Dual Influx Model of Thiogalactoside Accumulation in Escherichia coli

A model of thiogalactoside accumulation in Escherichia coli is described and supporting evidence is presented. The model states that thiogalactoside influx and efflux in the absence of energy coupling involves a reversible, carrier-mediated transfer, in which the carrier is the product of the y gene. Energy coupling brings about active transport by dissociating the carrier-thiogalactoside complex in the membrane with liberation of thiogalactoside inside the cell. Thereby a second, essentially irreversible influx path, dependent on cellular metabolism, is predicted. The supporting evidence includes the following. (a) A method was devised to estimate the fraction of membrane carriers loaded with thiogalactoside (thiomethylgalactoside, isopropylthiogalactoside, or thiodigalactoside) in the presence or absence of energy coupling. Energy coupling, which was altered experimentally by three different procedures, consistently decreased the fraction of loaded carriers, in accord with the model. (b) Temperature dependence studies of influx and efflux provided evidence for two influx pathways, one different from the efflux path and dependent on energy coupling, the other similar to the efflux path and independent of metabolism. (c) Studies of counterflow in the influx and efflux directions at various temperatures and the effects thereon of energy coupling were consistent with the existence of two influx paths and one efflux path, as predicted by the model.