Chromosome reorganization inCandida albicans1001 strain

N-acetylglucosamine transporter, Ngt1, undergoes sugar-responsive endosomal trafficking in Candida albicans

N-acetylglucosamine (GlcNAc), an important amino sugar at the infection sites of the fungal pathogen Candida albicans, triggers multiple cellular processes. GlcNAc import at the cell surface is mediated by GlcNAc transporter, Ngt1 which seems to play a critical role during GlcNAc signaling. We have investigated the Ngt1 dynamics that provide a platform for further studies aimed at understanding the mechanistic insights of regulating process(es) in C. albicans. The expression of this transporter is prolific and highly sensitive to even very low levels (˂2 µM) of GlcNAc. Under these conditions, Ngt1 undergoes phosphorylation-associated ubiquitylation as a code for internalization. This ubiquitylation process involves the triggering proteins like protein kinase Snf1, arrestin-related trafficking adaptors (ART) protein Rod1, and yeast ubiquitin ligase Rsp5. Interestingly, analysis of ∆snf1 and ∆rsp5 mutants revealed that while Rsp5 is promoting the endosomal trafficking of Ngt1-GFPɤ, Snf1 hinders the process. Furthermore, colocalization experiments of Ngt1 with Vps17 (an endosomal marker), Sec7 (a trans-Golgi marker), and a vacuolar marker revealed the fate of Ngt1 during sugar-responsive endosomal trafficking. ∆ras1 and ∆ubi4 mutants showed decreased ubiquitylation and delayed endocytosis of Ngt1. According to our knowledge, this is the first report which illustrates the mechanistic insights that are responsible for endosomal trafficking of a GlcNAc transporter in an eukaryotic organism.

Milestones in Candida albicans gene manipulation

In the United States, candidemia is one of the most common hospital-acquired infections and is estimated to cause 10,000 deaths per year. The species Candida albicans is responsible for the majority of these cases. As C. albicans is capable of developing resistance against the currently available drugs, understanding the molecular basis of drug resistance, finding new cellular targets, and further understanding the overall mechanism of C. albicans pathogenesis are important goals. To study this pathogen it is advantageous to manipulate its genome. Numerous strategies of C. albicans gene manipulation have been introduced. This review evaluates a majority of these strategies and should be a helpful guide for researchers to identify gene targeting strategies to suit their requirements.

Genomic plasticity of the human fungal pathogen Candida albicans

The genomic plasticity of Candida albicans, a commensal and common opportunistic fungal pathogen, continues to reveal unexpected surprises. Once thought to be asexual, we now know that the organism can generate genetic diversity through several mechanisms, including mating between cells of the opposite or of the same mating type and by a parasexual reduction in chromosome number that can be accompanied by recombination events (2, 12, 14, 53, 77, 115). In addition, dramatic genome changes can appear quite rapidly in mitotic cells propagated in vitro as well as in vivo. The detection of aneuploidy in other fungal pathogens isolated directly from patients (145) and from environmental samples (71) suggests that variations in chromosome organization and copy number are a common mechanism used by pathogenic fungi to rapidly generate diversity in response to stressful growth conditions, including, but not limited to, antifungal drug exposure. Since cancer cells often become polyploid and/or aneuploid, some of the lessons learned from studies of genome plasticity in C. albicans may provide important insights into how these processes occur in higher-eukaryotic cells exposed to stresses such as anticancer drugs.

Aneuploid chromosomes are highly unstable during DNA transformation of Candida albicans

Candida albicans strains tolerate aneuploidy, historically detected as karyotype alterations by pulsed-field gel electrophoresis and more recently revealed by array comparative genome hybridization, which provides a comprehensive and detailed description of gene copy number. Here, we first retrospectively analyzed 411 expression array experiments to predict the frequency of aneuploidy in different strains. As expected, significant levels of aneuploidy were seen in strains exposed to stress conditions, including UV light and/or sorbose treatment, as well as in strains that are resistant to antifungal drugs. More surprisingly, strains that underwent transformation with DNA displayed the highest frequency of chromosome copy number changes, with strains that were initially aneuploid exhibiting approximately 3-fold more copy number changes than strains that were initially diploid. We then prospectively analyzed the effect of lithium acetate (LiOAc) transformation protocols on the stability of trisomic chromosomes. Consistent with the retrospective analysis, the proportion of karyotype changes was highly elevated in strains carrying aneuploid chromosomes. We then tested the hypothesis that stresses conferred by heat and/or LiOAc exposure promote chromosome number changes during DNA transformation procedures. Indeed, a short pulse of very high temperature caused frequent gains and losses of multiple chromosomes or chromosome segments. Furthermore, milder heat exposure over longer periods caused increased levels of loss of heterozygosity. Nonetheless, aneuploid chromosomes were also unstable when strains were transformed by electroporation, which does not include a heat shock step. Thus, aneuploid strains are particularly prone to undergo changes in chromosome number during the stresses of DNA transformation protocols.

