Coupling of flagellar gene expression to flagellar assembly in salmonella enterica serovar typhimurium and Escherichia coli

How do organisms assess the degree of completion of a large structure, espe cially an extracellular structure such as a flagellum? Bacteria can do this . Mutants that lack key components needed early in assembly fail to express proteins that would normally be added at later assembly stages. In some ca ses, the regulatory circuitry is able to sense completion of structures bey ond the cell surface such as completion of the external hook structure. In Salmonella and Escherichia coli, regulation occurs at both transcriptional and posttranscriptional levels. One transcriptional regulatory mechanism in volves a regulatory protein, FlgM, that escapes from the cell land thus can no longer act) through a complete flagellum and is held inside when the st ructure has not reached a later stage of completion. FlgM prevents late fla gellar gene transcription by binding the flagellum-specific transcription f actor sigma (28). FlgM is itself regulated in response to the assembly of a n incomplete flagellum known as the hook-basal body intermediate structure Upon completion of the hook-basal body structure, FlgM is exported through this structure out of the cell. Inhibition of sigma (28)-dependent transcri ption is relieved, and genes required for the later assembly stages are exp ressed, allowing completion of the flagellar organelle. Distinct posttransc riptional regulatory mechanisms occur in response to assembly of the flagel lar type III secretion apparatus and of ring structures in the peptidoglyca n and lipopolysaccharide layers. The entire flagellar regulatory pathway is regulated in response to environmental cues. Cell cycle control and flagel lar development are codependent. We discuss how all these levels of regulat ion ensure efficient assembly of the flagellum in response to environmental stimuli.