FIGURE 8.

(A and B) Diagram showing how mutations can alter LacY selectivity for lactose and melibiose transport by preventing lactose binding to secondary sites within the transporter, thereby affecting lactose but not melibiose transit. (C) Diagram showing the condition for accelerated exchange or counterflow. Labeled lactose enters the LacY from the outside (periplasmic side). In contrast to the situation in A, it is prevented from binding to secondary binding sites within the central cavity by unlabeled lactose entering from the cytoplasmic side. This prevents labeled lactose reflux to the periplasmic side and consequently increases the apparent rate of labeled uptake. This will result in countertransport if the cytoplasmic lactose concentration is higher than the periplasmic concentration.