Bistable expression of WOR1, a master regulator of white-opaque switching in Candida albicans

Fungal sex and pathogenesis

Human fungal pathogens are associated with diseases ranging from dandruff and skin colonization to invasive bloodstream infections. The major human pathogens belong to the Candida, Aspergillus, and Cryptococcus clades, and infections have high and increasing morbidity and mortality. Many human fungal pathogens were originally assumed to be asexual. However, recent advances in genome sequencing, which revealed that many species have retained the genes required for the sexual machinery, have dramatically influenced our understanding of the biology of these organisms. Predictions of a rare or cryptic sexual cycle have been supported experimentally for some species. Here, I examine the evidence that human pathogens reproduce sexually. The evolution of the mating-type locus in ascomycetes (including Candida and Aspergillus species) and basidiomycetes (Malassezia and Cryptococcus) is discussed. I provide an overview of how sex is suppressed in different species and discuss the potential associations with pathogenesis.

Yeasts in the gut: from commensals to infectious agents

BACKGROUND

Controversy still surrounds the question whether yeasts found in the gut are causally related to disease, constitute a health hazard, or require treatment.

METHODS

The authors present the state of knowledge in this area on the basis of a selective review of articles retrieved by a PubMed search from 2005 onward. The therapeutic recommendations follow the current national and international guidelines.

RESULTS

Yeasts, mainly Candida species, are present in the gut of about 70% of healthy adults. Mucocutaneous Candida infections are due either to impaired host defenses or to altered gene expression in formerly commensal strains. The expression of virulence factors enables yeasts to form biofilms, destroy tissues, and escape the immunological attacks of the host. Yeast infections of the intestinal mucosa are of uncertain clinical significance, and their possible connection to irritable bowel syndrome, while plausible, remains unproved. Yeast colonization can trigger allergic reactions. Mucosal yeast infections are treated with topically active polyene antimycotic drugs. The adjuvant administration of probiotics is justified on the basis of positive results from controlled clinical trials.

CONCLUSION

The eradication of intestinal yeasts is advised only for certain clearly defined indications.

A delay model for noise-induced bi-directional switching

Many biological systems can switch between two distinct states. Once switched, the system remains stable for a period of time and may switch back to its original state. A gene network with bistability is usually required for the switching and stochastic effect in the gene expression may induce such switching. A typical bistable system allows one-directional switching, in which the switch from the low state to the high state or from the high state to the low state occurs under different conditions. It is usually difficult to enable bi-directional switching such that the two switches can occur under the same condition. Here, we present a model consisting of standard positive feedback loops and an extra negative feedback loop with a time delay to study its capability to produce bi-directional switching induced by noise. We find that the time delay in the negative feedback is critical for robust bi-directional switching and the length of delay affects its switching frequency.

White-opaque switching in Candida albicans

The human commensal yeast Candida albicans undergoes an epigenetic switch between two distinct types of cells, referred to as white and opaque. These two cell types differ in many respects, including their cell and colony morphologies, their metabolic states, their mating behaviors, their preferred niches in the host, and their interactions with the host immune system. Each of the two cell types is heritable for many generations and switching between them appears stochastic; however, environmental cues can significantly alter the frequency of switching. We review recent work on white-opaque switching, including the establishment of the transcriptional circuit underlying this switch, the identification of environmental signals that affect switching rates, newly discovered differences between the two types of cells, and the involvement of white-opaque switching in biofilm formation. We also review recent speculation on the evolution and adaptive value of white-opaque switching.

Genes selectively up-regulated by pheromone in white cells are involved in biofilm formation in Candida albicans

To mate, MTL-homozygous strains of the yeast pathogen Candida albicans must switch from the white to opaque phase. Mating-competent opaque cells then release pheromone that induces polarization, a G1 block and conjugation tube formation in opaque cells of opposite mating type. Pheromone also induces mating-incompetent white cells to become adhesive and cohesive, and form thicker biofilms that facilitate mating. The pheromone response pathway of white cells shares the upstream components of that of opaque cells, but targets a different transcription factor. Here we demonstrate that the genes up-regulated by the pheromone in white cells are activated through a common cis-acting sequence, WPRE, which is distinct from the cis-acting sequence, OPRE, responsible for up-regulation in opaque cells. Furthermore, we find that these white-specific genes play roles in white cell biofilm formation, and are essential for biofilm formation in the absence of an added source of pheromone, suggesting either an autocrine or pheromone-independent mechanism. These results suggest an intimate, complex and unique relationship between switching, mating and MTL-homozygous white cell biofilm formation, the latter a presumed virulence factor in C. albicans.

