Morphogenesis-independent regulation of actin transcript levels in the pathogenic yeast Candida albicans

Amplification of TLO Mediator Subunit Genes Facilitate Filamentous Growth in Candida Spp

Filamentous growth is a hallmark of C. albicans pathogenicity compared to less-virulent ascomycetes. A multitude of transcription factors regulate filamentous growth in response to specific environmental cues. Our work, however, suggests the evolutionary history of C. albicans that resulted in its filamentous growth plasticity may be tied to a change in the general transcription machinery rather than transcription factors and their specific targets. A key genomic difference between C. albicans and its less-virulent relatives, including its closest relative C. dubliniensis, is the unique expansion of the TLO (TeLOmere-associated) gene family in C. albicans. Individual Tlo proteins are fungal-specific subunits of Mediator, a large multi-subunit eukaryotic transcriptional co-activator complex. This amplification results in a large pool of ‘free,’ non-Mediator associated, Tlo protein present in C. albicans, but not in C. dubliniensis or other ascomycetes with attenuated virulence. We show that engineering a large ‘free’ pool of the C. dubliniensis Tlo2 (CdTlo2) protein in C. dubliniensis, through overexpression, results in a number of filamentation phenotypes typically associated only with C. albicans. The amplitude of these phenotypes is proportional to the amount of overexpressed CdTlo2 protein. Overexpression of other C. dubliniensis and C. albicans Tlo proteins do result in these phenotypes. Tlo proteins and their orthologs contain a Mediator interaction domain, and a potent transcriptional activation domain. Nuclear localization of the CdTlo2 activation domain, facilitated naturally by the Tlo Mediator binding domain or artificially through an appended nuclear localization signal, is sufficient for the CdTlo2 overexpression phenotypes. A C. albicans med3 null mutant causes multiple defects including the inability to localize Tlo proteins to the nucleus and reduced virulence in a murine systemic infection model. Our data supports a model in which the activation domain of ‘free’ Tlo protein competes with DNA bound transcription factors for targets that regulate key aspects of C. albicans cell physiology.

The ascomycete fungus Candida albicans is a leading cause of hospital-acquired bloodstream infections in the United States. Due to limited anti-fungal drug options, there is an approximately 40% mortality rate and over 10,000 deaths per year associated with systemic C. albicans infections. It is unknown why C. albicans is the primary cause of systemic Candidiasis, versus related ascomycetes such as Candida dubliniensis. The genomes of C. albicans and C. dubliniensis are remarkably similar, yet C. dubliniensis has reduced virulence and exhibits less phenotypic plasticity. A striking genomic difference between the fungi is the amplification of the TLO (TeLOmere-associated) genes in C. albicans, which encode a fungal-specific subunit of the Mediator co-activator complex. Amplification results in a large pool of ‘free’ (non-Mediator associated) Tlo protein in C. albicans that is absent in C. dubliniensis. Engineering a large ‘free’ pool of Tlo protein in C. dubliniensis, through overexpression, results in phenotypes common in C. albicans, yet typically absent in C. dubliniensis. Tlo proteins contain a potent transcriptional activation domain. Nuclear localization of the Tlo activation domain is necessary and sufficient for the TLO overexpression phenotypes. This study provides a mechanistic explanation for how TLO amplification in C. albicans may enhance its virulence.

Env7p Associates with the Golgin Protein Imh1 at the trans-Golgi Network in Candida albicans

A multitier regulation exists at the trans-Golgi network in all higher organisms. We report a palmitoylated protein kinase, Env7, that functions at the TGN interface by interacting with two more TGN-resident proteins, namely, Imh1 and Arl1. Palmitoylation seems to be important for the specific localization. This study focuses on the involvement of a ubiquitous protein kinase, whose substrates had not yet been reported from any organism, as an upstream signaling component that modulates the activity of the Imh1-Arl1 complex crucial for maintaining membrane asymmetry. Virulence is significantly diminished in an Env7 mutant. The functioning of this protein in C. albicans seems to be quite different from its nearest homologue in S. cerervisiae, which reflects the evolutionary divergence between these two organisms.

Vesicular dynamics is one of the very important aspects of cellular physiology, an imbalance of which leads to the disorders or diseases in higher eukaryotes. We report the functional characterization of a palmitoylated protein kinase from Candida albicans whose homologue in Saccharomyces cerevisiae has been reported to be involved in negative regulation of membrane fusion and was named Env7. However, the downstream target of this protein remains to be identified. Env7 in C. albicans (CaEnv7) could be isolated from the membrane fraction and localized to vesicular structures associated with the Golgi apparatus. Our work reports Env7 in C. albicans as a new player involved in maintaining the functional dynamics at the trans-Golgi network (TGN) by interacting with two other TGN-resident proteins, namely, Imh1p and Arl1p. Direct interaction could be detected between Env7p and the golgin protein Imh1p. Env7 is itself phosphorylated (Env7p) and phosphorylates Imh1 in vivo. An interaction between Env7 and Imh1 is required for the targeted localization of Imh1. CaEnv7 has a putative palmitoylation site toward both N and C termini. An N-terminal palmitoylation-defective strain retains its ability to phosphorylate Imh1 in vitro. An ENV7 homozygous mutant showed compromised filamentation in solid media and attenuated virulence, whereas an overexpressed strain affected cell wall integrity. Thus, Env7 plays a subtle but important role at the level of multitier regulation that exists at the TGN.

IMPORTANCE A multitier regulation exists at the trans-Golgi network in all higher organisms. We report a palmitoylated protein kinase, Env7, that functions at the TGN interface by interacting with two more TGN-resident proteins, namely, Imh1 and Arl1. Palmitoylation seems to be important for the specific localization. This study focuses on the involvement of a ubiquitous protein kinase, whose substrates had not yet been reported from any organism, as an upstream signaling component that modulates the activity of the Imh1-Arl1 complex crucial for maintaining membrane asymmetry. Virulence is significantly diminished in an Env7 mutant. The functioning of this protein in C. albicans seems to be quite different from its nearest homologue in S. cerervisiae, which reflects the evolutionary divergence between these two organisms.

N-acetylglucosamine kinase, HXK1 is involved in morphogenetic transition and metabolic gene expression in Candida albicans

Candida albicans, a common fungal pathogen which diverged from the baker's yeast Saccharomyces cerevisiae has the unique ability to utilise N-acetylglucosamine, an amino sugar and exhibits phenotypic differences. It has acquired intricate regulatory mechanisms at different levels in accordance with its life style. N-acetylglucosamine kinase, a component of the N-acetylglucosamine catabolic cascade is an understudied gene since Saccharomyces cerevisiae lacks it. We report HXK1 to act as both positive and negative regulator of transcription of genes involved in maintaining cellular homeostasis. It is involved in repression of hyphal specific genes in addition to metabolic genes. Its regulation of filamentation and GlcNAc metabolism is independent of the known classical regulators like EFG1, CPH1, RAS1, TPK2 or TUP1. Moreover, Hxk1-GFP is localised to cytoplasm, nucleus and mitochondria in a condition specific manner. By employing two-step affinity purification, we report the interaction of HXK1 with SIR2 under filamentation inducing conditions. Our work highlights a novel regulatory mechanism involved in filamentation repression and attempts to decipher the GlcNAc catabolic regulatory cascade in eukaryotes.

