Glutamate transport in Rhodobacter sphaeroides is mediated by a novel binding protein-dependent secondary transport system

Growth of a glutamate transport-deficient mutant of Rhodobacter sphaeroides on glutamate as sole carbon and nitrogen source can be restored by the addition of millimolar amounts of Na⁺. Uptake of glutamate (K_t of 0.2 μM) by the mutant strictly requires Na⁺ (K_m of 25 mM) and is inhibited by ionophores that collapse the proton motive force (pmf). The activity is osmotic-shock-sensitive and can be restored in spheroplasts by the addition of osmotic shock fluid. Transport of glutamate is also observed in membrane vesicles when Na⁺, a proton motive force, and purified glutamate binding protein are present. Both transport and binding is highly specific for glutamate. The Na⁺-dependent glutamate transporter of Rb. sphaeroides is an example of a secondary transport system that requires a periplasmic binding protein and may define a new family of bacterial transport proteins.