Monosomy of a specific chromosome determines L-sorbose utilization: a novel regulatory mechanism in Candida albicans

Genomic exploration of the hemiascomycetous yeasts: 10. Kluyveromyces thermotolerans

A genomic exploration of Kluyveromyces thermotolerans was performed by random sequence tag (RST) analysis. We sequenced 2653 RSTs corresponding to inserts sequenced from both ends. We performed a systematic comparison with a complete set of proteins from Saccharomyces cerevisiae, other completely sequenced genomes and SwissProt. We identified six mitochondrial genes and 1358-1496 nuclear genes by comparison with S. cerevisiae. In addition, 25 genes were identified by comparison with other organisms. This corresponds to about 24% of the estimated gene content of this organism. A lower level of conservation is observed with orthologues to genes of S. cerevisiae previously classified as orphans. Gene order was found to be conserved between S. cerevisiae and K. thermotolerans in 56.5% of studied cases.

Fine-resolution physical mapping of genomic diversity in Candida albicans

It has been suggested that Candida albicans, a diploid asexual fungus, achieves genetic diversity by genomic rearrangement. This important human pathogen may provide a system in which to analyze alternate routes to genomic diversity. C. albicans has a highly variable karyotype; its chromosomes contain a middle repeated DNA sequence called the Major Repeat Sequence (MRS), composed of subrepeats HOK, RPS, and RB2. RPS is tandemly repeated while the other subrepeats occur once in each MRS. Chromosome 7, the smallest of the eight chromosomes, has been previously mapped. The complete physical map of this chromosome was used to analyze chromosome 7 diversity in six strains, including two well-characterized laboratory strains (1006 and WO-1) and four clinical ones. We found four types of events to explain the genomic diversity: 1) Chromosome length polymorphism (CLP) results from expansion and contraction of the RPS; 2) reciprocal translocation occurs at the MRS loci; 3) chromosomal deletion; and (4) trisomy of individual chromosomes. These four phenomena play an important role in generating genomic diversity in C. albicans.

A single-transformation gene function test in diploid Candida albicans

The fungal pathogen Candida albicans is naturally diploid, and current gene disruption strategies require two successive transformations. We describe here a genetic construct (UAU1) for which two copies may be selected. Insertion of UAU1 into one genomic site, after a single transformation, allows selection for segregants with two copies of the insertion. Major classes of segregants are those carrying homozygous insertion mutations and allelic triplications, which have two insertion alleles and a wild-type allele. Thus nonessential and essential genes may be distinguished rapidly through PCR tests for homozygosis and triplication. We find that homozygous mutations may be isolated at three nonessential loci (ADE2, RIM20, and YGR189), while only allelic triplications were found at two essential loci (SNF1 and CDC28). We have unexpectedly isolated homozygous mutants with mutations at CDC25; they are viable but defective in filamentation on serum-containing medium. The UAU1 cassette is thus useful to assess rapidly the essentiality of C. albicans genes.

Induction of mating in Candida albicans by construction of MTLa and MTLalpha strains

Although the diploid fungus Candida albicans, a human pathogen, has been thought to have no sexual cycle, it normally possesses mating-type-like orthologs (MTL) of both of the Saccharomyces cerevisiae mating-type genes (MAT) a and alpha. When strains containing only MTLa or MTLalpha were constructed by the loss of one homolog of chromosome 5, the site of the MTL loci, MTLa and MTLalpha strains mated, but like mating types did not. Evidence for mating included formation of stable prototrophs from strains with complementing auxotrophic markers; these contained both MTL alleles and molecular markers from both parents and were tetraploid in DNA content and mononucleate.

Candida's arranged marriage

Biologists who study the fungus Candida albicans have always assumed that this organism reproduces asexually because they have not found evidence of mating, meiosis, or a haploid stage of the life cycle. However, as Gow et al. explain in a Perspective, sequencing of the C. albicans genome has revealed the existence of a possible mating type locus. This finding has now been extended to demonstrate actual mating in the fungus (Hull et al., Magee and Magee).