Detailed comparison of Candida albicans and Candida glabrata biofilms under different conditions and their susceptibility to caspofungin and anidulafungin

Candida biofilm development can be influenced by diverse factors such as substrate, culture medium, carbohydrate source and pH. We have analysed biofilm formation of Candida albicans SC5314 and Candida glabrata ATCC 2001 wild-type strains in the presence of different media (RPMI 1640 versus YNB) and using different pH values (pH 5.6 or 7.0). We determined adhesion and biofilm formation on polystyrene, changes in the expression of adhesin genes during these processes and the susceptibility of mature biofilms to echinocandins. Biofilms formed on polystyrene by both Candida species proved to be influenced strongly by the composition of the medium rather than pH. C. albicans and C. glabrata formed thicker biofilms in RPMI 1640 medium, whereas in YNB medium, both species manifested adhesion rather than characteristic multilayer biofilm architecture. The stimulated biofilm formation in RPMI 1640 medium at pH 7.0 corroborated positively with increased expression of adhesin genes, essential to biofilm formation in vitro, including ALS3 and EAP1 in C. albicans and EPA6 in C. glabrata. The thicker biofilms grown in RPMI 1640 medium were more tolerant to caspofungin and anidulafungin than YNB-grown biofilms. We also observed that mature C. glabrata biofilms were less susceptible in RPMI 1640 medium to echinocandins than C. albicans biofilms. Environmental conditions, i.e. medium and pH, can significantly affect not only biofilm architecture, but also the expression profile of several genes involved during the different stages of biofilm development. In addition, growth conditions may also influence the antifungal-susceptibility profile of fungal populations within biofilm structures. Therefore, before designing any experimental biofilm set-up, it is important to consider the potential influence of external environmental factors on Candida biofilm development.

Comparative transcript profiling of Candida albicans and Candida dubliniensis identifies SFL2, a C. albicans gene required for virulence in a reconstituted epithelial infection model

Candida albicans and Candida dubliniensis are closely related species displaying differences in virulence and genome content, therefore providing potential opportunities to identify novel C. albicans virulence genes. C. albicans gene arrays were used for comparative analysis of global gene expression in the two species in reconstituted human oral epithelium (RHE). C. albicans (SC5314) showed upregulation of hypha-specific and virulence genes within 30 min postinoculation, coinciding with rapid induction of filamentation and increased RHE damage. C. dubliniensis (CD36) showed no detectable upregulation of hypha-specific genes, grew as yeast, and caused limited RHE damage. Several genes absent or highly divergent in C. dubliniensis were upregulated in C. albicans. One such gene, SFL2 (orf19.3969), encoding a putative heat shock factor, was deleted in C. albicans. DeltaDeltasfl2 cells failed to filament under a range of hypha-inducing conditions and exhibited greatly reduced RHE damage, reversed by reintroduction of SFL2 into the DeltaDeltasfl2 strain. Moreover, SFL2 overexpression in C. albicans triggered hyphal morphogenesis. Although SFL2 deletion had no apparent effect on host survival in the murine model of systemic infection, DeltaDeltasfl2 strain-infected kidney tissues contained only yeast cells. These results suggest a role for SFL2 in morphogenesis and an indirect role in C. albicans pathogenesis in epithelial tissues.

Growth phase-dependent variations in transcript profiles for thioredoxin- and glutathione-dependent redox systems followed by budding and hyphal Candida albicans cultures

We report the absolute transcription profiles of 24 genes coding for putative thioredoxin (Trx)- and glutathione (GSH)-dependent redox system components, accompanying the Candida albicans batch culture growth, under either yeast or hyphal conditions. All mRNAs investigated (plus the housekeeping ACT1) displayed significant alterations in their steady-state copy number. Collectively, these quantifications show that: (1) the most abundant mRNAs during active growth coded for putative thiol peroxidases (TSA1, PRX1, AHP11 and AHP12) and for donor thioredoxin Trx1p, i.e. those five mRNAs represented >74% of all transcript molecules quantified in a late exponential phase; (2) the transcripts under study differed between budding and hyphal cells not only in their abundance but also in their profiles throughout the growth stages; (3) mRNA amounts for four GSH-transferase putative genes (GTT12, GTT13, GTT14 and GST3) increased in the stationary phase in yeast but not under filamentous conditions. Hydrogen peroxide resistance, plus GSH, GSSG and reactive oxygen species contents, throughout yeast and hyphal growth, were also studied, and the differences observed were related to the transcript profiles.

The application of tandem-affinity purification to Candida albicans

Tandem-affinity purification (TAP) tagging systems, developed by the research group of Bertrand Seraphin and others, are a means of isolating physiologically relevant protein and protein-nucleic acid complexes. Where complete (or nearly complete) genome sequence data are available for the organism from which the complexes are isolated, their components can be readily identified using mass spectrometry. The most widely used TAP-tag consists of a proximal calmodulin-binding peptide (CBP) and a distal repeated protein A IgG-binding domain with a cleavage site for the tobacco etch virus (TEV) protease positioned between this and the CBP. This tag is expressed as a co-translational fusion to the protein of interest. Purification is achieved under mild conditions through sequential affinity chromatography on IgG (eluting by proteolytic cleavage with TEV protease) and calmodulin (eluting by removal of Ca(2+) ions required for the interaction) resins. The approach has been hugely successful for categorizing the interactome of Saccharomyces cerevisiae. Here, we present vectors for carrying out TAP-tagging in Candida albicans and a protocol for purification of complexes containing TAP-tagged proteins.

Endoplasmic reticulum alpha-glycosidases of Candida albicans are required for N glycosylation, cell wall integrity, and normal host-fungus interaction

The cell surface of Candida albicans is enriched in highly glycosylated mannoproteins that are involved in the interaction with the host tissues. N glycosylation is a posttranslational modification that is initiated in the endoplasmic reticulum (ER), where the Glc(3)Man(9)GlcNAc(2) N-glycan is processed by alpha-glucosidases I and II and alpha1,2-mannosidase to generate Man(8)GlcNAc(2). This N-oligosaccharide is then elaborated in the Golgi to form N-glycans with highly branched outer chains rich in mannose. In Saccharomyces cerevisiae, CWH41, ROT2, and MNS1 encode for alpha-glucosidase I, alpha-glucosidase II catalytic subunit, and alpha1,2-mannosidase, respectively. We disrupted the C. albicans CWH41, ROT2, and MNS1 homologs to determine the importance of N-oligosaccharide processing on the N-glycan outer-chain elongation and the host-fungus interaction. Yeast cells of Cacwh41Delta, Carot2Delta, and Camns1Delta null mutants tended to aggregate, displayed reduced growth rates, had a lower content of cell wall phosphomannan and other changes in cell wall composition, underglycosylated beta-N-acetylhexosaminidase, and had a constitutively activated PKC-Mkc1 cell wall integrity pathway. They were also attenuated in virulence in a murine model of systemic infection and stimulated an altered pro- and anti-inflammatory cytokine profile from human monocytes. Therefore, N-oligosaccharide processing by ER glycosidases is required for cell wall integrity and for host-fungus interactions.

