Ribosomal RNA sequence shows Pneumocystis carinii to be a member of the fungi
https://www.nature.com/articles/334519a0
This study addresses the taxonomic uncertainty of Pneumocystis carinii, the most frequent opportunistic infection in AIDS, by analyzing its 16S-like rRNA sequences. Phylogenetic comparisons place the organism within the fungal lineage, rather than among protozoa as previously debated. These findings redefine its biological classification and provide a framework for understanding its evolutionary relationships and pathogenicity.
Pneumocystis carinii pneumonia is the most common opportunistic infection in AIDS1, and accounts for significant morbidity and mortality in these and other immunocompromised patients2. P. carinii is a eukaryotic microorganism of uncertain taxonomy that can infect numerous mammalian hosts. Developing from a small, unicellular 'trophozoite' into a 'cyst' containing eight 'sporozoites', its life cycle superficially resembles those seen both in the Protozoa and Fungi. Morphological and ultrastructural observations have lead some investigators to conclude that the organism is a protozoan3–6, while others have felt that it more closely resembles a fungus7–9. Phylogenetic relationships can be inferred from comparisons of macromolecular sequences10. Small subunit ribosomal RNAs (16S-like rRNAs) are well-suited for this purpose because they have the same function in all organisms and contain sufficient information to estimate both close and distant evolutionary relationships. Phylogenetic frameworks based upon such comparisons reveal that the plant, animal and fungal lineages are distinct from the diverse spectrum of protozoan lineages11. In this letter, phylogenetic analysis of Pneumocystis 16S-like rRNA demonstrates it to be a member of the Fungi.
