A 5S rRNA Gene in the Mitochondrial Genome of the Early Diverging Protist, Reclinomonas americana: Absence of the Same Gene in Red Algal Mitochondrial DNA.

B. Franz Lang*, Lynda Goff# and Michael W. Gray%

Program in Evolutionary Biology, Canadian Institute for Advanced Research

*Departement de biochimie, Universit de Montreal, Montreal, Quebec H3C, 3J7, Canada
#Department of Biology, University of California at Santa Cruz, Santa Cruz, CA 95064
%Department of Biochemistry, Dalhousie University, Halifax, Nova Scotia B3H 4H7, Canada

Except in the case of land plants, mitochondrial ribosomes apparently lack a 5S rRNA species, even though this small RNA is a component of all prokaryotic, chloroplast, and eukaryotic cytosol ribosomes. In plants, the mitochondrial 5S rRNA is encoded by mtDNA and differs in sequence from the 5S rRNA specified by plant nuclear and chloroplast genomes. A distinctive 5S rRNA component has not been found in the mitochondrial ribosomes of non-plant eukaryotes, nor has a 5S rRNA gene been identified in those non-plant mtDNAs characterized to date, with the notable exception of the chlorophycean alga, Prototheca wickerhamii. Here we report the presence of a 5S 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 also report sequence data from the mtDNAs of the red algaePorphyra purpurea and Gracilariopsis lemaneiformis, which we use to evaluate a recent claim (Leblanc et al. (1995) J. Mol. Biol. 250, 484-495) that a 5S rRNA gene exists in the mtDNA of a third rhodophyte alga, Chondrus crispus. The data we report here do not support the existence of a mitochondrial 5S rRNA gene in red algae. The current highly restricted phylogenetic distribution of identified mtDNA-encoded 5S rRNA genes suggests that a 5S rRNA gene was likely present in an ancestral proto-mitochondrial genome. If contemporary mtDNA-encoded 5S rRNA genes have all descended from this ancestral gene, then it follows that the mitochondrial 5S rRNA gene must have been lost multiple times during evolutionary diversification of the eukaryotic lineage