The tripartite tricarboxylate transporter (TTT) family.

Extracytoplasmic solute binding receptors are constituents of primary and secondary active transport systems. Previous studies have shown that the constituents of two such families (ABC and TRAP-T) occur in bacteria and archaea and have undergone minimal shuffling of constituents between systems during evolutionary history. We here show that a third family of binding receptor-dependent transporters, the tripartite tricarboxylate transporter (TTT) family, the prototype of which is the TctABC system of Salmonella typhimurium, occurs in many bacteria but not in archaea or eukaryotes. Phylogenetic analyses suggest that these systems have evolved from a primordial tripartite system with only two out of 39 possible examples of shuffling of constituents between systems. The occurrence of TctA homologues in many bacteria and archaea that apparently lack corresponding TctB and TctC homologues suggests that the appearance of tripartite systems was a relatively recent evolutionary invention that occurred after the divergence of archaea and eukaryotes from bacteria.