Candida albicans, like other microbial pathogens, form protective biofilms on host tissue, prosthetics and catheters. But C. albicans forms two types of biofilm, one by cells of majority strains that are heterozygous at the mating type locus, and another by white cells of minority strains that are homozygous at the mating type locus. These latter biofilms are enhanced by mating-competent minority opaque cells, a source of pheromone. The white cell biofilm response to pheromone is regulated by a pheromone response pathway that shares all of the upper components of the opaque cell mating response pathway, but targets a different transcription factor and activates different phase-specific downstream genes. Here we demonstrate that genes are up-regulated by pheromone in white cells through a common white cis-acting specific pheromone response element (WPRE), distinct from the element (OPRE) responsible for pheromone up-regulation of genes in opaque cells. In addition, the pheromone-induced white-specific genes play essential roles in biofilm formation. We further demonstrate that in the absence of minority opaque cells, the source of pheromone, majority white cells form biofilms through a process that is still dependent upon the same pheromone response pathway and targeted white-specific genes, suggesting either an autocrine system that involves the auto release of pheromone of the opposite mating type, or pheromone-independent activation. These observations indicate a unique interdependency of white cell biofilm formation, a presumed pathogenic trait, switching and mating in C. albicans.

The nuclear protein Sge1 of Fusarium oxysporum is required for parasitic growth

Dimorphism or morphogenic conversion is exploited by several pathogenic fungi and is required for tissue invasion and/or survival in the host. We have identified a homolog of a master regulator of this morphological switch in the plant pathogenic fungus Fusarium oxysporum f. sp. lycopersici. This non-dimorphic fungus causes vascular wilt disease in tomato by penetrating the plant roots and colonizing the vascular tissue. Gene knock-out and complementation studies established that the gene for this putative regulator, SGE1 (SIX Gene Expression 1), is essential for pathogenicity. In addition, microscopic analysis using fluorescent proteins revealed that Sge1 is localized in the nucleus, is not required for root colonization and penetration, but is required for parasitic growth. Furthermore, Sge1 is required for expression of genes encoding effectors that are secreted during infection. We propose that Sge1 is required in F. oxysporum and other non-dimorphic (plant) pathogenic fungi for parasitic growth.

Antigenic and phenotypic variations in fungi

Mechanisms to vary the phenotypic characteristics of fungi are diverse and can be important for their life cycle. This review summarizes phenotypic variability in fungi and divides this phenomenon into three topics: (i) morphological transitions, which are environmentally induced and involve the entire fungal population, (ii) reversible phenotypic switching between different colony morphologies, which is restricted to a small fraction of the population, and (iii) antigenic variation of surface antigens, which can be immuno-dominant epitopes happens in individual fungal cells.

Transcriptional loops meet chromatin: a dual-layer network controls white-opaque switching in Candida albicans

The human pathogen Candida albicans is able to undergo a reversible switch between two distinct cell types called white and opaque, which are considered different transcriptional states of cells harbouring identical genomes. The present model of switching regulation includes the bistable expression of a master switch gene that is controlled by multiple transcriptional feedback loops. Here, we show that chromatin-modifying enzymes constitute an additional important regulatory layer of morphogenetic switching. We identify eight chromatin modifiers as switching modulators. Extensive epistasis analysis maps them into at least two independent signalling pathways overlaying the known transcriptional network. Interestingly, we identify the conserved Set3/Hos2 histone deacetylase complex as a key regulator relying on the methylation status of histone H3 lysine 4 for switching modulation. Furthermore, we demonstrate that opaque to white switching is facilitated by the presence of adenine in vitro, but adenine has no effect on switching once the Set3/Hos2 complex is disrupted. Our observations postulate that chromatin modifications may serve as a means to integrate environmental or host stimuli through the underlying transcriptional circuits to determine cell fate in C. albicans.

