Quantitative analysis of proton-linked transport systems. Glutamate transport in Staphylococcus aureus.
1. The magnitude of the protonmotive force in respiring Staphylococcus aureus was measured over the range of extracellular pH from 5.6 to 7.8. 2. The membrane potential remains constant at 150 mV, inside-negative, but the pH gradient decreases from 2.1 units, inside-alkaline, at pH 5.6 to zero at pH 7.5 and above. 3. The accumulation of glutamate in the soluble cell pool is pH-independent at a value equivalent to 100 mV. 4. The results of experiments studying co-transport of protons are consistent with a proton/glutamate stoichiometry of 2 and electrogenic transport across the pH range examined. 5. The amount of glutamate uptake is the result of a kinetic steady state between influx and efflux pathways. 6. Evidence is presented for the regulation of this kinetic steady state by the response of the initial rate of uptake to changes in the protonmotive force.