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Pneumocystis carinii: sequence from ribosomal RNA implies a close relationship with fungi
S L Stringer1, J R Stringer, M A Blase
1University of Cincinnati College of Medicine, Department of Microbiology and Molecular Genetics, Ohio 45267-0524.
Abstract:
Pneumocystis carinii is the etiologic agent of a lethal pneumonia which occurs in patients with the acquired immune deficiency syndrome (AIDS) and other immunocompromised hosts. The basic biochemical and genetic characteristics of P. carinii are poorly understood and its taxonomic classification as a protozoan is uncertain. To address the taxonomic question, a method was developed for the extraction of total RNA from P. carinii. Denaturing agarose gel electrophoresis showed the two ribosomal RNA species of P. carinii to be similar in size to those of other lower eukaryotes, including Saccharomyces cerevisiae. Three portions of the small ribosomal RNA of P. carinii were sequenced by reverse transcription from oligonucleotide primers. Three hundred seventy-two nucleotides of sequence were obtained. The sequence derived from P. carinii RNA contained regions that previous phylogenetic studies have shown to be highly variable among species, as well as regions that are highly conserved. Comparison of the P. carinii sequence to corresponding sequences of organisms from other taxa showed the P. carinii sequence to be more similar to sequences from the fungi (Saccharomyces cerevisiae, Neurospora crassa, Candida albicans, and Cryptococcus diffuens) than to protozoan sequences. These data suggest that P. carinii is more closely related to fungi than to protozoa.
Insights
Pneumocystis carinii causes pneumonia in immunocompromised patients. Genetic analysis reveals this organism is more closely related to fungi than protozoa, clarifying its taxonomic classification.
Area of Science:
- Mycology
- Microbiology
- Molecular Biology
Background:
- Pneumocystis carinii is an opportunistic pathogen causing severe pneumonia in immunocompromised individuals, including those with acquired immune deficiency syndrome (AIDS).
- The precise taxonomic classification of P. carinii remains uncertain, with ongoing debate regarding its relationship to protozoa and fungi.
- Understanding the genetic and biochemical characteristics of P. carinii is crucial for its accurate classification and for developing targeted therapies.
Purpose of the Study:
- To investigate the taxonomic classification of Pneumocystis carinii using molecular techniques.
- To determine the phylogenetic relationship of P. carinii to other eukaryotic organisms.
Main Methods:
- Development of a method for total RNA extraction from P. carinii.
- Analysis of ribosomal RNA (rRNA) sizes using denaturing agarose gel electrophoresis.
- Sequencing of small ribosomal RNA portions (372 nucleotides) using reverse transcription and oligonucleotide primers.
- Phylogenetic comparison of P. carinii rRNA sequences with those of fungi and protozoa.
Main Results:
- Ribosomal RNA species of P. carinii were found to be similar in size to those of lower eukaryotes like Saccharomyces cerevisiae.
- Phylogenetic analysis of the sequenced rRNA regions indicated a closer relationship between P. carinii and fungi.
- The P. carinii sequence showed greater similarity to fungal sequences (Saccharomyces cerevisiae, Neurospora crassa, Candida albicans, Cryptococcus diffuens) than to protozoan sequences.
Conclusions:
- The molecular data strongly suggest that Pneumocystis carinii is evolutionarily more related to fungi than to protozoa.
- This finding helps resolve the long-standing taxonomic uncertainty surrounding P. carinii.
- The study provides a molecular basis for reclassifying P. carinii within the fungal kingdom.