Stress-induced phenotypic switching in Candida albicans

Candida albicans is both a common commensal and an opportunistic pathogen, being a prevalent cause of mucosal and systemic infections in humans. Phenotypic switching between white and opaque forms is a reversible transition that influences virulence, mating behavior, and biofilm formation. In this work, we show that a wide range of factors induces high rates of switching from white to opaque. These factors include different forms of environmental stimuli such as genotoxic and oxidative stress, as well as intrinsic factors such as mutations in DNA repair genes. We propose that these factors increase switching to the opaque phase via a common mechanism-inhibition of cell growth. To confirm this hypothesis, growth rates were artificially manipulated by varying expression of the CLB4 cyclin gene; slowing cell growth by depleting CLB4 resulted in a concomitant increase in white-opaque switching. Furthermore, two clinical isolates of C. albicans, P37005 and L26, were found to naturally exhibit both slow growth and high rates of white-opaque switching. Notably, suppression of the slow growth phenotype suppressed hyperswitching in the P37005 isolate. Based on the sensitivity of the switch to levels of the master regulator Wor1, we propose a model for how changes in cellular growth modulate white-opaque switching frequencies.

Hub genes with positive feedbacks function as master switches in developmental gene regulatory networks

MOTIVATION

Spatio-temporal regulation of gene expression is an indispensable characteristic in the development processes of all animals. 'Master switches', a central set of regulatory genes whose states (on/off or activated/deactivated) determine specific developmental fate or cell-fate specification, play a pivotal role for whole developmental processes. In this study on genome-wide integrative network analysis the underlying design principles of developmental gene regulatory networks are examined.

RESULTS

We have found an intriguing design principle of developmental networks: hub nodes, genes with high connectivity, equipped with positive feedback loops are prone to function as master switches. This raises the important question of why the positive feedback loops are frequently found in these contexts. The master switches with positive feedback make the developmental signals more decisive and robust such that the overall developmental processes become more stable. This finding provides a new evolutionary insight: developmental networks might have been gradually evolved such that the master switches generate digital-like bistable signals by adopting neighboring positive feedback loops. We therefore propose that the combined presence of positive feedback loops and hub genes in regulatory networks can be used to predict plausible master switches.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Bistable switches control memory and plasticity in cellular differentiation

Development of stem and progenitor cells into specialized tissues in multicellular organisms involves a series of cell fate decisions. Cellular differentiation in higher organisms is generally considered irreversible, and the idea of developmental plasticity in postnatal tissues is controversial. Here, we show that inhibition of mitogen-activated protein kinase (MAPK) in a human bone marrow stromal cell-derived myogenic subclone suppresses their myogenic ability and converts them into satellite cell-like precursors that respond to osteogenic stimulation. Clonal analysis of the induced osteogenic response reveals ultrasensitivity and an "all-or-none" behavior, hallmarks of a bistable switch mechanism with stochastic noise. The response demonstrates cellular memory, which is contingent on the accumulation of an intracellular factor and can be erased by factor dilution through cell divisions or inhibition of protein synthesis. The effect of MAPK inhibition also exhibits memory and appears to be controlled by another bistable switch further upstream that determines cell fate. Once the memory associated with osteogenic differentiation is erased, the cells regain their myogenic ability. These results support a model of cell fate decision in which a network of bistable switches controls inducible production of lineage-specific differentiation factors. A competitive balance between these factors determines cell fate. Our work underscores the dynamic nature of cellular differentiation and explains mechanistically the dual properties of stability and plasticity associated with the process.

