A Single-Domain FlgJ Contributes to Flagellar Hook and Filament Formation in the Lyme Disease Spirochete Borrelia burgdorferi

FlgJ plays a very important role in flagellar assembly. In the enteric bacteria,flgJ null mutants fail to produce the flagellar rods, hooks, and filaments but still assemble the integral membrane-supramembrane (MS) rings. These mutants are nonmotile. The FlgJ proteins consist of two functional domains. The N-terminal rod-capping domain acts as a scaffold for rod assembly, and the C-terminal domain acts as a peptidoglycan (PG) hydrolase (PGase), which allows the elongating flagellar rod to penetrate through the PG layer. However, the FlgJ homologs in several bacterial phyla (including spirochetes) often lack the PGase domain. The function of these single-domain FlgJ proteins remains elusive. Herein, a single-domain FlgJ homolog (FlgJBb ) was studied in the Lyme disease spirocheteBorrelia burgdorferi . Cryo-electron tomography analysis revealed that theflgJBb mutant still assembled intact flagellar basal bodies but had fewer and disoriented flagellar hooks and filaments. Consistently, Western blots showed that the levels of flagellar hook (FlgE) and filament (FlaB) proteins were substantially decreased in theflgJBb mutant. Further studies disclosed that the decreases of FlgE and FlaB in the mutant occurred at the posttranscriptional level. Microscopic observation and swarm plate assay showed that the motility of theflgJBb mutant was partially deficient. The altered phenotypes were completely restored when the mutant was complemented. Collectively, these results indicate that FlgJBb is involved in the assembly of the flagellar hook and filament but not the flagellar rod inB. burgdorferi . The observed phenotype is different from that offlgJ mutants in the enteric bacteria.