Role of Metabolic Energy in the Transport of β-Galactosides by Streptococcus lactis

Streptococcus lactis (ATCC 7962) accumulated thiomethyl-β-galactoside (TMG) and other galactosides against concentration gradients when the cells were supplied with a metabolizable substrate, such as glucose. The accumulated TMG was free and not phosphorylated. In the absence of glucose, TMG rapidly entered the cell to a concentration equal to that of the medium. Agents that uncouple oxidative phosphorylation abolished active transport but not the carrier-facilitated entry of TMG. Evidence that the transport carriers were functional in the absence of glucose or in the presence of uncoupling agents included the demonstration of counterflow, which depends on competitive inhibition for the carrier for exit.