The Nucleotide Sequence of the Small Ribosomal Subunit RNA of the Yeast Candida albicans and the Evolutionary Position of the Fungi among the Eukaryotes

Abstract

Summary Up to now the small ribosomal subunit RNA sequences of about 50 different eukaryotes have been published, of which only three belong to the fungi. We determined the complete srRNA sequence of the imperfect yeast Candida albicans. The sequence is 1788 nucleotides long and was determined at the DNA level using the dideoxy method with a set of primers specific for conserved sequences of small ribosomal subunit RNA. An evolutionary tree, comprising 58 organisms including C. albicans, was constructed. This tree shows a number of early diverging lineages such as a diplomonad, a microsporidian, an amoeba, slime molds, an euglenoid, kinetoplastids and sporozoans. Next within a relatively short time interval there is a radiation into a number of clusters composed of ciliates, metazoa, fungi and green plants. C. albicans was previously classified in the artificial taxon of imperfect fungi. The evolutionary tree presented in this paper clearly shows C. albicans to belong to the ascomycetous yeasts. An additional aim of this study was the refinement of the srRNA secondary structure model. Although the outline of this model is now well established, no consensus model exists in certain eukaryote-specific areas of high structural variability. The srRNA sequence of xC. albicans was fitted into the secondary structure model and the existence of a pseudoknot is proposed in one of these eukaryote-specific areas.