Analysis of RNA species of various sizes from stationary-phase planktonic yeast cells of Candida albicans

We initiated a comparison of Candida albicans stationary-phase gene expression with other growth states. The widely used hot acid phenol method for RNA extraction did not extract rRNA from late stationary-phase cells. The RNA from growing yeast cells, hyphae and biofilm was biased towards small-sized RNA. The 2 : 1 ratio between the two large rRNA bands was rarely obtained. Real-time reverse transcriptase PCR was used to determine mRNA extraction by several methods for OXR1, IRA2, RAD50, PNC1 and CHS2, which have 300 bp-8 kb coding regions, and ACT1, EFB1 and TDH3, sometimes used as internal standards. Only smaller-sized cDNA species were amplified from some extracts. Crushing cells with glass beads in liquid nitrogen before RNA extraction by the hot phenol method (CGB) yielded an unbiased distribution for rRNA and mRNA as verified by real-time reverse transcriptase PCR. With the CGB method, the large mRNA species, RAD50, IRA2 and OXR1, were present throughout the stationary phase, whereas the CSH2 transcript increased. The ACT1, EFB1 and TDH3 transcripts decreased in the stationary phase, making them unsuitable for standardization. The CGB method yielded high-quality RNA with the various growth conditions and permitted the comparison of stationary-phase transcripts with those obtained under other conditions.

Expression ofCaPTC7is developmentally regulated during serum-induced morphogenesis in the human fungal pathogen Candida albicans

Type 2C protein phosphatases (PP2C) represent a diversified protein phosphatase family and play various roles in cells. We previously identified and characterized a novel PP2C phosphatase encoded by the CaPTC7 gene in the human fungal pathogen Candida albicans . The CaPtc7p has 365 amino acids with a PP2C core domain at the C terminus and an additional 116-residue N-terminal sequence containing a mitochondrion-targeting sequence. Here, we show that CaPtc7p is indeed localized in the mitochondrion, the only eukaryotic PP2C phosphatase that has been directly shown to reside in the mitochondrion, suggesting its potential role in the regulation of mitochondrial physiology. Furthermore, we show that the expression of CaPTC7 at both transcriptional and protein levels is developmentally regulated during the serum-induced morphogenesis of C. albicans cells. However, disruption of the two alleles of CaPTC7 does not affect cell viability or filamentous development in C. albicans.

The actin gene in Paracoccidioides brasiliensis: organization, expression and phylogenetic analyses

PbrACT1, the gene responsible for the synthesis of actin in Paracoccidioides brasiliensis, was found as a single copy, organized into six exons and five introns. Its open reading frame (ORF) codes for a putative protein of 375 amino acids, with a molecular mass of 41.5 kDa and an isoelectric point of 5.6. Analysis of the nucleotide sequence revealed a high homology to other fungal actins, the presence of characteristic fungal actin sequences, and heat shock elements at the 5' untranslated region (UTR). Phylogenetic analyses with deduced amino acid sequences of fungal actins grouped P. brasiliensis within the phylum Ascomycota, order Onygenales, in concordance with a few previous reports. Patterns of expression through the temperature-induced morphological transitions from mycelial to yeast-like shapes and reverse, suggests that PbrACT1 is regulated in this process. The PbrACT1 gene sequence is available at the GenBank database under accession number AY383732.

Expression of TPK1 and TPK2 genes encoding PKA catalytic subunits during growth and morphogenesis in Candida albicans

The transcript levels of Candida albicans TPK1 and TPK2 genes, encoding PKA catalytic subunits, as well as phosphotransferase activity, were measured in the parental strain CAI4 and in homozygous tpk1Delta and tpk2Delta mutants during vegetative growth and during yeast-to-mycelial transition in N-acetylglucosamine liquid inducing medium at 37 degrees C. We observed two TPK2 transcripts, a major one of 1.8 kb and a minor one of 1.4 kb, and established by 3'-RACE that they originate from the recognition of the three polyadenylation signals present in the 3' untranslated region of the gene. During vegetative growth of CAI4 strain, the expression profiles of TPK1 and TPK2 varied similarly, reaching maximal expression at the late logarithmic phase. TPK1 mRNA levels were lower than those of TPK2 at all stages measured. In the corresponding homozygous tpk mutants, mRNA levels and the expression patterns of TPK1 and TPK2 were similar to those of CAI4, suggesting that the loss of one catalytic isoform is not compensated by overexpression of the other. Changes in PKA specific activity roughly correlated with fluctuations of mRNA expression levels. During yeast-to-mycelial transition, a sharp increase in TPK1 mRNA levels and in PKA-specific activity correlated with the onset of germ-tube formation in strain tpk2Delta. We also showed that tpk1Delta strain exhibited a delayed morphogenetic shift in comparison with CAI4 and tpk2Delta strains in several liquid inducing media, reinforcing the idea that Tpk1p is important for faster germ-tube appearance.

PMT family of Candida albicans: five protein mannosyltransferase isoforms affect growth, morphogenesis and antifungal resistance

Protein O-mannosyltransferases (Pmt proteins) initiate O-mannosylation of secretory proteins. The PMT gene family of the human fungal pathogen Candida albicans consists of PMT1 and PMT6, as well as three additional PMT genes encoding Pmt2, Pmt4 and Pmt5 isoforms described here. Both PMT2 alleles could not be deleted and growth of conditional strains, containing PMT2 controlled by the MET3- or tetOScHOP1-promoters, was blocked in non-permissive conditions, indicating that PMT2 is essential for growth. A homozygous pmt4 mutant was viable, but synthetic lethality of pmt4 was observed in combination with pmt1 mutations. Hyphal morphogenesis of a pmt4 mutant was defective under aerobic induction conditions, yet increased in embedded or hypoxic conditions, suggesting a role of Pmt4p-mediated O-glycosylation for environment-specific morphogenetic signalling. Although a PMT5 transcript was detected, a homozygous pmt5 mutant was phenotypically silent. All other pmt mutants showed variable degrees of supersensitivity to antifungals and to cell wall-destabilizing agents. Cell wall composition was markedly affected in pmt1 and pmt4 mutants, showing a significant decrease in wall mannoproteins. In a mouse model of haematogenously disseminated infection, PMT4 was required for full virulence of C. albicans. Functional analysis of the first complete PMT gene family in a fungal pathogen indicates that Pmt isoforms have variable and specific roles for in vitro and in vivo growth, morphogenesis and antifungal resistance.

APSES proteins regulate morphogenesis and metabolism in Candida albicans

Fungal APSES proteins regulate morphogenetic processes, including filamentation and differentiation. The human fungal pathogen Candida albicans contains two APSES proteins: the regulator Efg1p and its homologue Efh1p, described here. Overexpression of EFG1 or EFH1 led to similar phenotypes, including pseudohypha formation and opaque-white switching. An efh1 deletion generated no phenotype under most conditions but caused hyperfilamentation in an efg1 background under embedded or hypoxic conditions. This suggests cooperation of these APSES proteins in the suppression of an alternative morphogenetic signaling pathway. Genome-wide transcriptional profiling revealed that EFG1 and EFH1 regulate partially overlapping sets of genes associated with filament formation. Unexpectedly, Efg1p not only regulates genes involved in morphogenesis but also strongly influences the expression of metabolic genes, inducing glycolytic genes and repressing genes essential for oxidative metabolism. Using one- and two-hybrid assays, we further demonstrate that Efg1p is a repressor, whereas Efh1p is an activator of gene expression. Overall, the results suggest that Efh1p supports the regulatory functions of the primary regulator, Efg1p, and indicate a dual role for these APSES proteins in the regulation of fungal morphogenesis and metabolism.

