The c-di-GMP binding protein, YcgR, is the primary inhibitor of motor function in Salmonella and Escherichia coli.

E. coli and Salmonella enterica have multiple c-di-GMP cyclases and phosphodiesterases. Absence of a specific phosphodiesterase YhjH impairs motility in both bacteria. yhjH mutants have elevated c-di-GMP levels and require YcgR, a c-di- GMP-binding protein, for motility inhibition. This study demonstrates that YcgR interacts with the flagellar switch-complex proteins FliG and FliM, with the primary interaction site located within FliM. Interaction of YcgR with these proteins induces a CCW motor bias and reduces the efficiency of torque generation, thus inhibiting both chemotaxis and the speed of movement. In collaboration with David Blair’s group at the University of Utah, we propose a "backstop brake" model showing how both effects of YcgR on the motor can result from an initial disruption of the FliM/FliG interface, followed by destabilization and disorganization of the FliG C-terminal domain, which interacts with the stator protein MotA. Support for this order of events i.e. induction of a CCW bias followed by reduction of torque, is provided for S. enterica motors. Data from single motor analysis show that E. coli and S. enterica motors have inherently different properties, but that YcgR is solely responsible for disruption of motor function in both bacteria. This study also finds that E. coli and S. enterica employ c-di-GMP in additional and different pathways to impede motility. Inhibition of motility and chemotaxis may represent a bacterial strategy to prepare for sedentary existence by disfavoring migration away from a substrate on which a biofilm is to be formed.