Positive autoregulation of the yeast transcription factor Pdr3p, which is involved in control of drug resistance

Multilevel Regulation of Membrane Proteins in Response to Metal and Metalloid Stress: A Lesson from Yeast

In the face of flourishing industrialization and global trade, heavy metal and metalloid contamination of the environment is a growing concern throughout the world. The widespread presence of highly toxic compounds of arsenic, antimony, and cadmium in nature poses a particular threat to human health. Prolonged exposure to these toxins has been associated with severe human diseases, including cancer, diabetes, and neurodegenerative disorders. These toxins are known to induce analogous cellular stresses, such as DNA damage, disturbance of redox homeostasis, and proteotoxicity. To overcome these threats and improve or devise treatment methods, it is crucial to understand the mechanisms of cellular detoxification in metal and metalloid stress. Membrane proteins are key cellular components involved in the uptake, vacuolar/lysosomal sequestration, and efflux of these compounds; thus, deciphering the multilevel regulation of these proteins is of the utmost importance. In this review, we summarize data on the mechanisms of arsenic, antimony, and cadmium detoxification in the context of membrane proteome. We used yeast Saccharomyces cerevisiae as a eukaryotic model to elucidate the complex mechanisms of the production, regulation, and degradation of selected membrane transporters under metal(loid)-induced stress conditions. Additionally, we present data on orthologues membrane proteins involved in metal(loid)-associated diseases in humans.

Activity of the pleiotropic drug resistance transcription factors Pdr1p and Pdr3p is modulated by binding site flanking sequences

The transcription factors Pdr1p and Pdr3p regulate pleiotropic drug resistance (PDR) in Saccharomyces cerevisiae via the PDR responsive elements (PDREs) to modulate gene expression. However, the exact mechanisms underlying the differences in their regulons remain unclear. Employing genomic occupancy profiling (CUT&RUN), binding assays, and transcription studies, we characterized the differences in sequence specificity between transcription factors. Findings reveal distinct preferences for core PDRE sequences and the flanking sequences for both proteins. While flanking sequences moderately alter DNA binding affinity, they significantly impact Pdr1/3p transcriptional activity. Notably, both proteins demonstrated the ability to bind half sites, showing potential enhancement of transcription from adjacent PDREs. This insight sheds light on ways Pdr1/3p can differentially regulate PDR.

Unraveling the Half and Full Site Sequence Specificity of the Saccharomyces cerevisiae Pdr1p and Pdr3p Transcription Factors

The transcription factors Pdr1p and Pdr3p regulate pleotropic drug resistance (PDR) in Saccharomyces cerevisiae , via the PDR responsive elements (PDREs) to modulate gene expression. However, the exact mechanisms underlying the differences in their regulons remain unclear. Employing genomic occupancy profiling (CUT&RUN), binding assays, and transcription studies, we characterized the differences in sequence specificity between transcription factors. Findings reveal distinct preferences for core PDRE sequences and the flanking sequences for both proteins. While flanking sequences moderately alter DNA binding affinity, they significantly impact Pdr1/3p transcriptional activity. Notably, both proteins demonstrated the ability to bind half sites, showing potential enhancement of transcription from adjacent PDREs. This insight sheds light on ways Pdr1/3 can differentially regulate PDR.

De novo biosynthesis of rubusoside and rebaudiosides in engineered yeasts

High-sugar diet causes health problems, many of which can be addressed with the use of sugar substitutes. Rubusoside and rebaudiosides are interesting molecules, considered the next generation of sugar substitutes due to their low-calorie, superior sweetness and organoleptic properties. However, their low abundance in nature makes the traditional plant extraction process neither economical nor environmental-friendly. Here we engineer baker's yeast Saccharomyces cerevisiae as a chassis for the de novo production of rubusoside and rebaudiosides. In this process, we identify multiple issues that limit the production, including rate-liming steps, product stress on cellular fitness and unbalanced metabolic networks. We carry out a systematic engineering strategy to solve these issues, which produces rubusoside and rebaudiosides at titers of 1368.6 mg/L and 132.7 mg/L, respectively. The rubusoside chassis strain here constructed paves the way towards a sustainable, large-scale fermentation-based manufacturing of diverse rebaudiosides.