Protein A-tagging for purification of native macromolecular complexes from Candida albicans

Protein A-tagging has become an important tool in characterization of protein-protein interactions in many systems, allowing purification of multicomponent complexes under native conditions. Here we provide a set of vectors that allow protein A-tagging in Candida albicans, through addition of the tag to open reading frames. These vectors were successfully used to generate stably tagged proteins that were functional, shown to be localized appropriately or assembled into complexes. These new vectors comprise a useful addition to the C. albicans molecular toolbox.

The cAMP phosphodiesterase encoded by CaPDE2 is required for hyphal development in Candida albicans

The cAMP-dependent pathway, which regulates yeast-to-hypha morphogenesis in Candida albicans, is controlled by changes in cAMP levels determined by the processes of synthesis and hydrolysis. Both low- and high-affinity cAMP phosphodiesterases are encoded in the C. albicans genome. CaPDE2, encoding the high-affinity cAMP phosphodiesterase, has been cloned and shown to be toxic in Saccharomyces cerevisiae upon overexpression under pGAL1, but functional under the moderate pMET3. Deletion of CaPDE2 causes elevated cAMP levels and responsiveness to exogenous cAMP, higher sensitivity to heat shock, severe growth defects at 42 degrees C and highly reduced levels of EFG1 transcription. In vitro in hypha-inducing liquid medium CaPDE2, deletion prohibits normal hyphal, but not pseudohyphal growth. On solid medium capde2 mutants form aberrant hyphae, with fewer branches and almost no lateral buds, which are deficient in hypha-to-yeast reversion. The phenotypic defects of capde2 mutants show that the cAMP-dependent pathway plays specific roles in hyphal and pseudohyphal development, its regulatory role however, being greater in liquid than on solid medium in vitro. The increased expression of CaPDE2 after serum addition correlates well with a drop in cAMP levels following the initial rise in response to the hyphal inducer. These results suggest that Capde2p mediates a desensitization mechanism by lowering basal cAMP levels in response to environmental stimuli in C. albicans.

Adaptation of the Efg1p morphogenetic pathway in Candida albicans by negative autoregulation and PKA-dependent repression of the EFG1 gene

The Efg1p regulator protein permits hyphal morphogenesis in the human fungal pathogen Candida albicans. We have identified the major promoter of the EFG1 gene as a direct target of Efg1p, resulting in negative autoregulation of EFG1. Enhanced activity of protein kinase A (PKA) isoforms Tpk1p and Tpk2p or exogenous overexpression of EFG1 led to Efg1p-dependent down-regulation of the endogenous EFG1 promoter. Serial deletion analyses of the promoter region revealed that the TATA box region was required for EFG1 autoregulation. By chromatin immunoprecipitation we detected binding of Efg1p to the EFG1 transcriptional initiation region. Furthermore, Sin3p, a component of a specific histone deacetylase complex, was shown to bind to the EFG1 promoter. sin3 mutants grew as budding pseudohyphae and were unable to form true hyphae, similar to strains constitutively expressing EFG1. We propose that the PKA signalling pathway, in addition to its importance in the initial steps of filament formation, is part of a feedback loop that controls EFG1 expression allowing continued hypha formation in inducing conditions. This autoregulation of EFG1 expression is probably mediated through the Sin3p-containing histone deacetylation complex.

Stage-specific gene expression of Candida albicans in human blood

The pathogenic fungus Candida albicans commonly causes mucosal surface infections. In immunocompromised patients, C. albicans may penetrate into deeper tissue, enter the bloodstream and disseminate within the host causing life-threatening systemic infections. In order to elucidate how C. albicans responds to the challenge of a blood environment, we analysed the transcription profile of C. albicans cells exposed to human blood using genomic arrays and a cDNA subtraction protocol. By combining data obtained with these two methods, we were able to identify unique sets of different fungal genes specifically expressed at different stages of this model that mimics bloodstream infections. By removing host cells and incubation in plasma, we were also able to identify several genes in which the expression level was significantly influenced by the presence of these cells. Differentially expressed genes included those that are involved in the general stress response, antioxidative response, glyoxylate cycle as well as putative virulence attributes. These data point to possible mechanisms by which C. albicans ensures survival in the hostile environment of the blood and how the fungus may escape the bloodstream as an essential step in its systemic dissemination.

Two membrane proteins located in the Nag regulon of Candida albicans confer multidrug resistance

Pathogenic fungus Candida albicans can efficiently utilize the aminosugar N-acetylglucosamine (GlcNAc) as energy source. Since the mucosal membrane, the site of infection is rich in amino sugars, this specific adaptation is important for the establishment of infection. The genes encoding for the enzymes of the GlcNAc catabolic pathway, GlcNAc kinase (HXK1), GlcNAc-6-phosphate deacetylase (DAC1), and glucosamine-6-phosphate deaminase (NAG1), are present in a cluster, the Nag regulon, which is associated with virulence. In this study, we have characterized two genes, TMP1 and TMP2, present within the Nag regulon, upstream to DAC1. They encode two membrane associated sugar transporters of the major facilitator superfamily (MFS). The null mutant of TMP1 and TMP2 is able to grow in GlcNAc, implying that they are not involved in GlcNAc transport. However, it shows increased susceptibility to a number of unrelated antifungal compounds such as cycloheximide, 4-nitroquinoline-N-oxide, and 1-10 phenanthroline. Northern blot analysis revealed that TMP1 and TMP2 are upregulated in response to these drugs, suggesting that they function as multiple drug efflux pumps.

Gpa2, a G-protein alpha subunit required for hyphal development in Candida albicans

Candida albicans is able to respond to environmental changes by inducing a distinct morphological program, which is related to the ability to infect mammalian hosts. Although some of the signal transduction pathways involved in this response are known, it is not clear how the environmental signals are sensed and transmitted to these transduction cascades. In this work, we have studied the function of GPA2, a new gene from C. albicans, which encodes a G-protein alpha-subunit homologue. We demonstrate that Gpa2 plays an important role in the yeast-hypha dimorphic transition in the response of C. albicans to some environmental inducers. Deletion of both alleles of the GPA2 gene causes in vitro defects in morphological transitions in Spider medium and SLAD medium and in embedded conditions but not in medium containing serum. These defects cannot be reversed by exogenous addition of cyclic AMP. However, overexpression of HST7, which encodes a component of the filament-inducing mitogen-activated protein kinase (MAPK) cascade, bypasses the Gpa2 requirement. We have obtained different gain-of-function and loss-of-function mutant alleles of the GPA2 gene, which we have introduced in several C. albicans genetic backgrounds. Our results indicate that, in response to environmental cues, Gpa2 is required for the regulation of a MAPK signaling pathway.

