Primary structure and phylogenetic relationships of a malate dehydrogenasegene from Giardia lamblia

The lactate and malate dehydrogenases comprise a complex protein superfamil y with multiple enzyme homologues found in eubacteria, archaebacteria, and eukaryotes. In this study we describe the sequence and phylogenetic relatio nships of a malate dehydrogenase (MDH) gene from the amitochondriate diplom onad 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, mitochondr ial, and peroxisomal homologues. Further more, G. lamblia MDH is specifical ly related to a homologue from Trichomonas vaginalis. This MDH topology, to gether with published phylogenetic analyses of P-tubulin, chaperonin 60, va lyl-tRNA synthetase, and EF-1 alpha, suggests a sister-group relationship b etween diplomonads and parabasalids. Since these amitochondriate lineages c ontain genes encoding proteins which are characteristic of mitochondria and a-proteobacteria, their shared ancestry suggests that mitochondrial proper ties were lost in the common ancestor of both groups.