Cytosolic enzymes with a mitochondrial ancestry from the anaerobic chytridPiromyces sp. E2

The anaerobic chytrid Piromyces sp. E2 lacks mitochondria, but contains hyd rogen-producing organelles, the hydrogenosomes. We are interested in how th e adaptation to anaerobiosis influenced enzyme compartmentalization in this organism. Random sequencing of a cDNA library from Piromyces so. E2 result ed in the isolation of cDNAs encoding malate dehydrogenase, aconitase and a cetohydroxyacid reductoisomerase. Phylogenetic analysis of the deduced amin o acid sequences revealed that they are closely related to their mitochondr ial homologues from aerobic eukaryotes. However, the deduced sequences lack N-terminal extensions, which function as mitochondrial leader sequences in the corresponding mitochondrial enzymes from aerobic eukaryotes. Subcellul ar fractionation and enzyme assays confirmed that the corresponding enzymes are located in the cytosol. As anaerobic chytrids evolved from aerobic, mi tochondria-bearing ancestors, we suggest that, in the course of the adaptat ion from an aerobic to an anaerobic lifestyle, mitochondrial enzymes were r etargeted to the cytosol with the concomitant loss of their N-terminal lead er sequences.