The flhDC operon, or flagellar master operon, is under the control of numerous global regulatory signals that lead to the expression or inhibition of flagellar gene expression. Induction of the class I flhDC operon (class I on) produces FlhD and FlhC, which form a heteromultimeric complex, FlhD4C2, that acts to direct σ-dependent transcription from class II flagellar promoters and auto-repress flhDC transcription (class I off; class II on). Class II promoters direct the transcription of genes that are necessary for the structure and assembly of the hook–basal body (HBB) substructure. Upon HBB completion, late secretion substrates are exported from the cell and their cognate chaperones are released to regulate gene expression. FliT is an FlhD4C2 factor and prevents both FlhD4C2 auto-repression and the activation of class II promoters. The σ transcription factor directs the transcription of class III promoters, which include the filament structural genes and the genes of the chemosensory pathway (class I on; class II off; and class III on). Activation of class I transcription would re-initiate the flagellar regulon for a new round of flagellar gene expression. As drawn, the FliK and FlhA proteins are meant to reside within the C ring. The stoichiometries of Fluke, FlhA, FlhB, FliO, FliP, FliQ and FliR within the C ring are not known.