FIG. 1.

Proposed model for PTS-mediated regulation of vectorial sugar expulsion in L. brevis. Inducer expulsion in L. brevis is envisioned as a multicomponent signal transduction process. The galactose:H⁺ symporter GalP cotransports sugar and H⁺. H⁺ cotransport allows coupling of galactose uptake to the proton motive force. Upon addition of glucose (the extracellular effector), accumulation of glycolytic intermediates such as fructose 1,6-bisphosphate (FBP), gluconate-6-phosphate (Gnt 6-P), and 2-phosphoglycerate (2-PG) (the intracellular effectors) activate HPr(Ser) kinase (the sensor) to phosphorylate HPr on Ser-46 (the messenger), which binds to GalP (the target), resulting in uncoupling of sugar transport from H⁺ symport (the response). Consequently, the permease is converted from an active symporter to a facilitative diffusion uniporter that can no longer accumulate sugar substrates against a concentration gradient (inducer exclusion-expulsion).