The architecture, assembly, and evolution of a complex flagellar motor.

Bacterial flagella drive motility in many species, likely including the last bacterial common ancestor . Knowledge of flagellar assembly and function has mainly come from studies of and , which have simple flagellar motors . However, most flagellated bacteria possess complex motors with unique, species-specific adaptations whose mechanisms and evolution remain largely unexplored .

Tetrameric PilZ protein stabilizes stator ring in complex flagellar motor and is required for motility in .

Rotation of the bacterial flagellum, the first identified biological rotary machine, is driven by its stator units. Knowledge gained about the function of stator units has increasingly led to studies of rotary complexes in different cellular pathways. Here, we report that a tetrameric PilZ family protein, FlgX, is a structural component underneath the stator units in the flagellar motor of .

The common origin and degenerative evolution of flagella in .

Flagellar motility plays an important role in the environmental adaptation of bacteria and is found in more than 50% of known bacterial species. However, this important characteristic is sparsely distributed within members of the phylum , which constitutes one of the largest bacterial groups. It is unclear why this important fitness organelle is absent in most actinobacterial species and the origin of flagellar genes in other species.