Promotion or retardation of the growth of adenine auxotrophs ofCandida albicans by purines, pyrimidines and nucleosides

Molecular analysis of the Cryptococcus neoformans ADE2 gene, a selectable marker for transformation and gene disruption

Cryptococcus neoformans is an important fungal pathogen of man. The incidence of cryptococcal disease has increased dramatically in patients immunocompromised because of HIV infection, organ transplantation, or treatment with cytotoxic chemotherapy or corticosteroids. This organism is an excellent model for molecular dissection of fungal pathogenesis and virulence factors. Here we report the nucleotide sequence of the C. neoformans serotype D genomic ADE2 gene, which encodes a phosphoribosylaminoimidazole carboxylase required for purine biosynthesis. Importantly, this version of the ADE2 gene has been used as the selectable marker for virtually all gene disruptions by transformation and homologous recombination in C. neoformans. We compare the nucleotide and amino acid sequences of the ADE2 gene and product to other highly related adenine biosynthetic genes and enzymes from other yeasts and fungi. We also describe a series of convenient ADE2 cassettes for gene disruption construct preparation. Finally, we have identified the ade2 mutations in strains M001 and M049, adenine auxotrophic mutants derived from the serotype A strain H99. These mutant strains have served as recipients for targeted gene disruptions using the ADE2 gene. These studies should facilitate transformation and gene disruption approaches using the ADE2 selectable marker in this important human fungal pathogen.

The URA5 gene is necessary for histoplasma capsulatum growth during infection of mouse and human cells

The Histoplasma capsulatum URA5 gene, which has recently been cloned and disrupted by allelic replacement, encodes orotidine-5'-monophosphate pyrophosphorylase. Inactivation of URA5 by either targeted or UV mutagenesis results in disruption of the pyrimidine biosynthetic pathway and uracil auxotrophy. We examined the effect of uracil auxotrophy due to a ura5 mutation on H. capsulatum virulence in both cell culture and whole-animal models. Uracil auxotrophs of two H. capsulatum restriction fragment length polymorphism classes were found to be avirulent in cultured murine and human cells, as well as in mice. Moreover, virulence could be restored either by supplying a functional URA5 gene in trans or by supplying exogenous uracil during infection in vitro. These experiments demonstrate that the pyrimidine biosynthetic pathway is essential for H. capsulatum growth and virulence.

The gene encoding phosphoribosylaminoimidazole carboxylase (ADE2) is essential for growth of Cryptococcus neoformans in cerebrospinal fluid

A cryptococcal meningitis model in corticosteroid-treated rabbits was used to assess the requirement for the phosphoribosylaminoimidazole gene (ADE2) for virulence of Cryptococcus neoformans. A wild-type strain (H99), an ade2 auxotroph of H99 (M001), and a randomly selected prototrophic transformant of M001 (M001.1c) which had received the cloned ADE2 cDNA copy were inoculated intrathecally into immunosuppressed rabbits. While M001 was avirulent in the central nervous system model, virulence was completely restored to wild-type pathogenicity in the prototrophic transformant. This study identifies the pathogenic importance of an endogenous adenine pathway in this yeast and confirms that purine biosynthesis is a potential target for antifungal therapy. It also demonstrates that the virulence of C. neoformans can be molecularly changed and detected within a clinically relevant animal model.

Dosage of the smallest chromosome affects both the yeast-hyphal transition and the white-opaque transition of Candida albicans WO-1

The WO-1 strain of Candida albicans is capable of alternating between two highly distinct yeast cell types termed white and opaque (E. H. A. Rikkerrink, B. B. Magee, and P. T. Magee, J. Bacteriol. 170:895-899, 1988; B. Slutsky, M. Staebell, J. Anderson, L. Risen, M. Pfaller, and D. R. Soll, J. Bacteriol. 169:189-197, 1987). We have isolated WO-1 mutants that show a marked deficiency at being able to switch from the white form to the opaque form under conditions normally favorable for this transition. Pulsed-field electrophoresis demonstrated that one of the initial two spontaneous nonswitching mutants lacked the smallest chromosome that is normally present in WO-1. The availability of a WO-1 derivative whose only functional ADE2 gene is located on this small chromosome made possible, through the induction of chromosome nondisjunction, the isolation of numerous new mutants missing this chromosome as well as mutants containing two copies of the chromosome. Mutants missing the smallest chromosome showed a greatly diminished ability to produce opaque sectors and to produce germ tubes in the presence of human serum. Mutants containing two copies of the small chromosome showed an increased ability to produce germ tubes. These results indicate that this small chromosome carries one or more genes involved in both the white-opaque switch and the yeast-hyphal switch.

Pathogenicity of Candida albicans auxotrophic mutants in experimental infections

Auxotrophic and prototrophic control strain pairs of Candida albicans constructed by molecular biology methodologies were evaluated for pathogenicity in a systemic mouse model. Mutants that were auxotrophic for adenine, uracil, and heme each showed a lowered level of pathogenicity relative to control strains. It can be concluded from these experiments that decreased pathogenicity in each case is due to the auxotrophic mutation, because mutant and control strains were constructed so as to differ at a single locus. These observations suggest that new therapeutic agents for Candida infections might be designed based upon the inhibition of biosynthetic pathways that, in some cases, might be absent from the host.