A conserved mitogen-activated protein kinase pathway is required for mating in Candida albicans

Candida albicans had been thought to lack a mating process until the recent discovery of a mating type-like locus and mating between MTLa and MTL(alpha) strains. To elucidate the molecular mechanisms that regulate mating in C. albicans, we examined the function of Cph1 and its upstream mitogen-activated protein (MAP) kinase pathway in mating, as they are homologues of the pheromone-responsive MAP kinase pathway in Saccharomyces cerevisiae. We found that overexpressing CPH1 in MTLa, but not in MTLa/alpha strains, induced the transcription of orthologues of S. cerevisiae pheromone-induced genes and also increased mating efficiency. Furthermore, cph1 and hst7 mutants were completely defective in mating, and cst20 and cek1 mutants showed reduced mating efficiency, as in S. cerevisiae. The partial mating defect in cek1 results from the presence of a functionally redundant MAP kinase, Cek2. CEK2 complemented the mating defect of a fus3 kss1 mutant of S. cerevisiae and was expressed only in MTLa or MTL(alpha), but not in MTLa/alpha cell types. Moreover, a cek1 cek2 double mutant was completely defective in mating. Our data suggest that the conserved MAP kinase pathway regulates mating in C. albicans. We also observed that C. albicans mating efficiency was greatly affected by medium composition, indicating the potential involvement of nutrient-sensing pathways in mating in addition to the MAP kinase pathway.

Candida Albicans: a molecular revolution built on lessons from budding yeast

Candida albicans is an opportunistic fungal pathogen that is found in the normal gastrointestinal flora of most healthy humans. However, in immunocompromised patients, blood-stream infections often cause death, despite the use of anti-fungal therapies. The recent completion of the C. albicans genome sequence, the availability of whole-genome microarrays and the development of tools for rapid molecular-genetic manipulations of the C. albicans genome are generating an explosion of information about the intriguing biology of this pathogen and about its mechanisms of virulence. They also reveal the extent of similarities and differences between C. albicans and its benign relative, Saccharomyces cerevisiae.

Construction of FLAG tagging vectors for Candida albicans

We have constructed three new vectors for Candida albicans (pFLAG-Act1, pFLAG-Mal2, and pFLAG-Met3). The proteins can be expressed as C-terminal FLAG-tagged proteins under the control of different promoters (ACT1, MAL2, and MET3). To confirm the protein expression, we used the Renilla reniformis luciferase and the drug efflux pump Cdr1p of Candida albicans as reporters. The luciferase protein expressed by the MET3 promoter was found to have the strongest activity of the three promoters when cultured in a methionine-depleted synthetic medium. Cdr1p was expressed as a C-terminal FLAG-tagged protein using either these vectors or PCR-mediated integration. The fluconazole resistance was increased by the Cdr1p expression in a CDR1 homozygous disruptant. The expressed proteins were detected by Western blotting using the anti-FLAG antibody. We also constructed a Cdr1p-FLAG expressing strain, in which we directly tagged Cdr1p with FLAG on the genome loci, using a PCR-based integrative marker cassette that was amplified using the pFLAG vector. We then confirmed the protein expression by Western blotting. Thus, these new vectors are useful as C. albicans genetic tools.

DNA array studies demonstrate convergent regulation of virulence factors by Cph1, Cph2, and Efg1 in Candida albicans

Candida albicans, normally a human commensal, can cause fatal systemic infections under certain circumstances. Its unique ability to switch from yeast to hyphal growth in response to various environmental signals is inherent to its pathogenicity. Filamentation is regulated by multiple pathways including a Cph1-mediated mitogen-activated protein kinase pathway, an Efg1-mediated cAMP/PKA pathway, and a Cph2 pathway. To gain a general picture of how these various signaling pathways regulate differential gene expression during filamentation, we have constructed a partial C. albicans DNA array of 7,000 genes and used it to study the gene expression profiles using various mutants and growth conditions. By combining this novel technology with a new liquid medium in which cph1/cph1 is defective in filamentation, previously identified differentially expressed genes (ECE1, HWP1, HYR1, RBT1, SAPs5-6, and RBT4) are found to be regulated by all three pathways. In addition, two novel genes, DDR48 and YPL184, have been found to be differentially regulated during hyphal development and by all three pathways. This suggests that distinct filamentation signaling pathways converge to regulate a common set of differentially expressed genes. As one of the mechanisms for the observed convergence, we find that the transcription of a key regulator, TEC1, is regulated by Efg1 and Cph2. Importantly, most of the genes regulated by multiple filamentation pathways encode known virulence factors. Perhaps, C. albicans utilizes converging pathways to regulate its vital virulence factors to ensure its survival and pathogenicity in various host environments.

The basic helix-loop-helix transcription factor Cph2 regulates hyphal development in Candida albicans partly via TEC1

Candida albicans undergoes a morphogenetic switch from budding yeast to hyphal growth form in response to a variety of stimuli and growth conditions. Multiple signaling pathways, including a Cph1-mediated mitogen-activated protein kinase pathway and an Efg1-mediated cyclic AMP/protein kinase A pathway, regulate the transition. Here we report the identification of a basic helix-loop-helix transcription factor of the Myc subfamily (Cph2) by its ability to promote pseudohyphal growth in Saccharomyces cerevisiae. Like sterol response element binding protein 1, Cph2 has a Tyr instead of a conserved Arg in the basic DNA binding region. Cph2 regulates hyphal development in C. albicans, as cph2/cph2 mutant strains show medium-specific impairment in hyphal development and in the induction of hypha-specific genes. However, many hypha-specific genes do not have potential Cph2 binding sites in their upstream regions. Interestingly, upstream sequences of all known hypha-specific genes are found to contain potential binding sites for Tec1, a regulator of hyphal development. Northern analysis shows that TEC1 transcription is highest in the medium in which cph2/cph2 displays a defect in hyphal development, and Cph2 is necessary for this transcriptional induction of TEC1. In vitro gel mobility shift experiments show that Cph2 directly binds to the two sterol regulatory element 1-like elements upstream of TEC1. Furthermore, the ectopic expression of TEC1 suppresses the defect of cph2/cph2 in hyphal development. Therefore, the function of Cph2 in hyphal transcription is mediated, in part, through Tec1. We further show that this function of Cph2 is independent of the Cph1- and Efg1-mediated pathways.

Transcript analysis of 1003 novel yeast genes using high-throughput northern hybridizations

The expression of 1008 open reading frames (ORFs) from the yeast Saccharomyces cerevisiae has been examined under eight different physiological conditions, using classical northern analysis. These northern data have been compared with publicly available data from a microarray analysis of the diauxic transition in S.cerevisiae. The results demonstrate the importance of comparing biologically equivalent situations and of the standardization of data normalization procedures. We have also used our northern data to identify co-regulated gene clusters and define the putative target sites of transcriptional activators responsible for their control. Clusters containing genes of known function identify target sites of known activators. In contrast, clusters comprised solely of genes of unknown function usually define novel putative target sites. Finally, we have examined possible global controls on gene expression. It was discovered that ORFs that are highly expressed following a nutritional upshift tend to employ favoured codons, whereas those overexpressed in starvation conditions do not. These results are interpreted in terms of a model in which competition between mRNA molecules for translational capacity selects for codons translated by abundant tRNAs.

Polyadenylation of ribosomal RNA by Candida albicans

Candida albicans is the leading fungal pathogen in immunocompromised patients such as those with AIDS and malignancies. It is a polymorphic organism existing as a unicellular yeast or as filamentous forms that include pseudohyphae and true hyphae. While studying the early period of hyphal transformation, comparing cDNAs from yeast to those in early transition, we were surprised to find 25S rRNA represented frequently in our differential display assays, suggesting that our reverse transcription with poly-T primers was copying rRNA with extended poly-A 3' ends. We now report that both the yeast forms and germinating organisms polyadenylate some of their 25S rRNA transcripts. We also found a rapid and transient enhancement of this process upon stimulation with serum. These data indicate that 25S rRNA polyadenylation is part of the biological repertoire of C. albicans and its transient upregulation just prior to hyphal development raises the possibility of a regulatory role in this transition.

