Truncation of MalF results in lactose transport via the maltose transport system of Escherichia coli.

The active accumulation of maltose and maltodextrins by Escherichia coli is dependent on the maltose transport system. Several lines of evidence suggest that the substrate specificity of the system is not only determined by the periplasmic maltose-binding protein but that a further level of substrate specificity is contributed by the inner membrane integral membrane components of the system, MalF and MalG. We have isolated and characterized an altered substrate specificity mutant that transports lactose. The mutation responsible for the altered substrate specificity results in an amber stop codon at position 99 of MalF. The mutant requires functional MalK-ATPase activity and hydrolyzes ATP constitutively. It also requires MalG. The data suggest that in this mutant the MalG protein is capable of forming a low affinity transport path for substrate.