Abstract
Bacteria can use the second messenger c-di-GMP to adjust their motility in response to environmental cues. The protein YcgR, upon binding of c-di-GMP, interacts with the flagellar motor to affect the motor behavior. However, the full feature of the effects of c-di-GMP::YcgR on the flagellar motor remains unclear, and its interacting partners on the motor is still controversial. Here, we characterized the effects of c-di-GMP::YcgR on the torque-speed curve of the flagellar motor, one of the most important properties of the motor, finding that it affects the motor behavior throughout the full range of load conditions from zero to high loads by shifting the motor torque-speed curve downward. We also investigated the interacting partner on the motor through dynamical fluorescent studies, finding that c-di-GMP::YcgR mainly interacts with the motor-switch complex instead of the torque-generating units (stators). To directly test the behavioral consequence of elevated c-di-GMP levels, we measured the distribution of bacteria swimming near a surface, finding that elevated c-di-GMP levels promote bacterial aggregation on surfaces. The effects of c-di-GMP on bacterial motile behavior that we characterized here are consistent with the key role that c-di-GMP plays in the transition between motile and sedentary forms of bacterial life.