Primary structure and phylogenetic relationships of a malate dehydrogenase gene from Giardia lamblia.

The lactate and malate dehydrogenases comprise a complex protein superfamily with multiple enzyme homologues found in eubacteria, archaebacteria, and eukaryotes. In this study we describe the sequence and phylogenetic relationships of a malate dehydrogenase (MDH) gene from the amitochondriate diplomonad protist, Giardia lamblia. Parsimony, distance, and maximum-likelihood analyses of the MDH protein family solidly position G. lamblia MDH within a eukaryote cytosolic MDH clade, to the exclusion of chloroplast, mitochondrial, and peroxisomal homologues. Furthermore, G. lamblia MDH is specifically related to a homologue from Trichomonas vaginalis. This MDH topology, together with published phylogenetic analyses of beta-tubulin, chaperonin 60, valyl-tRNA synthetase, and EF-1alpha, suggests a sister-group relationship between diplomonads and parabasalids. Since these amitochondriate lineages contain genes encoding proteins which are characteristic of mitochondria and alpha-proteobacteria, their shared ancestry suggests that mitochondrial properties were lost in the common ancestor of both groups.