Recent developments in molecular genetics of Candida albicans

The frequency of opportunistic infections caused by the fungus Candida albicans is very high and is expected to continue to increase as the number of immunocompromised patients rises. Research initiatives to study the biology of this organism and elucidate its pathogenic determinants have therefore expanded significantly during the last 5-10 years. The past few years have also brought continuous improvement in the techniques to study gene function by gene inactivation and by regulated gene expression and to study gene expression and protein localization by using gene reporter systems. As steadily more genomic sequence information from this human fungal pathogen becomes available, we are entering a new era in antimicrobial research. However, many of the currently available molecular genetics tools are poorly adapted to a genome-wide functional analysis in C. albicans, and further development of these tools is hampered by the asexual and diploid nature of this organism. This review outlines recent advances in the development of molecular tools for functional analysis in C. albicans and summarizes current knowledge about the genomic and genetic variability of this important human fungal pathogen.

Divergence of eukaryotic secretory components: the Candida albicans homolog of the Saccharomyces cerevisiae ++Sec20 protein is N terminally truncated, and its levels determine antifungal drug resistance and growth

Sec20p is a component of the yeast Saccharomyces cerevisiae secretory pathway that does not have a close homolog in higher eukaryotic cells. To verify the function of Sec20p in other fungal species, we characterized the gene encoding a Sec20p homolog in the human fungal pathogen Candida albicans. The deduced protein has 27% identity with, but is missing about 100 N-terminal residues compared to S. cerevisiae Sec20p, which is part of the cytoplasmic tail interacting with the cytoplasmic protein Tip20p. Because a strain lacking both C. albicans SEC20 alleles could not be constructed, we placed SEC20 under transcriptional control of two regulatable promoters, MET3p and PCK1p. Repression of SEC20 expression in these strains prevented (MET3p-SEC20 allele) or retarded (PCK1p-SEC20 allele) growth and led to the appearance of extensive intracellular membranes, which frequently formed stacks. Reduced SEC20 expression in the PCK1p-SEC20 strain did not affect morphogenesis but led to a series of hypersensitivity phenotypes including supersensitivity to aminoglycoside antibiotics, to nystatin, to sodium dodecyl sulfate, and to cell wall inhibitors. These results demonstrate the occurrence and function of Sec20p in a fungal species other than S. cerevisiae, but the lack of the N-terminal domain and the apparent absence of a close TIP20 homolog in the C. albicans genome also indicate a considerable diversity in mechanisms of retrograde vesicle traffic in eukaryotes.

Repression of hyphal proteinase expression by the mitogen-activated protein (MAP) kinase phosphatase Cpp1p of Candida albicans is independent of the MAP kinase Cek1p

Cpp1p is a putative mitogen-activated protein (MAP) kinase phosphatase that suppresses Candida albicans hyphal formation at 25 degrees C through its probable substrate, the Cek1p filamentation MAP kinase. Here we report that expression of the serum-induced genes SAP4-6 and HYR1 increased several fold in hyphal forms of a cpp1/cpp1 null mutant, while the rate and extent of hyphal development up to 5 h were normal. Therefore, we provide evidence that Cpp1p represses hyphal gene expression by acting through a Cek1p-independent mechanism. SAP4-6 and HYR1 transcripts were undetectable in a null mutant of another key regulator of filamentation, Efg1p; thus, Efg1p and Cpp1p oppose each other during the expression of these genes in hyphal forms.

Crk1, a novel Cdc2-related protein kinase, is required for hyphal development and virulence in Candida albicans

Both mitogen-activated protein kinases and cyclin-dependent kinases play a role in hyphal development in Candida albicans. Using an oligonucleotide probe-based screen, we have isolated a new member of the Cdc2 kinase subfamily, designated Crk1 (Cdc2-related kinase). The protein sequence of Crk1 is most similar to those of Saccharomyces cerevisiae Sgv1 and human Pkl1/Cdk9. In S. cerevisiae, CRK1 suppresses some, but not all, of the defects associated with an sgv1 mutant. Deleting both copies of CRK1 in C. albicans slows growth slightly but leads to a profound defect in hyphal development under all conditions examined. crk1/crk1 mutants are impaired in the induction of hypha-specific genes and are avirulent in mice. Consistent with this, ectopic expression of the Crk1 kinase domain (CRK1N) promotes filamentous or invasive growth in S. cerevisiae and hyphal development in C. albicans. The activity of Crk1 in S. cerevisiae requires Flo8 but is independent of Ste12 and Phd1. Similarly, Crk1 promotes filamentation through a route independent of Cph1 and Efg1 in C. albicans. RAS1(V13) can also activate filamentation in a cph1/cph1 efg1/efg1 double mutant. Interestingly, CRK1N produces florid hyphae in ras1/ras1 strains, while RAS1(V13) generates feeble hyphae in crk1/crk1 strains.

A phosphatidylinositol 3-kinase of Candida albicans: molecular cloning and characterization

A phosphatidylinositol (PI) 3-kinase gene (CaVPS34) of the human pathogenic yeast Candida albicans was cloned by a PCR-based homology approach. The open reading frame encodes a 1020 amino acid protein with a calculated molecular weight of 118 kDa and a relative isoelectric point of 6.9. It shares 47% sequence identity with Saccharomyces cerevisiae Vps34p. Southern pattern indicated that CaVPS34 is probably present as a single copy gene per haploid genome in C. albicans. We localized the CaVPS34 gene on chromosome 1. Under all conditions tested a major CaVPS34 transcript of approximately 3. 5 kb could be detected. CaVPS34 mRNA levels increased during exponential growth up to 12-fold followed by a decline upon entry into stationary phase. The size of a 6xHis tag-CaVps34p fusion protein purified from Escherichia coli is in agreement with the calculated molecular mass of CaVps34p. It exhibits in vitro PI 3-kinase activity and produces only phosphatidylinositol 3-phosphate. The CaVPS34 gene under the control of its own promoter were not able to complement the temperature-sensitive growth of S. cerevisiae vps34. However, overexpression of CaVPS34 was sufficient to rescue the temperature-sensitive vps34 phenotype, suggesting a functional conservation in C. albicans.

Morphogenesis, adhesive properties, and antifungal resistance depend on the Pmt6 protein mannosyltransferase in the fungal pathogen candida albicans

Protein mannosyltransferases (Pmt proteins) initiate O glycosylation of secreted proteins in fungi. We have characterized PMT6, which encodes the second Pmt protein of the fungal pathogen Candida albicans. The residues of Pmt6p are 21 and 42% identical to those of C. albicans Pmt1p and S. cerevisiae Pmt6p, respectively. Mutants lacking one or two PMT6 alleles grow normally and contain normal Pmt enzymatic activities in cell extracts but show phenotypes including a partial block of hyphal formation (dimorphism) and a supersensitivity to hygromycin B. The morphogenetic defect can be suppressed by overproduction of known components of signaling pathways, including Cek1p, Cph1p, Tpk2p, and Efg1p, suggesting a specific Pmt6p target protein upstream of these components. Mutants lacking both PMT1 and PMT6 are viable and show pmt1 mutant phenotypes and an additional sensitivity to the iron chelator ethylenediamine-di(o-hydroxyphenylacetic acid). The lack of Pmt6p significantly reduces adherence to endothelial cells and overall virulence in a mouse model of systemic infection. The results suggest that Pmt6p regulates a more narrow subclass of proteins in C. albicans than Pmt1p, including secreted proteins responsible for morphogenesis and antifungal sensitivities.

