Measurement of transmembrane Δψ The Δψ-sensitive fluorophore Oxonol V [bis-(3-phenyl-5-oxoisoxazol-4-yl)pentamethine oxonol] (Cambridge Bioscience Ltd, Cambridge, UK) was used to determine if the MdtM-mediated antiport observed in the previous experiments
was electrogenic. Inverted vesicles were produced from TO114 cells transformed with pMdtM or pD22A as described previously , except that the vesicle resuspension buffer was made Cl–free by substitution of the 140 mM choline chloride component with 280 mM sorbitol  and by using H2SO4 rather than HCl to adjust buffer pH. Inverted vesicles produced from E. coli ICG-001 nmr BW25113 cells that retained the full complement of electrogenic Na+/H+ antiporters provided a positive control. Vesicles (500 μg/ml membrane protein) were added
to assay buffer (10 mM BTP, 5 mM MgSO4, 5 μM Oxonol V) that had its pH adjusted to 9.0 (for detection of electrogenic K+/H+ antiport) or 9.25 (for detection of electrogenic Na+/H+ antiport). The pH of the assay buffer used for positive control BW25113 vesicles was adjusted to 8.5 to ensure detection of electrogenic NhaA-catalysed Na+/H+ antiport activity . All vesicles were incubated on ice for 200 s prior to addition of 2 mM Tris-D-L-lactate to initiate respiration-dependent generation of Δψ, and the resultant quenching of Oxonol V fluorescence was monitored at 25°C using a PD0325901 cell line Fluoromax-4 fluorometer with an excitation wavelength of 599 nm and emission wavelength of 634 nm. Excitation and emission slit widths were set to 10 nm and 20 nm, respectively. Electrogenic antiport activity was estimated buy Ibrutinib on the basis of its ability to dissipate the established Δψ (recorded as a dequenching of the fluorescence signal) in response to addition of 100 mM Na+ gluconate or K+ gluconate to vesicles at the times indicated. Addition of 100 μM CCCP was used to abolish both Δψ and ΔpH components of the PMF. As a further control, 1 μM of the ionophore nigericin
(which at low concentrations selectively consumes ΔpH in the presence of K+ via electroneutral K+/H+ exchange) was added to vesicles of TO114 cells transformed with pMdtM. These vesicles were incubated in assay buffer that contained 50 mM K+ gluconate, and valinomycin (5 μM) was added to selectively abolish Δψ. Measurement of cytoplasmic pH The intracellular pH of E. coli whole-cell suspensions at various external alkaline pH values was determined by ratiometric fluorescence measurements of the acetoxymethyl ester derivative of the membrane-permeant, pH-sensitive fluorescent probe, 2,7-bis-(2-carboxyethyl)-5-carboxyfluorescein (BCECF-AM; Life Technologies Ltd, Paisley, UK) . Intracellular pH was correlated to fluorescence signal by recording the fluorescence emission intensity of BCECF at 530 nm upon excitation at 490 nm (the BCECF pH-dependent excitation wavelength) and at 440 nm (the BCECF pH-independent excitation wavelength).