UC San Diego

The Oligopeptide Transporter (OPT) family of peptide and iron-siderophore transporters includes members in both prokaryotes and eukaryotes but with restricted distribution in the latter domain. All functionally characterized peptide transporters segregate from the iron -siderophore transporters on a phylogenetic tree. Prokaryotic members derive from many different phyla, but they belong only to the iron-siderophore subdivision. This fact suggests, but does not prove, that this family arose in prokaryotes, and that the peptide transporters arose from iron-siderophore transporters in eukaryotes. Eukaryotic members are found only in fungi and plants with a single slime mold homologue clustering with the fungal proteins, suggestive of horizontal transfer from a fungus. OPT family proteins have 16, or occasionally 17 transmembrane spanning [alpha]-helical segments. We provide statistical evidence that the 16 TMS topology arose via three sequential duplication events followed by a gene fusion event for proteins with a seventeenth TMS. 2 TMSs [arrow right] 4 TMSs [arrow right] 8 TMSs [arrow right] 16 TMSs [arrow right] 17 TMSs. The seventeenth C- terminal TMS, which probably arose just once, is found in a restricted phylogenetic group of these homologues. Analyses for orthology revealed that a few phylogenetic clusters consist exclusively of orthologs, but most have undergone intermixing, suggestive of horizontal transfer. The results suggest that in this family, horizontal gene transfer was frequent among prokaryotes, rare among eukaryotes and totally absent between prokaryotes and eukaryotes as well as between plants and fungi. These observations provide evidence concerning the pathway taken for the evolution of this family. They also provide guides for future structural and functional analyses