A constitutively active GPCR governs morphogenic transitions in Cryptococcus neoformans

Sex in fungi is driven by peptide pheromones sensed through seven-transmembrane pheromone receptors. In Cryptococcus neoformans, sexual reproduction occurs through an outcrossing/heterothallic a- sexual cycle or an inbreeding/homothallic - unisexual mating process. Pheromone receptors encoded by the mating-type locus (MAT) mediate reciprocal pheromone sensing during opposite-sex mating and contribute to but are not essential for unisexual mating. A pheromone receptor-like gene, CPR2, was discovered that is not encoded by MAT and whose expression is induced during a- mating. cpr2 mutants are fertile but have a fusion defect and produce abnormal hyphal structures, whereas CPR2 overexpression elicits unisexual reproduction. When heterologously expressed in Saccharomyces cerevisiae, Cpr2 activates pheromone responses in the absence of any ligand. This constitutive activity results from an unconventional residue, Leu(222), in place of a conserved proline in transmembrane domain six; a Cpr2(L222P) mutant is no longer constitutively active. Cpr2 engages the same G-protein activated signalling cascade as the Ste3a/alpha pheromone receptors, and thereby competes for pathway activation. This study established a new paradigm in which a naturally occurring constitutively active G protein-coupled receptor governs morphogenesis in fungi.

Detection of protein-protein interactions through vesicle targeting

The detection of protein-protein interactions through two-hybrid assays has revolutionized our understanding of biology. The remarkable impact of two-hybrid assay platforms derives from their speed, simplicity, and broad applicability. Yet for many organisms, the need to express test proteins in Saccharomyces cerevisiae or Escherichia coli presents a substantial barrier because variations in codon specificity or bias may result in aberrant protein expression. In particular, nonstandard genetic codes are characteristic of several eukaryotic pathogens, for which there are currently no genetically based systems for detection of protein-protein interactions. We have developed a protein-protein interaction assay that is carried out in native host cells by using GFP as the only foreign protein moiety, thus circumventing these problems. We show that interaction can be detected between two protein pairs in both the model yeast S. cerevisiae and the fungal pathogen Candida albicans. We use computational analysis of microscopic images to provide a quantitative and automated assessment of confidence.

CO(2) regulates white-to-opaque switching in Candida albicans

To mate, Candida albicans must undergo homozygosis at the mating type-like locus MTL[1, 2], then switch from the white to opaque phenotype [3, 4]. Paradoxically, when opaque cells are transferred in vitro to 37 degrees C, the temperature of their animal host, they switch en masse to white [5-7], suggesting that their major niche might not be conducive to mating. It has been suggested that pheromones secreted by opaque cells of opposite mating type [8] or the hypoxic condition of host niches [9, 10] stabilize opaque cells. There is, however, an additional possibility, namely that CO(2), which achieves levels in the host 100 times higher than in air [11-13], stabilizes the opaque phenotype. CO(2) has been demonstrated to regulate the bud-hypha transition in C. albicans[14, 15], expression of virulence genes in bacteria [16], and mating events in Cryptococcus neoformans[14, 17]. We tested the possibility that CO(2) stabilizes the opaque phenotype, and found that physiological levels of CO(2) induce white-to-opaque switching and stabilize the opaque phenotype at 37 degrees C. It exerts this control equally under anaerobic and aerobic conditions. These results suggest that the high levels of CO(2) in the host induce and stabilize the opaque phenotype, thus facilitating mating.

Dynamics of the interlocked positive feedback loops explaining the robust epigenetic switching in Candida albicans

The two element mutual activation and inhibitory positive feedback loops are a common motifs that occur in many biological systems in both isolated and interlocked form, as for example, in the cell division cycle and thymus differentiation in eukaryotes. The properties of three element interlocked positive feedback loops that embeds both mutual activation and inhibition are studied in depth for their bistable properties by performing bifurcation and stochastic simulations. Codimension one and two bifurcations reveal important properties like robustness to parameter variations and adaptability under various conditions by its ability to fine tune the threshold to a wide range of values and to maintain a wide bistable regime. Furthermore, we show that in the interlocked circuit, mutual inhibition controls the decision to switch from OFF to ON state, while mutual activation enforces the decision. This view is supported through a concrete biological example Candida albicans, a human fungal pathogen that can exist in two distinctive cell types; one in the default white state and the other in an opaque form. Stochastic switching between these two forms takes place due to the epigenetic alternation induced by the transcriptional regulators in the circuit, albeit without any rearrangement of the nuclear chromosomes. The transcriptional regulators constitute interlocked mutual activation and inhibition feedback circuits that provide adaptable threshold and wide bistable regime. These positive feedback loops are shown to be responsible for robust noise induced transitions without chattering, persistence of particular phenotypes for many generations and selective exhibition of one particular form of phenotype when mutated. Finally, we propose for synthetic biology constructs to use interlocked positive feedback loops instead of two element positive feedback loops because they are better controlled than isolated mutual activation and mutual inhibition feedback circuits.

