High-Affinity C4-Dicarboxylate Uptake in Rhodobacter Capsulatus Is Mediated By A ‘Trap’ Transporter, A New Type of Periplasmic Secondary Transport System Widespread in Bacteria

Anoxygenic photosynthetic bacteria in the purple non-sulphur group have long been known to grow rapidly and with high yields on certain citric-acid cycle intermediates and their precursors, particularly pyruvate and the C4-dicarboxylates malate and succinate, under a wide variety of conditions. The transport of these molecules across the cytoplasmic membrane of Rhodobacter sphaeroideswas first studied by Gibson1, who concluded that transport of D,L-malate, succinate and fumarate was mediated by a common, inducible system of high-affinity. The closely related species Rhodobacter capsulatusalso possesses a high-affinity transport system for D and L-malate, succinate and fumarate, with a Ktvalue for L-malate of about 3 μM in intact cells2. This system has been studied in some detail in our laboratory and it has now become clear that it represents a distinct type of periplasmic transporter.