Nature 334, 519 - 522 (11 August 1988); doi:10.1038/334519a0

Ribosomal RNA sequence shows Pneumocystis carinii to be a member of the Fungi

Jeffrey C. Edman^*†, Joseph A. Kovacs^‡, Henry Masur^‡, Daniel V. Santi^§†, Hille J. Elwood^¶ & Mitchell L. Sogin^¶

^*Hormone Research Institute and Departments of ^†Laboratory Medicine, ^§Biochemistry and Biophysics and Pharmaceutical Chemistry, University of California, San Francisco, California 94143-0534, USA
^‡Department of Critical Care Medicine, National Institutes of Health, Bethesda, Maryland 20892, USA
^¶Division of Molecular and Cellular Biology, Department of Pediatrics, National Jewish Center for Immunology and Respiratory Medicine, Denver, Colorado 80206, USA

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 protozoan^3–6, while others have felt that it more closely resembles a fungus^7–9. 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.

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