Broadly conserved FlgV controls flagellar assembly and dissemination in mice.

Flagella propel pathogens through their environments yet are expensive to synthesize and are immunogenic. Thus, complex hierarchical regulatory networks control flagellar gene expression. Spirochetes are highly motile bacteria, but peculiarly in the Lyme spirochete , the archetypal flagellar regulator σ is absent. We rediscovered gene in as , a broadly-conserved gene in the flagellar superoperon alongside σ in many Spirochaetes, Firmicutes and other phyla, with distant homologs in Epsilonproteobacteria. We found that FlgV is localized within flagellar motors. lacking construct fewer and shorter flagellar filaments and are defective in cell division and motility. During the enzootic cycle, lacking survive and replicate in ticks but are attenuated for dissemination and infection in mice. Our work defines infection timepoints when spirochete motility is most crucial and implicates FlgV as a broadly distributed structural flagellar component that modulates flagellar assembly.