The evolution of combinatorial gene regulation in fungi

It is widely suspected that gene regulatory networks are highly plastic. The rapid turnover of transcription factor binding sites has been predicted on theoretical grounds and has been experimentally demonstrated in closely related species. We combined experimental approaches with comparative genomics to focus on the role of combinatorial control in the evolution of a large transcriptional circuit in the fungal lineage. Our study centers on Mcm1, a transcriptional regulator that, in combination with five cofactors, binds roughly 4% of the genes in Saccharomyces cerevisiae and regulates processes ranging from the cell-cycle to mating. In Kluyveromyces lactis and Candida albicans, two other hemiascomycetes, we find that the Mcm1 combinatorial circuits are substantially different. This massive rewiring of the Mcm1 circuitry has involved both substantial gain and loss of targets in ancient combinatorial circuits as well as the formation of new combinatorial interactions. We have dissected the gains and losses on the global level into subsets of functionally and temporally related changes. One particularly dramatic change is the acquisition of Mcm1 binding sites in close proximity to Rap1 binding sites at 70 ribosomal protein genes in the K. lactis lineage. Another intriguing and very recent gain occurs in the C. albicans lineage, where Mcm1 is found to bind in combination with the regulator Wor1 at many genes that function in processes associated with adaptation to the human host, including the white-opaque epigenetic switch. The large turnover of Mcm1 binding sites and the evolution of new Mcm1–cofactor interactions illuminate in sharp detail the rapid evolution of combinatorial transcription networks.

In explaining the diversity of organisms on Earth, it is increasingly evident that evolutionary changes in when and where genes are expressed provide a crucial source of variation. By using genome-wide transcription factor localization experiments in S. cerevisiae, K. lactis, and C. albicans, combined with comparative genomics across many more yeast species, we examined how a large combinatorial transcription circuit evolves over the course of hundreds of millions of years. Combinatorial regulation is pervasive in eukaryotic organisms and is thought to allow for increased specificity and integration of multiple signals in the control of gene expression. Our studies focused on one prolific combinatorial regulator, Mcm1, which, in combination with five cofactors, binds and regulates genes functioning in a diverse range of cellular processes in S. cerevisiae. We found evidence of massive network rewiring, including high rates of gain and loss of Mcm1 binding sites and the formation of new Mcm1–cofactor combinations and the breaking of old ones. We propose that the multiple protein–protein and protein–DNA interactions that specify transcription in combinatorial circuits allow for a richness of compensatory mutations and thereby provide ample opportunity for both adaptive and neutral evolution.

Experimental approaches combined with comparative genomics show how a large combinatorial transcription circuit has rewired over evolutionary time scales.

Positive feedback in cellular control systems

Feedback loops have been identified in a variety of regulatory systems and organisms. While feedback loops of the same type (negative or positive) tend to have properties in common, they can play distinctively diverse roles in different regulatory systems, where they can affect virulence in a pathogenic bacterium, maturation patterns of vertebrate oocytes and transitions through cell cycle phases in eukaryotic cells. This review focuses on the properties and functions of positive feedback in biological systems, including bistability, hysteresis and activation surges.

Exponential sensitivity of noise-driven switching in genetic networks

There is increasing experimental evidence that cells can utilize biochemical noise to switch probabilistically between distinct gene-expression states. In this paper, we demonstrate that such noise-driven switching is dominated by tails of probability distributions and is therefore exponentially sensitive to changes in physiological parameters such as transcription and translation rates. Exponential sensitivity limits the robustness of noise-driven switching, suggesting cells may use other mechanisms in order to switch reliably. We discuss our results in the context of competence in the bacterium Bacillus subtilis.