Protein kinase A encoded by TPK2 regulates dimorphism of Candida albicans

External signals induce the switch from a yeast to a hyphal growth form in the fungal pathogen Candida albicans. We demonstrate here that the catalytic subunit of a protein kinase A (PKA) isoform encoded by TPK2 is required for internal signalling leading to hyphal differentiation. TPK2 complements the growth defect of a Saccharomyces cerevisiae tpk1-3 mutant and Tpk2p is able to phosphorylate an established PKA-acceptor peptide (kemptide). Deletion of TPK2 blocks morphogenesis and partially reduces virulence, whereas TPK2 overexpression induces hyphal formation and stimulates agar invasion. The defective tpk2 phenotype is suppressed by overproduction of known signalling components, including Efg1p and Cek1p, whereas TPK2 overexpression reconstitutes the cek1 but not the efg1 phenotype. The results indicate that PKA activity of Tpk2p is an important contributing factor in regulating dimorphism of C. albicans.

Selection of genes repressed by cAMP that are induced by nutritional limitation in Saccharomyces cerevisiae

DNA-lacZ fusion libraries of yeast Saccharomyces cerevisiae were used to select genes coordinately regulated by the Ras-cAMP-cAPK signalling pathway. Sixteen new genes (AGP1, APE2, APE3, FPS1, GUT2, MDH2, PLB2, PYK2, RNR3, SUR1, UGA1, YHR033w, YBR006w, YHR143w, YMR086w and YOR173w) were found to be repressed by cAMP. Most of these genes encode for metabolic enzymes and are induced by nutritional limitations. These common properties suggest a role of this pathway in the metabolic adjustment of the cell to nutritional variations. The induction of 10 of these genes is reduced in the msn2,msn4 double mutant, which emphasizes the role of the Msn2/4p transcriptional activators in mediating the Ras-cAMP-cAPK signalling pathway. The Msn2p/Msn4p-independent expression of the six other genes suggests the existence of other regulatory systems under the control of this pathway.

Control of white-opaque phenotypic switching in Candida albicans by the Efg1p morphogenetic regulator

Phenotypic switching in Candida albicans spontaneously generates different cellular morphologies and is manifested in strain WO-1 by the reversible switching between the white and opaque phenotypes. We present evidence that phenotypic switching is regulated by the Efg1 protein, which is known as an essential element of hyphal development (dimorphism). Firstly, EFG1 is expressed specifically in cells of the white but not the opaque phenotype. During mass conversion from the opaque to the white phenotype, the EFG1 transcript level correlates with competence of switching of opaque cells to the white form. Secondly, overexpression of EFG1 by a PCK1p-EFG1 fusion forces opaque-phase cells to switch to the white form with a high level of efficiency. Thirdly, low-level expression of EFG1 in strain CAI-8 generates a cellular phenotype similar to that of opaque cells in that cells bud as short rods, which cannot be induced to form hyphae in standard conditions; such cells (unlike authentic opaque cells) lack typical surface "pimples." Importantly, the opaque-specific OP4 transcript is induced in the opaque-like cells generated by strain CAI8 as a response to low-level expression of EFG1. The results suggest that high EFG1 expression levels induce and maintain the white cell form while low EFG1 expression levels induce and maintain the opaque cell form. It is proposed that changes in EFG1 expression determine or contribute to phenotypic switching events in C. albicans.

Transcript analysis of 250 novel yeast genes from chromosome XIV

The European Functional Analysis Network (EUROFAN) is systematically analysing the function of novel Saccharomyces cerevisiae genes revealed by genome sequencing. As part of this effort our consortium has performed a detailed transcript analysis for 250 novel ORFs on chromosome XIV. All transcripts were quantified by Northern analysis under three quasi-steady-state conditions (exponential growth on rich fermentative, rich non-fermentative, and minimal fermentative media) and eight transient conditions (glucose derepression, glucose upshift, stationary phase, nitrogen starvation, osmo-stress, heat-shock, and two control conditions). Transcripts were detected for 82% of the 250 ORFs, and only one ORF did not yield a transcript of the expected length (YNL285w). Transcripts ranged from low (62%), moderate (16%) to high abundance (2%) relative to the ACT1 mRNA. The levels of 73% of the 206 chromosome XIV transcripts detected fluctuated in response to the transient states tested. However, only a small number responded strongly to the transients: eight ORFs were induced upon glucose upshift; five were repressed by glucose; six were induced in response to nitrogen starvation; three were induced in stationary phase; five were induced by osmo-stress; four were induced by heat-shock. These data provide useful clues about the general function of these ORFs and add to our understanding of gene regulation on a genome-wide basis.

Overlapping coding regions and trancriptional units of two essential chromosomal genes (CCT8, TRP1)in the fungal pathogen Candida albicans

Sequencing of the 3'-untranslated region of the CCT8 gene of the fungal pathogen Candida albicans revealed that the CCT8 coding region overlaps 13 bp with the coding region of the convergently orientated TRP1 gene. The same overlap was found in three strains with different genetic backgrounds. 3'-RACE was used to determine that the CCT8 and TRP1 transcripts extended significantly into the coding region of the adjacent gene, which also contained sequences encoding the poly(A) addition site. A strain retaining one wild-type CCT8/TRP1 locus on one chromosome and a deletion on the other homologous chromosome contained both CCT8 and TRP1 transcripts; this result indicates that both transcripts are synthesized from the same gene locus. The CCT8/TRP1 gene pair of C . albicans constitutes an extreme natural case of transcriptional overlap in a eukaryote. The results confirm that convergent overlapping transcription units are compatible with expression of the overlapping genes.

A Candida albicans chaperonin subunit (CaCct8p) as a suppressor of morphogenesis and Ras phenotypes in C. albicans and Saccharomyces cerevisiae

Saccharomyces cerevisiae and the pathogen Candida albicans can be induced to undergo morphogenesis from a yeast to a filamentous form. A C. albicans gene (CaCCT8) was identified encoding a subunit of the Cct chaperonin complex, whose expression prevents filament formation in both fungi without interfering with growth of the yeast form. In S. cerevisiae, pseudohyphal growth induced by Ras2Val19, by overproduction of Phd1p or by expression of the C. albicans EFG1 gene, was blocked by CaCct8p and its N-terminally deleted derivative CaCct8-delta1p; in contrast, pseudohyphal induction by other components (Cph1p, Cdc42p) could not be suppressed, indicating that morphogenesis per se is not inhibited. CaCCT8 expression also interfered with other Ras2pVal19 phenotypes, including heat sensitivity, lack of glycogen accumulation and lack of sporulation. In C. albicans, overproduction of CaCct8p effectively blocked hyphal morphogenesis induced by starvation conditions and by serum. The results suggest that the activity of a component in the Ras2p signal transduction pathway is suppressed by excess chaperonin subunits. This component may be a novel folding target for the Cct complex. In agreement with this hypothesis, disruption of one of the two CaCCT8 alleles in C. albicans led to defective hyphal morphogenesis.