The enigma of the major repeat sequence of Candida albicans

The major repeat sequence, discovered in the yeast Candida albicans, is a stretch of repeated DNA that occurs nine times in the haploid genome of this opportunistic fungal pathogen and probably a similar number of times in the genome of Candida dubliniensis. In C. albicans it constitutes 1-2% of the genome. Its occurrence is limited to those two species. Despite its major role as a genomic feature, its function, mode of expansion in size due to duplication of internal subunits, and its origin and mechanism of distribution throughout the genome are not understood, although it is associated with chromosome translocations, chromosome length polymorphisms and regulation of the yeast-hypha dimorphic transition. The polymorphism of the major repeat sequence has been exploited in epidemiology and taxonomic studies. This review describes its sequence, occurrence, use in epidemiology and examines the evidence for its role in chromosome dynamics.

The loss of parts of chromosome 7 followed by the insertion of URA cassette into RB2 on MRS in Candida albicans strain CAI-4

Clinical isolates of the medically important fungus Candida albicans show electrophoretic karyotype variations. Chromosome translocation is considered to be one of the possible mechanisms of karyotype variation and has been shown to occur very frequently at or near the unique repeated DNA sequences which comprise the Major Repeat Sequence (MRS) on the genome. The MRS consists of the repeated sequences RB2, RPS, and HOK. We previously showed the insertion at the RB2 region might initiate chromosome translocation in strain STN22u2 of C. albicans. To ask whether the insertion of a URA cassette into the RB2 but not into RPS and HOK causes chromosome translocation in C. albicans strains, we transformed three URA cassettes into strain CAI-4, which is commonly used as a host strain for gene knockout experiments. We found chromosome rearrangements followed the insertion of URA cassettes into RB2 in strain CAI-4. Three transformants had an extra chromosome showing the loss of the 7A and 7C region from one chromosome 7 homologue. The recombination occurred at or after the insertion of URA cassette into RB2. Insertion there seems to cause chromosome rearrangement and thus RB2 is considered one of the important elements for initiation of chromosome rearrangement.

Extensive chromosome rearrangements distinguish the karyotype of the hypovirulent species Candida dubliniensis from the virulent Candida albicans

Candida dubliniensis and Candida albicans, the most common human fungal pathogen, have most of the same genes and high sequence similarity, but C. dubliniensis is less virulent. C. albicans causes both mucosal and hematogenously disseminated disease, C. dubliniensis mostly mucosal infections. Pulse-field electrophoresis, genomic restriction enzyme digests, Southern blotting, and the emerging sequence from the Wellcome Trust Sanger Institute were used to determine the karyotype of C. dubliniensis type strain CD36. Three chromosomes have two intact homologues. A translocation in the rDNA repeat on chromosome R exchanges telomere-proximal regions of R and chromosome 5. Translocations involving the remaining chromosomes occur at the Major Repeat Sequence. CD36 lacks an MRS on chromosome R but has one on 3. Of six other C. dubliniensis strains, no two had the same electrophoretic karyotype. Despite extensive chromosome rearrangements, karyotypic differences between C. dubliniensis and C. albicans are unlikely to affect gene expression. Karyotypic instability may account for the diminished pathogenicity of C. dubliniensis.

Induction of SAP7 correlates with virulence in an intravenous infection model of candidiasis but not in a vaginal infection model in mice

SAP7 of Candida albicans is induced after vaginal infection of mice. Conversely, virulence during vaginal infection was not affected in a Deltasap7/Deltasap7 mutant strain. Only a partial virulence phenotype was detectable after intravenous injection. In conclusion, SAP7 expression does not correlate with C. albicans virulence in mice.