Environmental induction of white-opaque switching in Candida albicans

Candida albicans strains that are homozygous at the mating type locus (MTLa or MTLalpha) can spontaneously switch at a low frequency from the normal yeast cell morphology (white) to an elongated cell type (opaque), which is the mating-competent form of the fungus. The ability to switch reversibly between these two cell types also contributes to the pathogenicity of C. albicans, as white and opaque cells are differently adapted to specific host niches. We found that in strain WO-1, a strain in which genomic alterations have occurred, but not in other tested strains, switching from the white to the opaque phase can also be induced by environmental conditions. Transient incubation of white cells under anaerobic conditions programmed the cells to switch en masse to the opaque phase. The anaerobic induction of white-opaque switching was controlled by the transcription factor CZF1, which in heterozygous MTLa/alpha cells regulates filamentous growth under embedded, hypoxic conditions. Intriguingly, passage of white cells of strain WO-1 through the mouse intestine, a host niche in which the cells are likely to be exposed to anaerobic conditions, resulted in a strongly increased frequency of switching to the opaque phase. These results demonstrate that white-opaque switching is not only a spontaneous process but, in combination with genomic alterations, can also be induced by environmental signals, suggesting that switching and mating of C. albicans may occur with high efficiency in appropriate niches within its human host.

Temperature-induced switch to the pathogenic yeast form of Histoplasma capsulatum requires Ryp1, a conserved transcriptional regulator

Histoplasma capsulatum, a fungal pathogen of humans, switches from a filamentous spore-forming mold in the soil to a pathogenic budding-yeast form in the human host. This morphologic switch, which is exhibited by H. capsulatum and a group of evolutionarily related fungal pathogens, is regulated by temperature. Using insertional mutagenesis, we identified a gene, RYP1 (required for yeast phase growth), which is required for yeast-form growth at 37 degrees C. ryp1 mutants are constitutively filamentous irrespective of temperature. Ryp1 is a member of a family of fungal proteins that includes Wor1, a master transcriptional regulator of the white-opaque transition required for mating in Candida albicans. Ryp1 associates with its own upstream regulatory region, consistent with a direct role in transcriptional control, and both the protein and its transcript accumulate to high levels in wild-type yeast-phase cells. Microarray analysis demonstrated that Ryp1 is required for the expression of the vast majority of yeast-specific genes, including two genes linked to virulence. Thus, Ryp1 appears to be a critical transcriptional regulator of a temperature-regulated morphologic switch in H. capsulatum.

Differential phagocytosis of white versus opaque Candida albicans by Drosophila and mouse phagocytes

The human fungal pathogen Candida albicans resides asymptomatically in the gut of most healthy people but causes serious invasive diseases in immunocompromised patients. Many C. albicans strains have the ability to stochastically switch between distinct white and opaque cell types, but it is not known with certainty what role this switching plays in the physiology of the organism. Here, we report a previously undescribed difference between white and opaque cells, namely their interaction with host phagocytic cells. We show that both Drosophila hemocyte-derived S2 cells and mouse macrophage-derived RAW264.7 cells preferentially phagocytose white cells over opaque cells. This difference is seen both in the overall percentage of cultured cells that phagocytose white versus opaque C. albicans and in the average number of C. albicans taken up by each phagocytic cell. We conclude that susceptibility to phagocytosis by cells of the innate immune system is an important distinction between white and opaque C. albicans, and propose that one role of switching from the prevalent white form into the rarer opaque form may be to allow C. albicans to escape phagocytosis.

Morphogenesis in Candida albicans

Candida albicans is termed a dimorphic fungus because it proliferates in either a yeast form or a hyphal form. The switch between these forms is the result of a complex interplay of external and internal factors and is coordinated in part by polarity-regulating proteins that are conserved among eukaryotic cells. However, yeast and hyphal cells are not the only morphological states of C. albicans. The opaque form required for mating, the pseudohyphal cell, and the chlamydospore represent distinct cell types that form in response to specific genetic or environmental conditions. In addition, hyperextended buds can form as a result of various cell cycle-related stresses. Recent studies are beginning to shed light on some of the molecular controls regulating the various morphogenetic forms of this fascinating human pathogen.