Multiple functions of Pmt1p-mediated protein O-mannosylation in the fungal pathogen Candida albicans

Protein mannosylation by Pmt proteins initiates O-glycosylation in fungi. We have identified the PMT1 gene and analyzed the function of Pmt1p in the fungal human pathogen Candida albicans. Mutants defective in PMT1 alleles lacked Pmt in vitro enzymatic activity, showed reduced growth rates, and tended to form cellular aggregates. In addition, multiple specific deficiencies not known in Saccharomyces cerevisiae (including defective hyphal morphogenesis; supersensitivity to the antifungal agents hygromycin B, G418, clotrimazole, and calcofluor white; and reduced adherence to Caco-2 epithelial cells) were observed in pmt1 mutants. PMT1 deficiency also led to faster electrophoretic mobility of the Als1p cell wall protein and to elevated extracellular activities of chitinase. Homozygous pmt1 mutants were avirulent in a mouse model of systemic infection, while heterozygous PMT1/pmt1 strains showed reduced virulence. The results indicate that protein O-mannosylation by Pmt proteins occurs in different fungal species, where PMT1 deficiency can lead to defects in multiple cellular functions.

The TATA-binding protein (TBP) from the human fungal pathogen Candida albicans can complement defects in human and yeast TBPs

Candida albicans is the major fungal pathogen in humans, yet little is known about transcriptional regulation in this organism. Therefore, we have isolated, characterized, and expressed the C. albicans TATA-binding protein (TBP) gene (TBP1), because this general transcription initiation factor plays a key role in the activation and regulation of eukaryotic promoters. Southern and Northern blot analyses suggest that a single C. albicans TBP1 locus is expressed at similar levels in the yeast and hyphal forms of this fungus. The TBP1 open reading frame is 716 bp long and encodes a functional TBP of 27 kDa. C. albicans TBP is capable of binding specifically to a TATA box in vitro, substituting for the human TBP to activate basal transcription in vitro, and suppressing the lethal delta spt15 mutation in Saccharomyces cerevisiae. The predicted amino acid sequences of TBPs from C. albicans and other organisms reveal a striking pattern of C-terminal conservation and N-terminal variability: the C-terminal DNA-binding domain displays at least 80% amino acid sequence identity to TBPs from fungi, flies, nematodes, slime molds, plants, and humans. Sequence differences between human and fungal TPBs in the DNA-binding domain may represent potential targets for antifungal therapy.

Regulation of chitin synthesis during dimorphic growth of Candida albicans

Candida albicans has three genes encoding chitin synthase enzymes. In wild-type strains, the expression of CHS2 and CHS3 peaked 1-2 h after the induction of hyphal growth, whilst mRNA levels in a non-germinative strain, CA2, remained low under the same conditions. CHS1 gene expression did not peak during germ tube formation but remained at low levels in both yeast and hyphal growth. The pattern of gene expression did not predict the changes in measured chitin synthase activities or changes in chitin content during dimorphic transition. Chitin synthase activity increased steadily, and did not peak shortly after germ tube induction, and activity profiles were similar in germ-tube-competent and germ-tube-negative strains. The phenotype of a delta chs2 null mutant suggested that CHS2 encoded the major enzyme activity in vitro and was largely responsible for elevated chitin synthase activities in microsomal preparations from hyphal cells compared to yeast cells. However, CaChs3p was responsible for synthesis of most chitin in both yeast and hyphae. Three independent chitin assays gave markedly different estimates of the relative chitin content of yeast and hyphae and wild-type and chs mutants. Only one of the methods gave a significantly higher chitin content for hyphal compared to yeast cell walls and a lower chitin content for hyphae of the delta chs2 null mutant compared to the parental strain.

Characterization and regulation of the genes encoding ribosomal proteins L39 and S7 of the human pathogen Candida albicans

Genes encoding the Candida albicans ribosomal proteins L39 and S7 (RPL39, RPS7) were isolated and sequenced. From RPL39 cDNA a single intron interrupting the fifth codon in the genomic sequence could be deduced. Two homologous RPL39 genes in Saccharomyces cerevisiae contain a single intron in a conserved position. In contrast, C. albicans RPS7 was found to lack an intron, while both S. cerevisiae homologs are interrupted by single introns. The deduced L39 and S7 proteins contained 67% and 83% identical residues compared to the S. cerevisiae homologs. During hyphal induction the RPL39, RPS7 and RPL29 transcript levels increased three- to six-fold relative to ribosomal RNA, while ACT1 and RPS33 control transcripts were not regulated extensively. As suggested by unaltered transcript stabilities during hyphal induction, this regulation occurs on the transcriptional level; a conserved 18 bp palindromic sequence (5'-TTAGGGCTATAGCCCTAA-3'), which is present in the promoter regions of the RPL39 and RPS7 genes, may be involved in regulation.

Sequence and promoter regulation of the PCK1 gene encoding phosphoenolpyruvate carboxykinase of the fungal pathogen Candida albicans

The PCK1 gene encoding PEP carboxykinase (Pck1) of the fungal pathogen Candida albicans was isolated and sequenced. The deduced Pck1 protein has high homology to ATP-dependent Pck1 proteins in other species, especially to Pck1 of Saccharomyces cerevisiae (70% homology), but not to GTP-dependent Pck1 proteins. PCK1 transcript levels were efficiently repressed by glucose and derepressed (induced) on gluconeogenetic carbon sources. PCK1 regulation occurs on the level of transcription, as demonstrated by a fusion of the PCK1 promoter to the LAC4 reporter gene, yielding derepressed/repressed expression ratios of > 100. Homologous sequences in the PCK1 promoters of C. albicans and S. cerevisiae were identified. The PCK1 promoter may be useful to efficiently regulate expression and thereby test the function of genes in C. albicans.

Temperature-related expression of the vacuolar aspartic proteinase (APR1) gene and beta-N-acetylglucosaminidase (HEX1) gene during Candida albicans morphogenesis

Expression of the Candida albicans vacuolar aspartic proteinase (APR1) and beta-N-acetylglucosaminidase (HEX1) genes was studied when carbon-starved cells of strains ATCC 10261 and A72 were induced to grow as yeast or as germ tube-forming cells. Amounts of APR1 mRNA were similar under yeast or germ tube growth conditions. However, more APR1 mRNA was present in cells grown at 28 degrees C than in cells grown at 37 degrees C. The Apr1 enzyme activity of cell-free extracts was not affected by cellular morphology, culture pH or growth temperature. Amounts of HEX1 mRNA were also higher in N-acetylglucosamine (GlcNAc)-induced cells grown at 28 degrees C than in cells grown at 37 degrees C. There was slightly more HEX1 mRNA in cells grown at pH 4.5 than in cells grown at pH 6.7. The beta-N-acetylglucosaminidase activities of GlcNAc-grown cells correlated with the amounts of HEX1 mRNA and were higher when cells were grown at a lower temperature and at a lower pH. Although a similar temperature- and pH-dependent pattern of HEX1 mRNA expression was seen in cells grown on glucose, the enzyme activities in cell-free extracts were all very low. These data indicate that the APR1 and HEX1 genes play no direct role in the dimorphic transition of C. albicans and that transcription of both genes appears to be temperature regulated when the cells are released from carbon starvation. The expression of HEX1 mRNA is in part under the control of culture pH and translation of HEX1 mRNA seems to be regulated by glucose.