The switch complex proteins, FliG, FliM, and FliN, are essential for assembly, counterclockwise/clockwise (CCW/CW) switching, and rotation of the flagellar motor. The switch complex comprises about 30 copies of FliG, about 30 copies of FliM, and greater-than 100 copies of FliN. A better understanding of the structure of the switch complex is essential if we are to understand how these proteins participate in flagellar assembly, CCW/CW switching, and torque generation.; The focus of this dissertation is structural and functional analysis of the switch-complex proteins of the bacterial flagellar motor. Chapter 1 is a general introduction to bacterial motility and chemotaxis. Chapters 2 and 3 deal with the structure of the C-terminal two-thirds of the flagellar rotor protein FliG. Chapter 4 presents the structure of the bulk of the protein FliN. Chapter 5 presents a speculative model for the switch complex based on all of the available structural information and suggests some future directions for study of the switch complex proteins.
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