Archaebacterial relationships of the phosphoenolpyruvate carboxykinase gene reveal mosaicism of Giardia intestinalis core metabolism

A gene encoding a putative GTP-specific phosphoenolpyruvate carboxykinase has been cloned and sequenced from the type I amitochondriate protist Giardia intestinalis. The deduced amino acid sequence is related most closely to homologs from hyper thermophilic archaebacteria and only more distantly to homologs from Eubacteria and Metazoa. Most enzymes of Giardia core metabolism however, are related more closely to eubactetial and metazoan homologs. An archaebacterial relationship has been noted previously for the unusual acetyl-CoA synthetase (ADP-forming) of this organism. The results suggest that phosphoenolpyruvate carboxykinase and acetyl-CoA synthetase have been acquired from different sources than most enzymes of Giardia core metabolism.