Microevolutionary changes and chromosomal translocations are more frequent at RPS loci in Candida dubliniensis than in Candida albicans

The Cd1 fingerprinting probe of Candida dubliniensis, which is extremely effective in identifying microevolutionary changes in infecting populations, generates hybridization patterns that are similar to those generated by the Candida albicans fingerprinting probe Ca3. Since Ca3 recognizes microevolutionary changes through the repeat sequence RPS, it was suggested that Cd1 also contains a RPS-like element. To test this possibility, the C. albicans RPS unit was used as a probe, and an RPS-like element, RPSd1, was cloned from C. dubliniensis. The sequence of RPSd1 was 76% homologous to that of the C. albicans RPS unit RPS620 and the organization, including the non-RPS 3' and 5' bordering sequences, was highly similar. This analysis revealed additional copies of the repeat extraalt element and short additional repeat (SAR) sequences in both RPSd1 and RPS620 not previously identified in the latter. This analysis has allowed us to develop a model of RPSd1 organization and to revise the model for RPS620 organization. An estimate of the average frequency of reorganization (duplication and deletion) per RPS unit in C. dubliniensis was similar to that for C. albicans, but the estimate of frequency of reorganization per C. dubliniensis genome was higher, most probably as a result of the higher estimated average number of RPS clusters in C. dubliniensis. These results demonstrate that the microevolutionary changes identified by the Cd1 fingerprinting probe are based on the reorganization of RPS-like elements and are, therefore, similar to the microevolutionary changes identified by the Ca3 probe of C. albicans. Linkage analysis of pairs of markers situated on either side of an RPS cluster on chromosome 7 further revealed frequent recombination between non-homologous chromosomes at the RPS cluster in C. dubliniensis strains, but not in C. albicans strains, suggesting that RPS clusters may function as recombination hot spots in C. dubliniensis.

Variation of isoenzyme and RAPD patterns in Candida albicans morphological mutants with altered colony ultrastructure

Molecular typing methods were applied to characterize four stable morphological mutants [1] isolated from a UV-induced unstable mutant colony of Candida albicans. The wild-type strain (ATCC 64385), the intermediate unstable mutant and its four morphologically altered derivatives revealed the same electrophoretic karyotypes. Of the five isoenzymes tested (catalase, malate dehydrogenase, glutamate dehydrogenase, acid phosphatase and 3-glucosidase), glutamate dehydrogenase displayed a different enzyme pattern (with an extra band of lower mobility) in the morphological mutants. In contrast, the random amplification DNA polymorphism patterns of the mutant strains differed in all cases from that of the parental strain. Different primers revealed various degrees of DNA polymorphism; one of them (OPC-8) proved to be useful for differentiation between all examined strains. Differences in genetic alterations between spontaneous and induced mutants, and the applicability of different molecular markers to analyse the consequences of induced mutagenesis in C. albicans are discussed.

Extensive chromosome translocation in a clinical isolate showing the distinctive carbohydrate assimilation profile from a candidiasis patient

Variation of the electrophoretic karyotype is common among clinical strains of Candida albicans and chromosome translocation is considered one of the causes of karyotypic variation. Such chromosome translocations may be a mechanism to confer phenotypic diversity on the imperfect fungus C. albicans. A clinical strain, TCH23, from a vaginal candidiasis patient shows distinct carbohydrate assimilation profile, serotype B, no chlamydospore formation and an atypical karyotype (Asakura et al., 1991). To examine the taxonomic relationship among C. albicans, Candida dubliniensis and this strain, we sequenced the internal transcribed spacer 1 (ITS1) of nuclear ribosomal DNA. The ITS1 sequence of TCH23 was identical with that of C. albicans but not of C. dubliniensis. Thus, strain TCH23 was classified as a variant of C. albicans with an atypical phenotype. The chromosomal DNAs of this strain were resolved into 13 bands on pulse-field gel electrophoresis (PFGE). Using DNA probes located at or near both ends of each chromosome of C. albicans, we investigated the chromosome organization of this strain. Referring to the SfiI map of C. albicans 1006 (Chu et al., 1993), we found that seven chromosomal DNA bands in strain TCH23 were reciprocal chromosome translocations. One homologue from chromosomes 1, 2 and 6 and both homologues from chromosomes 4 and 7 participated in these events. One translocation product was composed of three SfiI fragments, one each from chromosomes 2, 4 and 7. We deduced the breakpoints of chromosome translocation from the physical map of this strain; between 1J and 1J1, between 2A and 2U, both ends of 4F2, between 6C and 6O and both ends of 7F.

