A pleiotropic role of FlaG in regulating the cell morphogenesis and flagellar homeostasis at the cell poles of Treponema denticola.

FlaG homologue has been found in several bacteria including spirochetes; however, its function is poorly characterised. In this report, we investigated the role of TDE1473, a putative FlaG, in the spirochete Treponema denticola, a keystone pathogen of periodontitis. TDE1473 resides in a large gene operon that is controlled by a σ70 -like promoter and encodes a putative FlaG protein of 123 amino acids. TDE1473 can be detected in the periplasmic flagella (PFs) of T. denticola, suggesting that it is a flagella-associated protein. Consistently, in vitro studies demonstrate that the recombinant TDE1473 interacts with the PFs in a dose-dependent manner and that such an interaction requires FlaA, a flagellar filament sheath protein. Deletion of TDE1473 leads to long and less motile mutant cells. Cryo-electron tomography analysis reveal that the wild-type cells have 2-3 PFs with nearly homogenous lengths (ranging from 3 to 6 µm), whereas the mutant cells have less intact PFs with disparate lengths (ranging from 0.1 to 9 µm). The phenotype of T. denticola TDE1473 mutant reported here is different from its counterparts in other bacteria, which provides insight into further understanding the role of FlaG in the regulation of bacterial cell morphogenesis and flagellation.