Proton-coupled β-galactoside translocation in non-metabolizingEscherichia coli

Abstract

Acid-base and electrogenic processes coupled to the flux of β-galactosides into non-metabolizing cells ofEscherichia coli have been studied.

When β-glactoside was added to non-metabolizing suspensions ofE. coli, the pH of the suspension medium increased, indicating that the β-galactoside travelled in with acid equivalents. When the cells were made permeable to K⁺ ions, this inflow of acid equivalents was accompanied by an equal outflow of K⁺ ions, indicating that each acid equivalent carried one positive charge across the membrane, and corresponded to an H⁺ ion going in or an OH⁻ ion coming out. The effective movement of H⁺ ions, caused either by a pH difference or by an electrical potential difference across the membrane of the cells, was specifically facilitated by the presence of β-galactoside. These effects of β-galactoside were abolished by N-ethyl maleimide, which is known to inhibit the specific β-galactoside translocation.

The possible involvement of a Na⁺-β-galactoside symporter was ruled out by showing that the galactoside-induced inflow of acid was practically independent of Na⁺ ion concentration in the range 0.05–50.0 mM, and that Na⁺ ions did not flow into the bacteria under the influence of a β-galactoside concentration gradient.

It is concluded that the β-galactoside translocation inE. coli is probably mediated by a β-galactoside-H⁺ symporter or by a β-galactoside/OH⁻ antiporter.