VARIATIONS IN THE NUMBER OF RIBOSOMAL DNA UNITS IN MORPHOLOGICAL MUTANTS AND NORMAL STRAINS OF CANDIDA-ALBICANS AND IN NORMAL STRAINS OF SACCHAROMYCES-CEREVISIAE

Naturally occurring strains of Candida albicans are opportunistic path ogens that lack a sexual cycle and that are usually diploids with eigh t pairs of chromosomes. C. albicans spontaneously gives rise to a high frequency of colonial morphology mutants with altered electrophoretic karyotypes, involving one or more of their chromosomes. However, the most frequent changes involve chromosome VIII, which contains the gene s coding for ribosomal DNA (rDNA) units. We have used restriction frag ment lengths to analyze the number and physical array of the rDNA unit s on chromosome VIII in four normal clinical strains and seven morphol ogical mutants derived spontaneously from one of the clinical isolates . HindIII does not cleave the rDNA repeats and liberates the tandem rD NA cluster from each homolog of chromosome VIII as a single fragment, whereas the cleavage at a single site by NotI reveals the size of the single rDNA unit. All clinical strains and morphological mutants diffe red greatly in the number of rDNA units per cluster and per cell. The four clinical isolates differed additionally among themselves by the s ize of the single rDNA unit. For a total of 25 chromosome VIII homolog s in a total of 11 strains considered, the variability of chromosome V III was exclusively due to the length of rDNA clusters (or the number of rDNA units) in approximately 92% of the cases, whereas the others i nvolved other rearrangements of chromosome VIII. Only slight variation s in the number of rDNA units were observed among 10 random C albicans subclones and 10 random Saccharomyces cerevisiae subclones grown for a prolonged time at 22-degrees-C. However, when grown faster at optima l temperatures of 37 and 30-degrees-C, respectively, both fungi accumu lated higher numbers of rDNA units, suggesting that this condition is selected for in rapidly growing cells. The morphological mutants, in c omparison with the C. albicans subclones, contained a markedly wider d istribution of the number of rDNA units, suggesting that a distinct pr ocess may be involved in altering the number of rDNA units in these mu tants.