Interlocking transcriptional feedback loops control white-opaque switching in Candida albicans

The human pathogen Candida albicans can assume either of two distinct cell types, designated “white” and “opaque.” Each cell type is maintained for many generations; switching between them is rare and stochastic, and occurs without any known changes in the nucleotide sequence of the genome. The two cell types differ dramatically in cell shape, colony appearance, mating competence, and virulence properties. In this work, we investigate the transcriptional circuitry that specifies the two cell types and controls the switching between them. First, we identify two new transcriptional regulators of white-opaque switching, Czf1 and white-opaque regulator 2 (Wor2). Analysis of a large set of double mutants and ectopic expression strains revealed genetic relationships between CZF1, WOR2, and two previously identified regulators of white-opaque switching, WOR1 and EFG1. Using chromatin immunoprecipitation, we show that Wor1 binds the intergenic regions upstream of the genes encoding three additional transcriptional regulators of white-opaque switching (CZF1, EFG1, and WOR2), and also occupies the promoters of numerous white- and opaque-enriched genes. Based on these interactions, we have placed these four genes in a circuit controlling white-opaque switching whose topology is a network of positive feedback loops, with the master regulator gene WOR1 occupying a central position. Our observations indicate that a key role of the interlocking feedback loop network is to stably maintain each epigenetic state through many cell divisions.

The opportunistic fungal pathogen Candida albicans can switch between two heritable states—the “white” and “opaque” states. These two cell types differ in many characteristics, including cell structure, mating competence, and virulence. Recent studies of the molecular mechanism of regulating the white-opaque switch identified a master transcriptional regulator, Wor1. In this study, we identified two transcriptional regulators, Czf1 and Wor2, as new regulators of white-opaque switching. By constructing a series of single and double mutants and by examining where the master regulator Wor1 binds throughout the genome, we generated a molecular model of the bistable switch that regulates white-opaque switching. The regulatory model consists of interlocking positive feedback loops, which mutually reinforce one another and stabilize the opaque state. These results show how an organism can exist in two distinctive, heritable states without changes in the nucleotide sequence of its genome.

Analysis of mutations inCandida albicans indicates that the four genes in a circuit controlling white-opaque switching form a network of positive feedback loops, with the master regulatorWOR1 occupying a central position.

A genetic code alteration is a phenotype diversity generator in the human pathogen Candida albicans

BACKGROUND

The discovery of genetic code alterations and expansions in both prokaryotes and eukaryotes abolished the hypothesis of a frozen and universal genetic code and exposed unanticipated flexibility in codon and amino acid assignments. It is now clear that codon identity alterations involve sense and non-sense codons and can occur in organisms with complex genomes and proteomes. However, the biological functions, the molecular mechanisms of evolution and the diversity of genetic code alterations remain largely unknown. In various species of the genus Candida, the leucine CUG codon is decoded as serine by a unique serine tRNA that contains a leucine 5'-CAG-3'anticodon (tRNA(CAG)(Ser)). We are using this codon identity redefinition as a model system to elucidate the evolution of genetic code alterations.

METHODOLOGY/PRINCIPAL FINDINGS

We have reconstructed the early stages of the Candida genetic code alteration by engineering tRNAs that partially reverted the identity of serine CUG codons back to their standard leucine meaning. Such genetic code manipulation had profound cellular consequences as it exposed important morphological variation, altered gene expression, re-arranged the karyotype, increased cell-cell adhesion and secretion of hydrolytic enzymes.

CONCLUSION/SIGNIFICANCE

Our study provides the first experimental evidence for an important role of genetic code alterations as generators of phenotypic diversity of high selective potential and supports the hypothesis that they speed up evolution of new phenotypes.

Rational design of memory in eukaryotic cells

The ability to logically engineer novel cellular functions promises a deeper understanding of biological systems. Here we demonstrate the rational design of cellular memory in yeast that employs autoregulatory transcriptional positive feedback. We built a set of transcriptional activators and quantitatively characterized their effects on gene expression in living cells. Modeling in conjunction with the quantitative characterization of the activator-promoter pairs accurately predicts the behavior of the memory network. This study demonstrates the power of taking advantage of components with measured quantitative parameters to specify eukaryotic regulatory networks with desired properties.

