| The protozoan parasite Entamoeba histolytica lives in the anaerobic colonic lumen and lacks the mitochondrial associated proteins of oxidative phosphorylation and the Krebs cycle. However, the research presented here indicates E. histolytica is not a primitive eukaryote that diverged prior to the uptake of the mitochondrial endosymbiont. First, E. histolytica has a mitochondrion-derived organelle that not only contains the mitochondrial chaperonin Hsp60, but also double stranded DNA, and it is bound by a double membrane. Second, the amebic genes encoding the prokaryote-like, ‘primitive’ fermentation enzymes such as malic enzyme, acetyl-CoA synthetase, alcohol dehydrogenases (ADH1 and ADHE), and iron-only (Fe-) hydrogenase were obtained by lateral gene transfer. All the genes encoding these fermentation enzymes appear to have been derived from different eubacterial and archael sources. While lateral gene transfer among prokaryotes is well documented, this is one of the first reports of extensive lateral gene transfer from prokaryotes to a eukaryote. The discovery of an amebic Fe-hydrogenase was unexpected, as this enzyme has only been found in the hydrogenosomes of eukaryotes and is rare among prokaryotes. Hydrogenosomes and mitochondria appear to share a common ancestor, and the gene encoding the Fe-hydrogenase may have been derived from the ancestral endosymbiont. However, research presented here indicates the gene was indeed obtained via an independent lateral transfer event and not obtained from the endosymbiont. Third, E. histolytica encodes conventional eukaryotic heat shock proteins like a subunit of the chaperonin containing TCP-1 (CCTζ) and an endoplasmic reticulum Hsp70 (Bip). These heat shock proteins function as expected in both heat shock and encystation. Fourth, E. histolytica has made novel modifications of genes typical of eukaryotes such as a unique HSP70 (Hsp70-u) with homology to neither cytosolic, endoplasmic reticulum, nor mitochondrial Hsp70s. In addition a heat shock induced inositol 1,3,4-trisphosphate 5/6-kinase with a novel 3-kinase activity was discovered. E. histolytica appears to have adapted to a parasitic lifestyle in an anaerobic niche by discarding genes of fruitless function and modifying or obtaining genes that increase its chances of survival. |
