Fig 2.

Substrate specificity assay of CitP. (A) Citrate consumption by resting cells of L. lactis IL1403(pFL3) in the presence of 0.2 mM l-lactate (▼), 2 mM oxaloacetate (○), or no further additions (●). (B) Schematic of transport of citrate by CitP operating in the H⁺-symport mode in the presence of nonpermeative dicarboxylates. Oxaloacetate (ox²⁻) cannot enter the membrane by passive diffusion but competes with citrate (Hcit²⁻) outside the cell. (C) Schematic of the shuttle mechanism in the presence of permeative monocarboxylates. l-Lactate added outside the cells allows CitP to operate in the fast Hcit²⁻-lac⁻ exchange mode by reentering the cell in the permeative protonated state (Hlac).