The heterofermentative lactic acid bacterium Lactobacillus brevis transports galactose and the nonmetabolizable galactose analogue thiomethyl--galactoside (TMG) by a permease-catalyzed sugar:H⁺ symport mechanism. Addition of glucose to L. brevis cells loaded with [¹⁴C]TMG promotes efflux and prevents accumulation of the galactoside, probably by converting the proton symporter into a uniporter. Such a process manifests itself physiologically in phenomena termed inducer expulsion and exclusion. Previous evidence suggested a direct allosteric mechanism whereby the phosphocarrier protein, HPr, phosphorylated at serine-46 [HPr(Ser-P)], binds to the galactose:H⁺ symporter to uncouple sugar transport from proton symport. To elucidate the molecular mechanism of inducer control in L. brevis, we have cloned the genes encoding the HPr(Ser) kinase, HPr, enzyme I, and the galactose:H⁺ symporter. The sequences of these genes were determined, and the relevant phylogenetic trees are presented. Mutant HPr derivatives in which the regulatory serine was changed to either alanine or aspartate were constructed. The cloned galP gene was integrated into the chromosome of Bacillus subtilis, and synthesis of the mutant HPr proteins in this organism was shown to promote regulation of GalP, as expected for a direct allosteric mechanism. We have thus reconstituted inducer control in an organism that does not otherwise exhibit this phenomenon. These results are consistent with the conclusion that inducer exclusion and expulsion in L. brevis operates via a multicomponent signal transduction mechanism wherein the presence of glycolytic intermediates such as fructose 1,6-bisphosphate (the intracellular effector), derived from exogenous glucose (the extracellular effector), activates HPr(Ser) kinase (the sensor) to phosphorylate HPr on Ser-46 (the messenger), which binds to the galactose:H⁺ symporter (the target), resulting in uncoupling of sugar transport from proton symport (the response). This cascade allows bacteria to quickly respond to changes in external sugar concentrations. Understanding the molecular mechanism of inducer control advances our knowledge of the link between metabolic and transport processes in bacteria.