Solvent-isotope and pH effects on flagellar rotation in Escherichia coli

We studied changes in speed of the flagellar rotary motor of Escherichia co li when tethered cells or cells carrying small latex spheres on flagellar s tubs were shifted from H2O to D2O or subjected to changes in external pH. I n the high-torque, low-speed regime, solvent isotope effects were found to be small; in the low-torque, high-speed regime, they were large. The bounda ries between these regimes were close to those found earlier in measurement s of the torque-speed relationship of the flagellar rotary motor (Berg and Turner, 1993, Biophys. J. 65:2201-2216; Chen and Berg, 2000, Biophys. J., 7 8:1036-1041). This observation provides direct evidence that the decline in torque at high speed is due primarily to limits in rates of proton transfe r. However, variations of speed (and torque) with shifts of external pH (fr om 4.7 to 8.8) were small for both regimes. Therefore, rates of proton tran sfer are not very dependent on external pH.