Organization of the flagellar biosynthesis transcriptional hierarchy. The early genes flhDC are transcribed from the class I promoter and encode the master transcriptional regulator FlhD4C2 which is able to bind within the class II promoter to activate transcription of the middle assembly genes in a σ70-dependent manner. The middle assembly genes encode the hook-basal body structure which spans the inner and outer membrane, the sigma factor FliA (σ28) and the anti-sigma factor FlgM. Once the hook-basal body is fully assembled, FlgM is exported through the hook-basal body allowing FliA to activate transcription of the late assembly genes from the class 3 promoter. Late assembly genes encode flagellin and proteins required for flagellar rotation and chemotaxis.