A single-copy suppressor of the Saccharomyces cerevisae late-mitotic mutants cdc15 and dbf2 is encoded by the Candida albicans CDC14 gene

The Saccharomyces cerevisiae CDC15, DBF2, TEM1 and CDC14 genes encode regulatory proteins that play a crucial role in the latest stages of the M phase of the cell cycle. By complementation of a S. cerevisiae cdc15-lyt1 mutant with a Candida albicans centromeric-based genomic library, we have isolated a homologue of the protein phosphatase-encoding gene CDC14. The sequence analysis of the C. albicans CDC14 gene reveals a putative open reading frame of 1626 base pairs interrupted by an intron located close to the 5' region. Analysis of C. albicans cDNA proved that the intron is processed in vivo. The CaCDC14 gene shares 49% of amino acid sequence identity with the S. cerevisiae CDC14 gene, 46% with Schizosaccharomyces pombe homologue, 35% with Caenorhabditis elegans and 37% and 38% with human CDC14A and CDC14B genes, respectively. As expected, the C. albicans CDC14 gene complemented a S. cerevisiae cdc14-1 mutant. We found that this gene was able to efficiently suppress not only a S.cerevisiae cdc15-lyt1 mutant but also a dbf2-2 mutant in a low number of copies and allowed growth, although very slightly, of a tem1 deletant. Overexpression of the human CDC14A and CDC14B genes complemented, although very poorly, S. cerevisiae cdc15-lyt1 and dbf2-2 mutants, suggesting a conserved function of these genes throughout phylogeny. The sequence of CaCDC14 was deposited in the EMBL database under Accession No. AJ243449.

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.

The TEA/ATTS transcription factor CaTec1p regulates hyphal development and virulence in Candida albicans

The temporal and spatial expression of stage-specific genes during morphological development of fungi and higher eukaryotes is controlled by transcription factors. In this study, we report the cloning and functional analysis of the Candida albicans TEC1 (CaTEC1) gene, a new member of the TEA/ATTS family of transcription factors that regulates C. albicans virulence. The promoters of the type 4, 5 and 6 proteinase isogenes (SAP4-6) contain repetitive TEA/ATTS consensus sequence motifs. This finding suggests a possible role for a homologue of Saccharomyces cerevisiae TEC1 during the activation of proteinase gene expression in C. albicans. CaTEC1 is predominantly expressed in the hyphal form of C. albicans. In vitro, serum-induced hyphal formation as well as evasion from MPhi after phagocytosis is suppressed in catec1/catec1 mutant cells. Furthermore, expression of the proteinase isogenes SAP4-6 is no longer inducible in these mutant cells. The deletion of the CaTEC1 gene attenuates virulence of C. albicans in a systemic model of murine candidiasis, although both mutant and revertant cells that were prepared from infected tissues or the vaginal mucosa grew in a hyphal morphology in vivo. CaTEC1 complements the pseudohyphal and invasive growth defect of haploid and diploid S. cerevisiae tec1/tec1 mutant cells and strongly activates the promoter of FLO11, a gene required for pseudohyphal growth. This study provides the first evidence pointing to an essential role for a member of the TEA/ATTS transcription factor family that had so far only been ascribed to function during development as a virulence regulator in microbial pathogenesis.

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.

