A MITOCHONDRIAL-LIKE CHAPERONIN-60 GENE IN GIARDIA-LAMBLIA - EVIDENCETHAT DIPLOMONADS ONCE HARBORED AN ENDOSYMBIONT RELATED TO THE PROGENITOR OF MITOCHONDRIA

Diplomonads, parabasalids, as represented by trichomonads, and microsp oridia are three protist lineages lacking mitochondria that branch ear lier than all other eukaryotes in small subunit rRNA and elongation fa ctor phylogenies, The absence of mitochondria and plastids in these or ganisms suggested that they diverged before the origin of these organe lles, However, recent discoveries of mitochondrial-like heat shock pro tein 70 and/or chaperonin 60 (cpn60) genes in trichomonads and microsp oridia imply that the ancestors of these two groups once harbored mito chondria or their endosymbiotic progenitors, In this report, we descri be a mitochondrial-like cpn60 homolog from the diplomonad parasite Gia rdia lamblia. Northern and Western blots reveal that the expression of cpn60 is independent of cellular stress and, except during excystatio n, occurs throughout the G. lamblia life cycle, Phylogenetic analyses position the G. lamblia cpn60 in a clade that includes mitochondrial a nd hydrogenosomal cpn60 proteins, The most parsimonious interpretation of these data is that the cpn60 gene was transferred from the endosym biotic ancestors of mitochondria to the nucleus early in eukaryotic ev olution, before the divergence of the diplomonads and trichomonads fro m other extant eukaryotic lineages, A more complicated explanation req uires that these genes originated from distinct alpha-proteobacterial endosymbioses that formed transiently within these protist lineages.