The morphogenetic regulator Czf1p is a DNA-binding protein that regulates white–opaque switching in Candida albicans

Czf1p has been demonstrated to regulate the switch between the yeast-cell morphology and filamentous morphologies of the human fungal pathogen Candida albicans. The predicted amino acid sequence of Czf1p contains a zinc-cluster motif similar to the DNA-binding domains of proteins such as Saccharomyces cerevisiae Gal4p, suggesting that Czf1p is a DNA-binding protein. Czf1p also demonstrates genetic interaction and a two-hybrid interaction with a second regulator of C. albicans cellular morphology, Efg1p. During growth in contact with an agar matrix, Efg1p has a negative effect on filamentation and Czf1p antagonizes this effect. In addition to regulating cellular morphology, Efg1p plays a role in regulating the cell-type switch between the commonly observed white phase of C. albicans and the opaque, mating-competent phase. While overexpression of EFG1 stimulates the switch from opaque to white, the results reported here demonstrate that overexpression of CZF1 promotes the reverse switch, from white to opaque. We also demonstrate that Czf1p binds CZF1 promoter DNA in vitro. Therefore, for the regulation of both contact-dependent filamentation and white-opaque switching, Czf1p and Efg1p have opposing functions.

Environmental sensing and signal transduction pathways regulating morphopathogenic determinants of Candida albicans

Candida albicans is an opportunistic fungal pathogen that is found in the normal gastrointestinal flora of most healthy humans. However, under certain environmental conditions, it can become a life-threatening pathogen. The shift from commensal organism to pathogen is often correlated with the capacity to undergo morphogenesis. Indeed, under certain conditions, including growth at ambient temperature, the presence of serum or N-acetylglucosamine, neutral pH, and nutrient starvation, C. albicans can undergo reversible transitions from the yeast form to the mycelial form. This morphological plasticity reflects the interplay of various signal transduction pathways, either stimulating or repressing hyphal formation. In this review, we provide an overview of the different sensing and signaling pathways involved in the morphogenesis and pathogenesis of C. albicans. Where appropriate, we compare the analogous pathways/genes in Saccharomyces cerevisiae in an attempt to highlight the evolution of the different components of the two organisms. The downstream components of these pathways, some of which may be interesting antifungal targets, are also discussed.

Positive-feedback loops as a flexible biological module

BACKGROUND

Bistability in genetic networks allows cells to remember past events and to make discrete decisions in response to graded signals. Bistable behavior can result from positive feedback, but feedback loops can have other roles in signal transduction as well.

RESULTS

We introduced positive feedback into the budding-yeast pheromone response to convert it into a bistable system. In the presence of feedback, transient induction with high pheromone levels caused persistent pathway activation, whereas at lower levels a fraction of cells became persistently active but the rest inactivated completely. We also generated mutations that quantitatively tuned the basal and induced expression levels of the feedback promoter and showed that they qualitatively changed the behavior of the system. Finally, we developed a simple stochastic model of our positive-feedback system and showed the agreement between our simulations and experimental results.

CONCLUSIONS

The positive-feedback loop can display several different behaviors, including bistability, and can switch between them as a result of simple mutations.

Positive feedback, stochasticity and genetic competence

A single gene, regulating its own expression via a positive feedback loop, constitutes a common motif in gene regulatory networks and signalling cascades. Recent experiments on the development of competence in the bacterial population B. subtilis show that the autoregulatory genetic module by itself can give rise to two types of cellular states. The states correspond to the low and high expression states of the master regulator ComK. The high expression state is attained when the ComK protein level exceeds a threshold value leading to a full activation of the autostimulatory loop. Stochasticity in gene expression drives the transitions between the two stable states. In this paper, we explain the appearance of bimodal protein distributions in B. subtilis cell population in the framework of three possible scenarios. In two of the cases, bistability provides the basis for binary gene expression. In the third case, the system is monostable in a deterministic description and stochasticity in gene expression is solely responsible for the appearance of the two expression states.