Phenotypic switching in Cryptococcus neoformans results in changes in cellular morphology and glucuronoxylomannan structure

Cryptococcus neoformans strains exhibit variability in their capsular polysaccharide, cell morphology, karyotype, and virulence, but the relationship between these variables is poorly understood. A hypovirulent C. neoformans 24067A isolate, which usually produces smooth (SM) colony types, was found to undergo phenotypic switching and to produce wrinkled (WR) and pseudohyphal (PH) colony types at frequencies of approximately 10(-4) to 10(-5) when plated on Sabouraud agar. Cells from these colony types had large polysaccharide capsules and PH morphology, respectively. Scanning electron microscopy showed that different colony types were the result of altered cellular packing in the colony. Phenotypic switching was associated with quantitative and qualitative changes in capsular polysaccharide. Specifically, the glucuronoxylomannan (GXM) of the WR polysaccharide differed in the proportion of structural reporter groups and in increased xylose residue content linked at the 4 to 0 position. The relative virulence of the colony types was WR > PH > SM, as measured by CFU in rat lungs after intratracheal infection. Karyotype instability was observed in strain 24067A and involved primarily two chromosomes. Colonies with an alternative colony type exhibited more karyotype changes, which did not revert to the original karyotype in reverted colonies. In summary, this study revealed that phenotypic switching in C. neoformans (i) can produce WR colonies consisting of cells with either large capsule or PH morphology, (ii) is associated with production of structurally different GXM, (iii) is commonly associated with karyotype changes, (iv) can produce cells of PH morphology, and (v) can increase the virulence of a strain. Hence, phenotypic switching is an adaptive mechanism linked to virulence that can generate cell types with very different biological characteristics.

Appearance and properties of L-sorbose-utilizing mutants of Candida albicans obtained on a selective plate

This is the first report that adaptive mutagenesis can arise by chromosomal nondisjunction, a phenomenon previously associated exclusively with DNA alterations. We previously uncovered a novel regulatory mechanism in Candida albicans in which the assimilation of an alternative sugar, l-sorbose, was determined by copy number of chromosome 5, such that monosomic strains utilized l-sorbose, whereas disomic strains did not. We present evidence that this formation of monosomy of chromosome 5, which is apparently a result of nondisjunction, appeared with increased frequencies after a selective condition was applied, i.e., by adaptive mutagenesis. The rate of formation of l-sorbose-utilizing mutants per viable cell per day ranged from 10(-6) at the initial time of detection to 10(-2) after 4 days of incubation on the selective plate.

Specific chromosome alterations in fluconazole-resistant mutants of Candida albicans

The exposure of Candida albicans to fluconazole resulted in the nondisjunction of two specific chromosomes in 17 drug-resistant mutants, each obtained by an independent mutational event. The chromosomal changes occurred at high frequencies and were related to the duration of the drug exposure. The loss of one homologue of chromosome 4 occurred after incubation on a fluconazole medium for 7 days. A second change, the gain of one copy of chromosome 3, was observed after exposure for 35 or 40 days. We found that the mRNA levels of ERG11, CDR1, CDR2, and MDR1, the candidate fluconazole resistance genes, remained either the same or were diminished. The lack of overexpression of putative drug pumps or the drug target indicated that some other mechanism(s) may be operating. The fluconazole resistance phenotype, electrophoretic karyotypes, and transcript levels of mutants were stable after growth for 112 generations in the absence of fluconazole. This is the first report to demonstrate that resistance to fluconazole can be dependent on chromosomal nondisjunction. Furthermore, we suggest that a low-level resistance to fluconazole arising during the early stages of clinical treatment may occur by this mechanism. These results support our earlier hypothesis that changes in C. albicans chromosome number is a common means to control a resource of potentially beneficial genes that are related to important cellular functions.

Phenotypic switching in Candida albicans is controlled by a SIR2 gene

We report the cloning of a gene from the human fungal pathogen Candida albicans with sequence and functional similarity to the Saccharomyces cerevisiae SIR2 gene. Deletion of the gene in C. albicans produces a dramatic phenotype: variant colony morphologies arise at frequencies as high as 1 in 10. The morphologies resemble those described previously as part of a phenotypic switching system proposed to contribute to pathogenesis. Deletion of SIR2 also produces a high frequency of karyotypic changes. These and other results are consistent with a model whereby Sir2 controls phenotypic switching and chromosome stability in C.albicans by organizing chromatin structure.