Parasexual genetics of Torulopsis glabrata

Prototrophic hybrids were generated in the asexual yeast Torulopsis glabrata by the fusion of spheroplasts derived from parent strains which bore complementing auxotrophic markers. The DNA content (per cell) of two hybrids was essentially that predicted by summing the corresponding parental values. UV irradiation of these two hybrids resulted in the formation of sectored colonies with genetic properties consistent with their origin by either mitotic recombination or chromosomal nondisjunction.

Integrative transformation of Candida albicans, using a cloned Candida ADE2 gene

Candida albicans is a diploid dimorphic yeast with no known sexual cycle. The development of a DNA transformation system would greatly improve the prospects for genetic analyses of this yeast. Plasmids were isolated from a Candida Sau3A partial library which complements the ade2-1 and ade2-5 mutations in Saccharomyces cerevisiae. These plasmids contain a common region, part of which, when subcloned, produces ade2 complementation. Among the small number of auxotrophs previously isolated in C. albicans, red adenine-requiring mutants had been identified by several groups. In two of these strains, the cloned Candida DNA transformed the mutants to ADE+ at frequencies of 0.5 to 5 transformants per micrograms of DNA. In about 50% of the transformants, plasmid DNA sequences became stably integrated into the host genome and, in the several cases analyzed by Southern hybridization, the DNA was integrated at the site of the ADE2 gene in one of the chromosomal homologs.

The genetics of medically important fungi

The present review is concerned with recent progress in basic genetic investigations with a variety of fungi which are pathogenic for man and animals. The principles and strategies involved in undertaking genetic investigations of sexual species and of asexual species are discussed. Progress in genetic analysis of Cryptococcus neoformans made possible by the discovery of its sexual phase is described in detail, as is progress in development of parasexual methods of analysis in Candida albicans. The genetic bases of virulence and drug resistance are discussed for those few species in which these phenotypes have been investigated. Suggestions for future research, including the application of recent advances in molecular biology to the study of pathogenic fungi, are presented.

Integrative transformation of Candida albicans, using a cloned Candida ADE2 gene

Candida albicans is a diploid dimorphic yeast with no known sexual cycle. The development of a DNA transformation system would greatly improve the prospects for genetic analyses of this yeast. Plasmids were isolated from a Candida Sau3A partial library which complements the ade2-1 and ade2-5 mutations in Saccharomyces cerevisiae. These plasmids contain a common region, part of which, when subcloned, produces ade2 complementation. Among the small number of auxotrophs previously isolated in C. albicans, red adenine-requiring mutants had been identified by several groups. In two of these strains, the cloned Candida DNA transformed the mutants to ADE+ at frequencies of 0.5 to 5 transformants per micrograms of DNA. In about 50% of the transformants, plasmid DNA sequences became stably integrated into the host genome and, in the several cases analyzed by Southern hybridization, the DNA was integrated at the site of the ADE2 gene in one of the chromosomal homologs.

Parasexual genetic analysis of Candida albicans by spheroplast fusion

Doubly auxotrophic strains of Candida albicans were selected from mutagenized cultures. Spheroplasts prepared from the auxotrophic strains were fused with polyethylene glycol. Prototrophic derivatives formed by this fusion protocol from auxotrophic strains were selected by complementation on minimal medium. These prototrophs had a cell volume twice that of the original strain and were shown to be heterozygous at four loci. Prototrophs obtained by this procedure infrequently gave rise to auxotrophic recombinants whose cell volume remained twice that of the original strain. It is suggested that these auxotrophic recombinants arise from mitotic crossing-over. This paper is the first report of a parasexual cycle in C. albicans.

Hybridization of Candida albicans through fusion of protoplasts

Protoplasts of complementing auxotrophs of Candida albicans can fuse in the presence of polyethylene glycol and generate prototrophic cells. The yields of prototrophs from fusion mixtures depend greatly on the particular combinations of auxotrophies involved but not on other features of the strain backgrounds of protoplasts. The initial cellular products of fusions isolated on selective media are heterokaryons which replicate slowly but also segregate single parental nuclei into blastospores in high frequency. Karyogamy within heterokaryons produces hybrid nuclei which, on segregation, give rise to rapidly growing, uninucleate substrains. Analyses of the substrains show that hybrid nuclei either stabilize as diploid or undergo random loss of chromosomes to stabilize at various levels of aneuploidy prior to segregation. Chromosome losses and radiation induced mitotic crossing-over can effect recombination for parental auxotrophic markers in hybrids; patterns of recombination for ader and arg markers provide the first documented example of chromosomal linkage in C. albicans. Thus, protoplast fusions offer opportunities otherwise unavailable for applying the incisive tools of genetic recombination to analysis of this important, asexual yeast.

Heterozygosity and segregation in Candida albicans

Strain FC18 of the pathogenic yeast Candida albicans is heterozygous at a gene required for synthesis of methionine and cysteine, and yields auxotrophic derivatives by induced mitotic segregation. Derivatives of FC18 are heterozygous at other genes. Genetic analysis of C. albicans is feasible using induced mitotic recombination.