SUMMARY How do organisms assess the degree of completion of a large structure, especially 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 cases, the regulatory circuitry is able to sense completion of structures beyond 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 involves a regulatory protein, FlgM, that escapes from the cell (and thus can no longer act) through a complete flagellum and is held inside when the structure has not reached a later stage of completion. FlgM prevents late flagellar gene transcription by binding the flagellum-specific transcription factor ς28. FlgM is itself regulated in response to the assembly of an 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 ς28-dependent transcription is relieved, and genes required for the later assembly stages are expressed, allowing completion of the flagellar organelle. Distinct posttranscriptional regulatory mechanisms occur in response to assembly of the flagellar type III secretion apparatus and of ring structures in the peptidoglycan and lipopolysaccharide layers. The entire flagellar regulatory pathway is regulated in response to environmental cues. Cell cycle control and flagellar development are codependent. We discuss how all these levels of regulation ensure efficient assembly of the flagellum in response to environmental stimuli.

SummaryThe flagellar-specific anti-sigma factor, FlgM, inhibits the expression of late flagellar genes until the hookbasal body structure is assembled and competent for export of the flagellins and hook-associated proteins (flagellar late proteins). FlgM monitors this assembly checkpoint by being a substrate for export via the hook-basal body structure, which includes a type III protein secretion complex. Amino acid sequence alignment of late-secreted flagellar proteins identified a region of homology present in the aminoterminus of FlgM and the other late flagellar proteins, but not in flagellar proteins secreted earlier during flagellar biosynthesis. Single amino acid substitutions at specific positions within this motif decreased the export of FlgM. Deletion of this region (S3-P11) resulted in lower intracellular FlgM levels, but did not prevent recognition and export by the flagellarspecific secretion system. Mutations were isolated in a second region of FlgM spanning residues K27 to A65 that exhibited increased anti-28 activity. These FlgM 'hyperinhibitor' mutants were secreted less than wild-type FlgM. Mutations that interfere with the secretion of FlgM without abolishing anti-28 activity have a negative effect upon the secretion of a Histagged FlgM mutant that lacks anti-28 activity. Models are proposed to explain the dominant negative phenotype of the FlgM secretion mutants reported in this study.