SWI/SNF and the histone chaperone Rtt106 drive expression of the Pleiotropic Drug Resistance network genes

The Pleiotropic Drug Resistance (PDR) network is central to the drug response in fungi, and its overactivation is associated with drug resistance. However, gene regulation of the PDR network is not well understood. Here, we show that the histone chaperone Rtt106 and the chromatin remodeller SWI/SNF control expression of the PDR network genes and confer drug resistance. In Saccharomyces cerevisiae, Rtt106 specifically localises to PDR network gene promoters dependent on transcription factor Pdr3, but not Pdr1, and is essential for Pdr3-mediated basal expression of the PDR network genes, while SWI/SNF is essential for both basal and drug-induced expression. Also in the pathogenic fungus Candida glabrata, Rtt106 and SWI/SNF regulate drug-induced PDR gene expression. Consistently, loss of Rtt106 or SWI/SNF sensitises drug-resistant S. cerevisiae mutants and C. glabrata to antifungal drugs. Since they cooperatively drive PDR network gene expression, Rtt106 and SWI/SNF represent potential therapeutic targets to combat antifungal resistance.

UME6 Is Involved in the Suppression of Basal Transcription of ABC Transporters and Drug Resistance in the ρ+ Cells of Saccharomyces cerevisiae

In Saccharomycescerevisiae, the Rpd3L complex contains a histone deacetylase, Rpd3, and the DNA binding proteins, Ume6 and Ash1, and acts as a transcriptional repressor or activator. We previously showed that RPD3 and UME6 are required for the activation of PDR5, which encodes a major efflux pump, and pleiotropic drug resistance (PDR) in ρ0/− cells, which lack mitochondrial DNA. However, there are inconsistent reports regarding whether RPD3 and UME6 are required for Pdr5-mediated PDR in ρ+ cells with mitochondrial DNA. Since PDR5 expression or PDR in the ρ+ cells of the rpd3Δ and ume6Δ mutants have primarily been examined using fermentable media, mixed cultures of ρ+ and ρ0/− cells could be used. Therefore, we examined whether RPD3 and UME6 are required for basal and drug-induced PDR5 transcription and PDR in ρ+ cells using fermentable and nonfermentable media. UME6 suppresses the basal transcription levels of the ABC transporters, including PDR5, and drug resistance in ρ+ cells independent of the carbon source used in the growth medium. In contrast, RPD3 is required for drug resistance but did not interfere with the basal PDR5 mRNA levels. UME6 is also required for the cycloheximide-induced transcription of PDR5 in nonfermentable media but not in fermentable media.

RPD3 and UME6 are involved in the activation of PDR5 transcription and pleiotropic drug resistance in ρ0 cells of Saccharomyces cerevisiae

BACKGROUND

In Saccharomyces cerevisiae, the retrograde signalling pathway is activated in ρ0/- cells, which lack mitochondrial DNA. Within this pathway, the activation of the transcription factor Pdr3 induces transcription of the ATP-binding cassette (ABC) transporter gene, PDR5, and causes pleiotropic drug resistance (PDR). Although a histone deacetylase, Rpd3, is also required for cycloheximide resistance in ρ0/- cells, it is currently unknown whether Rpd3 and its DNA binding partners, Ume6 and Ash1, are involved in the activation of PDR5 transcription and PDR in ρ0/- cells. This study investigated the roles of RPD3, UME6, and ASH1 in the activation of PDR5 transcription and PDR by retrograde signalling in ρ0 cells.

RESULTS

ρ0 cells in the rpd3∆ and ume6∆ strains, with the exception of the ash1∆ strain, were sensitive to fluconazole and cycloheximide. The PDR5 mRNA levels in ρ0 cells of the rpd3∆ and ume6∆ strains were significantly reduced compared to the wild-type and ash1∆ strain. Transcriptional expression of PDR5 was reduced in cycloheximide-exposed and unexposed ρ0 cells of the ume6∆ strain; the transcriptional positive response of PDR5 to cycloheximide exposure was also impaired in this strain.

CONCLUSIONS

RPD3 and UME6 are responsible for enhanced PDR5 mRNA levels and PDR by retrograde signalling in ρ0 cells of S. cerevisiae.

Transcription factors and ABC transporters: from pleiotropic drug resistance to cellular signaling in yeast

Budding yeast Saccharomyces cerevisiae survives in microenvironments utilizing networks of regulators and ATP-binding cassette (ABC) transporters to circumvent toxins and a variety of drugs. Our understanding of transcriptional regulation of ABC transporters in yeast is mainly derived from the study of multidrug resistance protein networks. Over the past two decades, this research has not only expanded the role of transcriptional regulators in pleiotropic drug resistance (PDR) but evolved to include the role that regulators play in cellular signaling and environmental adaptation. Inspection of the gene networks of the transcriptional regulators and characterization of the ABC transporters has clarified that they also contribute to environmental adaptation by controlling plasma membrane composition, toxic-metal sequestration, and oxidative stress adaptation. Additionally, ABC transporters and their regulators appear to be involved in cellular signaling for adaptation of S. cerevisiae populations to nutrient availability. In this review, we summarize the current understanding of the S. cerevisiae transcriptional regulatory networks and highlight recent work in other notable fungal organisms, underlining the expansion of the study of these gene networks across the kingdom fungi.

