Regulation of Flagellar Assembly. Much like its assembly, regulation of flagellar genes is hierarchal and based on three promoter classes in Gram-negative enteric bacteria. At the top of the flagellar gene hierarchy is the flagellar master operon flhDC expressed from a class I promoter. The decision to assemble flagella is dependent on flhDC expression which is highly regulated in response to changes in the environment, including nutrient availability, pH, osmolarity, and temperature (130-134). The regulation of flhDC will be discussed in detail later in this chapter. The FlhD and FlhC proteins encoded by the flhDC operon comprise the hetero-hexameric FlhD4C2 transcriptional regulator. FlhD4C2 binds large asymmetric repeats present in class II promoters to support σ70-dependent transcription of these genes. Genes transcribed from class II promoters (9 operons, 38 genes) encode proteins of the hook and basal body substructures (HBB) and regulators which drive the expression of class III promoters. These regulators include fliA (σ28), flgM (anti-σ28), flgN, and fliT. The regulator σ28 drives the expression of class III promoters (16 operons, 29 genes) encoding flagellar filament structural proteins, hook associated proteins, the flagellar motor, and chemotactic proteins (135). The class II regulators can have dual roles as secretion chaperones and transcriptional regulators. Chaperones were once thought to prevent proteolysis of a substrate before secretion or direct localization of a substrate to the secretion machinery. Alas, a mechanism has emerged in which secretion chaperones function as transcriptional regulators allowing for strict calibration in the assembly of complex structures such as a flagellum. Secretion chaperones that are dual regulators in the flagellar transcriptional cycle are FliA (σ28), FlgN, and FliT (116, 118). Each affects transcription of flagellar genes in a different manner following secretion of their substrates at the HBB. The secretion chaperone FliA drives transcription of class III promoters, and the secreted substrate of FliA is FlgM (anti-σ28). Both are expressed from class II and class III promoters (118, 136). Class II-expressed FlgM serves as an internal checkpoint in flagellar assembly by binding to FliA in the cytoplasm and preventing interaction of σ28 with RNA polymerase (137, 138). Once the HBB is completed, the flagellar T3SS changes its affinity for HBB structural proteins to late secreted substrates. Following secretion of FlgM by FliA the ratio o...
