THE ESCHERICHIA-COLI CITRATE CARRIER CITT - A MEMBER OF A NOVEL EUBACTERIAL TRANSPORTER FAMILY RELATED TO THE 2-OXOGLUTARATE MALATE TRANSLOCATOR FROM SPINACH-CHLOROPLASTS/

Under anoxic conditions in the presence of an oxidizable cosubstrate s uch as glucose or glycerol, Escherichia coli converts citrate to aceta te and succinate. Two enzymes are specifically required for the fermen tation of the tricarboxylic acid, i.e., a citrate uptake system and ti trate lyase. Here we report that the open reading frame (designated ci tT) located at 13.90 min on the E. coli chromosome between rna and the citrate lyase genes encodes a citrate carrier. E. coli transformed wi th a plasmid expressing citT was capable of aerobic growth on citrate, which provides convincing evidence for a function of CitT as a citrat e carrier. Transport studies with cell suspensions of the transformed strain indicated that CitT catalyzes a homologous exchange of citrate or a heterologous exchange against succinate, fumarate, or tartrate. S ince succinate is the end product of citrate fermentation in E. coli, it is likely that CitT functions in vivo as a citrate/succinate antipo rter. Analysis of the primary sequence showed that CitT (487 amino aci ds, 53.1 kDa) is a highly hydrophobic protein with 12 putative transme mbrane helices. Sequence comparisons revealed that CitT is related to the 2-oxoglutarate/malate translocator (SODiT1 gene product) from spin ach chloroplasts and five bacterial gene products, none of which has y et been functionally characterized. It is suggested that the E. coli C itT protein is a member of a novel family of eubacterial transporters involved in the transport of di- and tricarboxylic acids.