Where and how many: evolutionary diversification of a molecular switch regulating flagellar patterns

Flagella are rotating organelles of locomotion that enable bacteria to navigate their environments. They are positioned at various locations and in differing numbers across the bacterial surface, a characteristic known as the "flagellation pattern." Surprisingly, many of these diverse patterns are regulated by a conserved molecular switch composed of the GTP-binding protein FlhF and the ATPase FlhG, with FlhG stimulating the GTPase activity of FlhF. The evolutionary origins of FlhF and FlhG can be traced to the signal recognition particle (SRP) system and the MinD-dependent cell division machinery, respectively. Here, we review current knowledge on the mechanisms by which the conserved FlhF/FlhG switch controls flagellation patterns across different bacterial species. This system exemplifies how evolution repurposes ancient cellular machineries to control new functions, highlighting the adaptability of protein-based regulatory networks.