FliA

FliA, also known as sigma factor 28 or σ28, is an alternative sigma factor used by bacteria to direct RNA polymerase to promoters of late flagellar genes. In Escherichia coli and many related bacteria, the flagellar gene network is organized into a regulatory cascade, and FliA specifically governs the transcription of class 3 flagellar genes required for final assembly of the flagellum and its motor.

FliA activity is tightly regulated. The master regulator FlhDC activates fliA expression, placing FliA within the positive regulatory hierarchy for flagellar synthesis. However, FliA is inhibited by the anti-sigma factor FlgM until the basal body and hook structures are assembled. Once the export apparatus for flagellar components becomes functional and assembly progresses, FlgM is secreted, freeing FliA to activate transcription of FliA-dependent genes, including fliC (flagellin) and other late structural and motor components necessary for a functional flagellum.

Functionally, FliA-driven gene expression is essential for flagellum formation and bacterial motility, enabling swimming and swarming behaviors. In addition to its role in motility, FliA can influence interactions with hosts and environmental niches in certain species, and its regulon has been reported to include a limited set of non-flagellar genes in some bacteria.

Evolutionarily, FliA is widely conserved among flagellated bacteria, particularly within the Proteobacteria. While the core mechanism—σ28 directing RNA polymerase to late flagellar promoters and being kept inactive by FlgM until assembly—is broadly conserved, the exact set of FliA targets can vary between species.