GENES ENCODING THE SAME 3 SUBUNITS OF RESPIRATORY COMPLEX-II ARE PRESENT IN THE MITOCHONDRIAL-DNA OF 2 PHYLOGENETICALLY DISTANT EUKARYOTES

Although mitochondrial DNA is known to encode a limited number (<20) o f the polypeptide components of respiratory complexes I, III, IV, and V, genes for components of complex II [succinate dehydrogenase (ubiqui none); succinate:ubiquinone oxidoreductase, EC 1.3.5.1] are conspicuou sly lacking in mitochondrial genomes so far characterized. Here we sho w that the same three subunits of complex II are encoded in the mitoch ondrial DNA of two phylogenetically distant eukaryotes, Porphyra purpu rea (a photosynthetic red alga) and Reclinomonas americana (a heterotr ophic zooflagellate). These complex II genes, sdh2, sdh3, and sdh4, ar e homologs, respectively, of Escherichia coli sdhB, sdhC, and sdhD. In E. coli, sdhB encodes the iron-sulfur subunit of succinate dehydrogen ase (SDH), whereas sdhC and sdhD specify, respectively, apocytochrome b(558) and a hydrophobic 13-kDa polypeptide, which together anchor SDH to the inner mitochondrial membrane. Amino acid sequence similarities indicate that sdh2, sdh3, and sdh4 were originally encoded in the pro tomitochondrial genome and have subsequently been transferred to the n uclear genome in most eukaryotes. The data presented here are consiste nt with the view that mitochondria constitute a monophyletic lineage.