unique genetic control mechanism regulating this structural variation is discussed.