| Description |
The bacterial flagellar motor is a remarkably complex system, comprising thousands of protein subunits and responsible for motility in numerous species. Of these proteins, FlhE is not universally conserved and is relatively understudied. Usually found cotranscribed with flhB and flhA in the flhBAE operon, the flhE gene encodes a 14 kDa, 130 amino acid protein that is present in several peritrichously flagellated Gram-negative species. Deletion of chromosomal flhE in E. coli results in very unusual cellular morphologies, accompanied by an abrogation in swarming (surface-associated) motility and a severe defect in swimming motility. All of these phenomena are relieved upon introduction of full-length flhE from a plasmid, but not by an N-terminally truncated version of the gene (Δ1-17) lacking the signal sequence that targets FlhE to the periplasm. Defects are complemented even at a basal level of expression suggesting that FlhE is present in low-copy number, though overexpression of the protein harms cells and results in a growth defect. Both the growth and morphology defects are remedied in ΔflhE mutants with the overexpression of FlgM, an anti-σ28 factor responsible for repressing late-stage flagellar components. This suggests that at least one of these components plays a role in the defects seen in ΔflhE cells. These and other observations support a model in which FlhE is responsible for keeping the basal body pushed up against the peptidoglycan (PG) layer, allowing for proper assembly and export of the motor and filament. In the absence of this stabilizing interaction, structures that normally assemble outside the cell (the hook and filament) assemble in the periplasm, leading to distortion of the cell envelope and consequent growth defects. |
