Sucrose transport by the Escherichia coli lactose carrier

Several lines of evidence suggest that sucrose is transported by the lactose carrier of Escherichia coli. Entry of sucrose was monitored by an osmotic method which involves exposure of cells to a hyperosmotic solution of disaccharide (250 mM). Such cells shrink (optical density rises), and if the solute enters the cell, there is a return toward initial values (optical density falls). By this technique sucrose was found to enter cells at a rate approximately one third that of lactose. In addition, the entry of [14C]sucrose was followed by direct analysis of cell contents after separation of cells from the medium by centrifugation. Sucrose accumulated within the cell to a concentration 160% of that in the external medium. The addition of sucrose to an anaerobic suspension of cells resulted in a small alkalinization of the external medium. These data are consistent with the view that the lactose carrier can accumulate sucrose by a proton cotransport system. The carrier exhibits a very low affinity for the disaccharide (150 mM) but a moderately rapid Vmax.