Candida albicans is the most frequently isolated fungal pathogen in humans. It occurs in two distinct biotypes differentiated by their ability to assimilate sucrose. The strains unable to assimilate sucrose have been identified at C. albicans, var. stellatoidea. The stellatoidea variety was first isolated from vaginal mucosa in the 1940's in the United States but was rarely found among the isolates obtained from immunocompetent individuals. Since 1980, increasingly frequent isolations of the sucrose negative variety has been reported from HIV positive patients. Our previous studies indicated that the stellatoidea variety not only differs from the type variety in its ability to assimilate sucrose but also in its karyotype, linkage map and virulence in the animal model. When the sucrose negative phenotype is reverted to sucrose positive, it also gains the ability to assimilate glycerool, methyl-D-glucoside and maltose, and has increased virulence in the animal model. These phenomenon suggest a mutation in the regulatory gene(s) and resembles the phenotypic characteristics of Saccharomyces snf1 mutants. We have chosen to study SNF1 genes in two biotypes of C. albicans. SNF1 gene encodes a serine-theonine protein kinase which was first identified as a gene essential for sucrose utilization in Saccharomyces cerevisiae. Futher studies suggested the SNF1 is responsible for derepression of the SUC2 gene expression which is necessary for the assimilation of sucrose and also plays a role in the regulation of lipid metabolism, cell cycle, sporulation and most likely other important mechanisms in the two biotypes of C. albicans. Using the PCR method, Candida SNF1 gene was isolated and characterized. Candida gene had high similiarity at the amino acid level with that of Saccharomyces and was able to complement the snf1 phenotype of Saccharomyces. The SNF1 gene was assigned to chromosome # 5 in C. albicans and to two different chromosomes in the Stellatoidea variety. To investigate the role of SNF1 in Candida, disruption of the two homologs was attempted using the URA3-hisG cassettes which had been successfully applied for deletions of other genes in C. albicans. We disrupted one homolog and found that the snf1 mutation is recessive as is the case in Saccharomyces. The results up to this point indicate that the SNF1 gene is highly conserved in fungi and that the two biotypes of C. albicans are different in the SNF1 gene at the chromosomal level.
