The supercoiled forms of the flagellar filaments are thought to be constructed from a mixture of two distinct subunit conformations arranged in a regular manner. We analyzed the structure of one of the two straight flagellar filaments, each of which is built up with all its subunits in one of the two conformations. The filament we studied was isolated from the strain SJW1655 of Salmonella typhimurium and had a right-handed helical symmetry With recent advancements in electron cryomicroscopy such as a liquid helium temperature stage for frozen hydrated specimens and a stable field emission source, and also by averaging high resolution data with a proper correction of the contrast transfer function, the density distribution map of this straight flagellar filament was generated in far more detail than before by including data up to 9 Å resolution. The structure shows a densely packed core region from about 15 to 55 Å in radius, where a pair of concentric tubular features of high density is present without well-defined subunit boundaries, and an outer part from 55 to 115 Å, where the subunits are mostly well separated from each other. The outer tube in the core region, from 35 to 55 Å in radius, contains many rod-like features with near-axial orientation and closest lateral distances of around 10 Å, which are most likely to represent the α-helical bundles that were predicted in our previous report. In the inner tube, from 15 to 30 Å in radius, the rod-like features are less clear. Between the inner and outer tubes are the short spoke-like densities, which are radially tilted and are connecting the two tubes. The outer part, from 55 to 115 Å, contains an axially elongated column density and a slewed projection with a narrow neck region. When compared with the other straight filament having left-handed helical symmetry, this outer part does not show any significant changes in orientation, suggesting that the switch in the subunit conformation and packing involved in the polymorphic transitions is quite subtle and only occurs within the core region. Reassignment of each structural domain to the amino acid sequence is suggested, based on the volume of each domain, which was determined rather precisely by a proper correction of the contrast transfer function for both amplitudes and phases.
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