A 5 S rRNA gene is present in the mitochondrial genome of the protist Reclinomonas americana but is absent from red algal mitochondrial DNA.

Except in the case of land plants, mitochondrial ribosomes apparently lack a 5 S rRNA species, even though this small RNA is a component of all prokaryotic, chloroplast and eukaryotic cytosol ribosomes. In plants, the mitochondrial 5 S rRNA is encoded by mtDNA and differs in sequence from the 5 S rRNA specified by plant nuclear and chloroplast genomes. A distinctive 5 S rRNA component has not been found in the mitochondrial ribosomes of non-plant eukaryotes and, with the notable exception of the chlorophycean alga, Prototheca wickerhamii, a 5 S rRNA gene has not been identified in those non-plant mtDNAs characterized to date. Here, we report the presence of a 5 S rRNA gene in the mtDNA of the heterotrophic flagellate Reclinomonas americana. This unicellular eukaryote is a member of the jakobid flagellates, an early-diverging group of protists that share ultrastructural characteristics with the retortamonads, primitive protists that lack mitochondria. We report sequence data from the mtDNAs of the red algae Porphyra purpurea and Gracilariopsis lemaneiformis, which we use to evaluate a recent claim that a 5 S rRNA gene exists in the mtDNA of a third rhodophyte alga, Chondrus crispus. Our results lead us to the opposite conclusion: that a 5 S rRNA gene is not encoded by red algal mtDNA. In view of the accumulating evidence favoring a monophyletic origin of the mitochondrial genome, it is likely that a 5 S rRNA gene was present in an ancestral proto-mitochondrial genome, and that contemporary mtDNA-encoded 5 S rRNA genes have all descended from this ancestral gene. Considering the highly restricted phylogenetic distribution of identified mtDNA-encoded 5 S rRNA genes, it follows that the mitochondrial 5 S rRNA gene must have been lost multiple times during evolutionary diversification of the eukaryotic lineage.