High-frequency occurrence of chromosome translocation in a mutant strain of Candida albicans by a suppressor mutation of ploidy shift

Significant occurrence of high-ploidy cells is commonly observed among many Candida albicans strains. We isolated two isogenic strains, STN21 and STN22, each from a half sector of a colony obtained after mild UV-irradiation of a Arg(-) derivative of CBS5736. The two strains were different from each other in ploidy states and chromosome organization. Although cells of STN22 were homogeneous in size and had a single nucleus, high-ploidy cells, with either a single large nucleus or several nuclei, were present together with apparently normal cells with a single nucleus in the cell population of STN21. Flow cytometry showed that STN22 was a stable diploid; however, STN21 seemed to be the mixture of different ploidy states, including diploid and tetraploid. The phenotype of STN21 containing high-ploidy cells is referred to here as the Sps(-) phenotype (suppressor of ploidy shift). STN22 showed a typical electrophoretic karyotype similar to strain 1006 in C. albicans. However, an extra chromosomal band appeared in some clones of STN21 at high frequency. By assignment of several DNA probes, this extra chromosome was shown to be a translocation of the 7F-7G portion of chromosome 7 with the 470 kb DNA segment containing H SfiI fragment from chromosome 4. Thus, this extra chromosome is a hybrid of 4H and 7F-7G. Since the isogenic Sps(+) strain STN22 exhibited no extra chromosome bands, a correlation is suggested between the Sps(-) phenotype and the occurrence of chromosome translocations.

Translation elongation factor 2 is encoded by a single essential gene in Candida albicans

Translation elongation factor 2 (eEF2) is a large protein of more than 800 amino acids which establishes complex interactions with the ribosome in order to catalyze the conformational changes needed for translation elongation. Unlike other yeasts, the pathogenic fungus Candida albicans was found to have a single gene encoding this factor per haploid genome, located on chromosome 2. Expression of this locus is essential for vegetative growth, as evidenced by placing it under the control of a repressible promoter. This C. albicans gene, named EFT2, was cloned and sequenced (EMBL accession number Y09664). Genomic and cDNA sequence analysis identified common transcription initiation and termination signals and an 842 amino acid open reading frame (ORF), which is interrupted by a single intron. Despite some genetic differences, CaEFT2 was capable of complementing a Saccharomyces cerevisiae Deltaeft1 Deltaeft2 null mutant, which lacks endogenous eEF2, indicating that CaEFT2 can be expressed from its own promoter and its intron can be correctly spliced in S. cerevisiae.

Cloning and sequence of a 3.835 kbp DNA fragment containing the HIS4 gene and a fragment of a PEX5-like gene from Candida albicans

We have isolated the Candida albicans HIS4 (CaHIS4) gene by complementation of a his4-34 Saccharomyces cerevisiae mutant. The sequenced DNA fragment contains a putative ORF of 2514 bp, whose translation product shares a global identity of 44% and 55% to the His4 protein homologs of S. cerevisiae and Kluyveromyces lactis, respectively. Analysis of CaHIS4 sequence suggests that, similarly to S. cerevisiae HIS4, it codes for a polypeptide having three separate enzymatic activities (phosphoribosyl-AMP cyclohydrolase, phosphoribosyl-ATP pyrophosphohydrolase and histidinol dehydrogenase) which reside in different domains of the protein. A C. albicans his4 strain is complemented with this gene when using a C. albicans-S. cerevisiae-Escherichia coli shuttle vector, thus enabling the construction of a host system for C. albicans genetic manipulation. In addition, upstream of the sequenced CaHIS4 sequence, we have found the 3'-terminal half of a gene encoding a PEX5-like protein.

Cloning of Candida albicans SEC14 gene homologue coding for a putative essential function

The yeast SEC14 gene product is required for the transport of proteins from the Golgi complex. We have cloned the homologous Candida albicans SEC14 gene (CaSEC14) by functional complementation of a Saccharomyces cerevisiae thermosensitive mutant, sec14ts. Some putative TATA boxes have been identified in CaSEC14 and, contrary to S. cerevisiae SEC14, no introns were found in the Candida homologue. Sequence analysis revealed that CaSec14p is a 301 amino acid protein, 67% identical to S. cerevisiae and Kluyveromyces iactis Sec14p, and 61% identical to the 300 amino-terminal residues of Yarrowia lipolytica Sec14p. Hydrophatic profile analysis of CaSec14p suggests a soluble protein without transmembrane domains as has been described for the S. cerevisiae counterpart. While it was easy to disrupt one allele of SEC14 in C. albicans, repeated attempts to disrupt the second allele were unsuccessful, thus suggesting that the gene could be essential for vegetative growth in C. albicans.