BINDING-PROTEIN DEPENDENT TRANSPORT OF C4-DICARBOXYLATES IN RHODOBACTER-CAPSULATUS

The characteristics of malate transport into aerobically grown cells of the purple photosynthetic bacterium Rhodobacter capsulatus were determined. A single transport system was distinguished kinetically which displayed a K(t) value of 2.9 +/- 1.2 mu-M and V(max) of 43 +/- 6 nmol . min-1 . mg-1 protein. Competition experiments indicated that the metabolically related C4-dicarboxylates succinate and fumarate are also transported by this system. Malate uptake was sensitive to osmotic shock and evidence from the binding of radiolabelled malate and succinate to periplasmic protein fractions indicated that transport is mediated by a dicarboxylate binding protein. The activity of the transport system was studied as a function of external and internal pH and it was found that a marked activation of uptake occurred at intracellular pH values greater than 7. The use of a high affinity binding protein dependent system to transport a major carbon and energy source suggests that Rhodobacter capsulatus would be capable of obtaining growth sustaining quantities of C4-dicarboxylates even if these were present at very low concentrations in the environment.