ANAEROBIC FUMARATE TRANSPORT IN ESCHERICHIA-COLI BY AN FNR-DEPENDENT DICARBOXYLATE UPTAKE SYSTEM WHICH IS DIFFERENT FROM THE AEROBIC DICARBOXYLATE UPTAKE SYSTEM

Escherichia coli grown anaerobically with fumarate as electron acceptor is able to take up C4-dicarboxylates by a specific transport system. The system differs in all tested parameters from the known aerobic C4-dicarboxylate transporter. The anaerobic transport system shows higher transport rates (95-mu-mol/g [dry weight] per min versus 30-mu-mol/g/min) and higher K(m)s (400 versus 30-mu-M) for fumarate than for the aerobic system. Mutants lacking the aerobic dicarboxylate uptake system are able to grow anaerobically at the expense of fumarate respiration and transport dicarboxylates with wild-type rates after anaerobic but not after aerobic growth. Transport by the anaerobic system is stimulated by preloading the bacteria with dicarboxylates. The anaerobic transport system catalyzes homologous and heterologous antiport of dicarboxylates, whereas the aerobic system operates only in the unidirectional mode. The anaerobic antiport is measurable only in anaerobically grown bacteria with fnr+ backgrounds. Additionally, the system is inhibited by incubation of resting bacteria with physiological electron acceptors such as O2, nitrate, dimethyl sulfoxide, and fumarate. The inhibition is reversed by the presence of reducing agents. It is suggested that the physiological role of the system is a fumarate/succinate antiport under conditions of fumarate respiration.