Schematic representation of the flagella biosynthesis regulon. In E. coli K12 the master regulator FlhDC activates the transcription of seven operons, one of which encodes the secondary activator FliA. FliA in turn activates the operons that are regulated by FlhDC, as well as additional operons. Efficient shutdown of flagella synthesis in the non-motile bacteria S. flexneri and Y. pestis is achieved by the loss of the major activator FlhDC. Nonsense mutations in genes of the regulated operons are then gradually accumulated.