EVIDENCE THAT EUKARYOTIC TRIOSEPHOSPHATE ISOMERASE IS OF ALPHA-PROTEOBACTERIAL ORIGIN

We have cloned and sequenced genes for triosephosphate isomerase (TPI) from the gamma-proteobacterium Francisella tularensis, the green non- sulfur bacterium Chloroflexus aurantiacus, and the alpha-proteobacteri um Rhizobium etli and used these in phylogenetic analysis with TPI seq uences from other members of the Bacteria, Archaea, and Eukarya. These analyses show that eukaryotic TPI genes are most closely related to t he homologue from the alpha-proteobacterium and most distantly related to archaebacterial homologues. This relationship suggests that the TP I genes present in modern eukaryotic genomes were derived from an alph a-proteobacterial genome (possibly that of the protomitochondrial endo symbiont) after the divergence of Archaea and Eukarya. Among these euk aryotic genes are some from deeply branching, amitochondrial eukaryote s (namely Giardia), which further suggests that this event took place quite early in eukaryotic evolution.