A COMMON EVOLUTIONARY ORIGIN FOR MITOCHONDRIA AND HYDROGENOSOMES

Trichomonads are among the earliest eukaryotes to diverge from the mai n line of eukaryotic descent, Keeping with their ancient nature, these facultative anaerobic protists lack two ''hallmark'' organelles found in most eukaryotes: mitochondria and peroxisomes. Trichomonads do, ho wever, contain an unusual organelle involved in carbohydrate metabolis m called the hydrogenosome. Like mitochondria, hydrogenosomes are doub le-membrane bounded organelles that produce ATP using pyruvate as the primary substrate. Hydrogenosomes are, however, markedly different fro m mitochondria as they lack DNA, cytochromes and the citric acid cycle , Instead, they contain enzymes typically found in anaerobic bacteria and are capable of producing molecular hydrogen. We show here that hyd rogenosomes contain heat shock proteins, Hsp70, Hsp60, and Hsp10, with signature sequences that are conserved only in mitochondrial and alph a-Gram-negative purple bacterial Hsps, Biochemical analysis of hydroge nosomal Hsp60 shows that the mature protein isolated from the organell e lacks a short, N-terminal sequence, similar to that observed for mos t nuclear-encoded mitochondrial matrix proteins, Moreover, phylogeneti c analyses of hydrogenosomal Hsp70, Hsp60, and Hsp10 show that these p roteins branch within a monophyletic group composed exclusively of mit ochondrial homologues. These data establish that mitochondria and hydr ogenosomes have a common eubacterial ancestor and imply that the earli est-branching eukaryotes contained the endosymbiont that gave rise to mitochondria in higher eukaryotes.