CONSERVATION OF MITOCHONDRIAL TARGETING SEQUENCE FUNCTION IN MITOCHONDRIAL AND HYDROGENOSOMAL PROTEINS FROM THE EARLY-BRANCHING EUKARYOTES CRITHIDIA, TRYPANOSOMA AND TRICHOMONAS

Kinetoplastid protozoa are the earliest-branching eukaryotes to posses s a true mitochondrion. This organelle is host to a variety of intrigu ing and unique features, including RNA editing. We examined the charac teristics of protein import into mitochondria of Trypanosoma brucei. D ihydrofolate reductase (DHFR) carrying a yeast mitochondrial targeting signal was correctly translocated into trypanosome mitochondria in vi vo, as were DHFR fusion proteins bearing two unusually short (7-9 amin o acids) presequences from trypanosomatids. The short trypanosomal tar geting signals were functional in Saccharomyces cerevisiae as well, bu t their targeting efficiency was lower and processing was absent. Tric homonads branched even earlier than kinetoplastids in eukaryotic evolu tion and contain energy-generating organelles called hydro genosomes. The origin of hydrogenosomes has been controversial, but most evidence suggests that they are related to mitochondria. Putative hydrogenosom al targeting signals from Trichomonas vaginalis are short (5-12 amino acids). Three such sequences were capable of targeting a passenger pro tein to mitochondria both in yeast and in trypanosomes, and one of the hydrogenosomal presequences was effi ciently processed in both organi sms. These findings suggest a resemblance between the import machineri es of mitochondria and hydrogenosomes.