Differential Regulation of the Multiple Flagellins in Spirochetes

The expression of flagellin genes in most bacteria is typically regulated by the flagellum-specific sigma28 factor FliA, and an anti-sigma28 factor, FlgM. However, the regulatory hierarchy in several bacteria that have multiple flagellins is more complex. In these bacteria, the flagellin genes are often transcribed by at least two different sigma factors. The flagellar filament in spirochetes consists of one to three FlaB core proteins and at least one FlaA sheath protein. Here, the genetically amenable bacteriumBrachyspira hyodysenteriae was used as a model spirochete to investigate the regulation of its four flagellin genes,flaA ,flaB1 ,flaB2 , andflaB3 . We found that theflaB1 andflaB2 genes are regulated by sigma28 , whereas theflaA andflaB3 genes are controlled by sigma70 . The analysis of a flagellar motor switchfliG mutant further supported this proposition; in the mutant, the transcription offlaB1 andflaB2 was inhibited, but that offlaA andflaB3 was not. In addition, the continued expression offlaA andflaB3 in the mutant resulted in the formation of incomplete flagellar filaments that were hollow tubes and consisted primarily of FlaA. Finally, our recent studies have shown that each flagellin unit contributes to the stiffness of the periplasmic flagella, and this stiffness directly correlates with motility. The regulatory mechanism identified here should allow spirochetes to change the relative ratio of these flagellin proteins and, concomitantly, vary the stiffness of their flagellar filament.