The nucleotide sequence of the 25S rRNA-encoding gene fromCandida albicans

Ribosomal RNA sequences have been shown to be a target of choice for the interand intraspecies identification and classification of a number of microorganisms (For example: 1, 2). However, no known large subunit rRNAs (LSU) sequence of pathogenic yeasts have yet been obtained. Therefore, during the course of an epidemiologic study of recurrent vulvovaginal candidiasis, we decided to determine the sequence of the 25S rRNA of C.albicons, the most common pathogenic yeast. Complete DNA repetitive unit encoding rRNAs in two C.albicans strains were cloned, via their unique Bgin site. Subclones were submitted to the exonuclease HI deletion procedure (Stratagene). The sequence of the DNA strand, corresponding to the 25S rRNA of each strain, was determined using the Sequenase procedure (USB). The total length of the 25S rRNA of the two strains is either 3361 bp or 3363 bp. The insertion responsible for this difference of two nucleotides is located in one of the 6 expansion segments found in Saccharomyces cerevisiae and known to be specific to eucaryotic LSU rRNA (3). Comparison between the two C.albicans strains also revealed 7 nucleotide's substitutions, 5 of them being again located in the expansion segments. An homology analysis of the 3363 bp C. albicans sequence with the other known LSU from fungi was performed using Clustal V software (4). This analysis revealed an homology of 91.0% with S.cerevisiae (5), 91.2% with S.carlsbergiensis (6), and 77.7% with Mucor racemosus (7). This lower homology with Mucor was mostly the result of a 47.3% homology in the 6 expansion segments; the homology between C.albicans and the two species of Saccharomyces for those same regions was also lower than for the rest of the sequence and reached only 74.6%. Thus, by this criterium, the dimorphic yeast C.albicans (Deuteromycetes) appears to be more closely related to the Ascomycetes Saccharomyces which develops as a yeast than to a Phycomycetes like Mucor which primarily develops as a mold.