EVIDENCE FOR THE BACTERIAL ORIGIN OF GENES ENCODING FERMENTATION ENZYMES OF THE AMITOCHONDRIATE PROTOZOAN PARASITE ENTAMOEBA-HISTOLYTICA

Entamoeba histolytica is an amitochondriate protozoan parasite with nu merous bacterium-like fermentation enzymes including the pyruvate:ferr edoxin oxidoreductase (POR), ferredoxin (FD), and alcohol dehydrogenas e E (ADHE). The goal of this study was to determine whether the genes encoding these cytosolic E. histolytica fermentation enzymes might der ive from a bacterium by horizontal transfer, as has previously been su ggested for E. histolytica genes encoding heat shock protein 60, nicot inamide nucleotide transhydrogenase, and superoxide dismutase. In this study, the E. histolytica por gene and the adhE gene of a second amit ochondriate protozoan parasite, Giardia lamblia, were sequenced, and t heir phylogenetic positions were estimated in relation to POR, ADHE, a nd FD cloned from eukaryotic and eubacterial organisms. The E. histoly tica par gene encodes a 1,620-amino-acid peptide that contained conser ved iron-sulfur- and thiamine pyrophosphate-binding sites. The predict ed E. histolytica POR showed fewer positional identities to the POR of G. lamblia (34%) than to the POR of the enterobacterium Klebsiella pn eumoniae (49%), the cyanobacterium Anabaena sp. (44%), and the protozo an Trichomonas vaginalis (46%), which targets its POR to anaerobic org anelles called hydrogenosomes. Maximum-likelihood, neighbor-joining, a nd parsimony analyses also suggested as less likely E. histolytica POR sharing more recent common ancestry with G. lamblia POR than with POR of bacteria and the T. vaginalis hydrogenosome. The G. lamblia adhE e ncodes an 888-amino-acid fusion peptide with an aldehyde dehydrogenase at its amino half and an iron-dependent (class 3) ADH at its carboxy half The predicted G. lamblia ADHE showed extensive positional identit ies to ADHE of Escherichia coli (49%), Clostridium acetobutylicum (44% ), and E. histolytica (43%) and lesser identities to the class 3 ADH o f eubacteria and yeast (19 to 36%). Phylogenetic analyses inferred a c loser relationship of the E. histolytica ADHE to bacterial ADHE then t o the G. lamblia ADHE. The 6-kDa FD of E. histolytica and G. lamblia w ere most similar to those of the archaebacterium Methanosarcina barker i and the delta-purple bacterium Desulfovibrio desulfuricans, respecti vely, while the 12-kDa FD of the T. vaginalis hydrogenosome was most s imilar to the 12-kDa PD of gamma-purple bacterium Pseudomonas putida. E. histolytica genes (and probably G. lamblia genes) encoding fermenta tion enzymes therefore likely derive from bacteria by horizontal trans fer, although it is not clear from which bacteria these amebic genes d erive. These are the first nonorganellar fermentation enzymes of eukar yotes implicated to have derived from bacteria.