Adaptation and evolution in flagellar motors

The bacterial flagellar motor (BFM) is a membrane-embedded, ion-driven rotary nanomachine responsible for the motility of the majority of known bacterial species. Torque in the BFM is generated at the interface between the motor's rotor and transmembrane stator units, which are tethered to the peptidoglycan layer of the bacterial cell wall. Each stator unit contains an ion-binding site, allowing the motor to use the energy from the passage of ions (usually protons or sodium) down a transmembrane gradient for torque generation. Recent high-resolution structures of the BFM's stator units have provided considerable new insight into the possible mechanism of torque generation in the motor; these new structures also necessitate a re-examination of several predictions made by previous models. Here, we present a model for torque generation in the E. coli flagellar motor rooted in this new structural understanding. Further, using both structural information and experiments on motor dynamics from several bacterial species, we analyze how adaptation under various selective pressures has shaped the diversity of flagellar motors across bacterial lineages.