Energy transduction in the bacterial flagellar motor. Effects of load and pH

The effect of load and pH on the relation between proton potential and flagellar rotation has been studied in cells of a smooth-swimming Streptococcus strain. The driving potential, speeds of free-swimming bacteria, and rotation rates of bacteria tethered to glass by a single flagellum were measured. The relation between rotation rate of tethered bacteria and potential was remarkably linear up to nearly -200 mV. The relation between swimming speed and potential exhibited both saturation and threshold, as previously observed in other species. The form of these relations depended on pH. The equivalence of the electrical and chemical potential components of the proton potential in enabling swimming depended on the voltage. Our observations may be most simply accommodated by a kinetic scheme that links transmembrane proton transits to a tightly coupled work cycle. The properties of this scheme were elucidated by computer simulations of the experimental plots. These simulations indicated that the protonable groups that participate in the rate limiting reactions have a fractional electrical distance between three-fourths to all of the way toward the cytoplasm with a corresponding mean proton binding affinity of 10(-7.3)-10(-7.0) M, respectively.