Differentially Regulated Transcription Factors and ABC Transporters in a Mitochondrial Dynamics Mutant Can Alter Azole Susceptibility of Aspergillus fumigatus

Azole resistance of the fungal pathogen Aspergillus fumigatus is an emerging problem. To identify novel mechanisms that could mediate azole resistance in A. fumigatus, we analyzed the transcriptome of a mitochondrial fission/fusion mutant that exhibits increased azole tolerance. Approximately 12% of the annotated genes are differentially regulated in this strain. This comprises upregulation of Cyp51A, the azole target structure, upregulation of ATP-binding cassette (ABC) superfamily and major facilitator superfamily (MFS) transporters and differential regulation of transcription factors. To study their impact on azole tolerance, conditional mutants were constructed of seven ABC transporters and 17 transcription factors. Under repressed conditions, growth rates and azole susceptibility of the mutants were similar to wild type. Under induced conditions, several transcription factor mutants showed growth phenotypes. In addition, four ABC transporter mutants and seven transcription factor mutants exhibited altered azole susceptibility. However, deletion of individual identified ABC transporters and transcription factors did not affect the increased azole tolerance of the fission/fusion mutant. Our results revealed the ability of multiple ABC transporters and transcription factors to modulate the azole susceptibility of A. fumigatus and support a model where mitochondrial dysfunctions trigger a drug resistance network that mediates azole tolerance of this mold.

Sequence modification of the master regulator Pdr1 interferes with its transcriptional autoregulation and confers altered azole resistance in Candida glabrata

The transcriptional regulator Pdr1 plays a positive role in regulating azole drug resistance in Candida glabrata. Previous studies have shown the importance of the carboxyl (C)-terminal sequence of Pdr1 in fulfilling its function, as this region mediates interactions between Pdr1 and the co-activator Gal11A and is crucial for activation of Pdr1 targets. However, mechanisms of how Pdr1 is regulated, especially implication of its C-terminus in the regulatory activity, remain uncharacterized. In this study, we unexpectedly observed that the C-terminal modification of Pdr1 in an azole-resistant clinical isolate harboring a single GOF mutation, resulted in adverse effects such as decreased expression levels of Pdr1, downregulation of Pdr1 targets and azole hypersensitivity. Importantly, the C-terminal 3 × FLAG tagging significantly decreased the binding of Pdr1 to the pleiotropic drug response elements in its own promoter, promoted an irregular cellular mislocalization and thereby disrupted the transcriptional autoregulation of this master regulator. Unexpectedly, the aberrant cytoplasmic localization caused a non-functional interaction with Gal11A, a co-activator involved in drug resistance. Based on these findings, we proposed that C-terminal sequence of Pdr1 is vital for its stability and functionality, and targeting regulation of this region may represent a promising future strategy for combating C. glabrata infection and drug resistance.

Unveiling the transcriptional control of pleiotropic drug resistance in Saccharomyces cerevisiae: Contributions of André Goffeau and his group

Studies in the yeast Saccharomyces cerevisiae have provided much of the basic detail underlying the organization and regulation of multiple or pleiotropic drug resistance gene network in eukaryotic microbes. As with many aspects of yeast biology, the initial observations that drove the eventual molecular characterization of multidrug resistance gene were provided by genetics. This review focuses on contributions from the laboratory of Dr. André Goffeau that uncovered key aspects of the transcriptional regulation of these multidrug resistance genes. André's group made many seminal discoveries that helped lead to the current picture we have of how eukaryotic microbes respond to and deal with a variety of antifungal agents. The importance of the transcriptional contribution to antifungal drugs is illustrated by the large number of drug resistant mutants found in several yeast species that lead to increased activity of transcriptional regulators. The characterization of the Saccharomyces cerevisiae PDR1 gene by the Goffeau group provided the first molecular basis explaining the link between this hyperactive transcription factor and drug resistance.

Multiple interfaces control activity of the Candida glabrata Pdr1 transcription factor mediating azole drug resistance

The Cys₆Zn₂ DNA-binding domain transcription factor Pdr1 is a central regulator of drug resistance in the pathogenic yeast Candida glabrata. In this review, I discuss the multiple control mechanisms modulating the function of this positive transcriptional regulator. Available data suggest that Pdr1 activity is restrained by multiple negative inputs that can be lost by either mutagenesis of the protein or loss of trans-acting factors. Although extensive data are available on the C. glabrata transactivator as well as its cognate proteins in Saccharomyces cerevisiae, the physiological rationale underlying the regulation of these factors remains to be understood.

Importance of Proteasome Gene Expression during Model Dough Fermentation after Preservation of Baker's Yeast Cells by Freezing

Freeze-thaw stress causes various types of cellular damage, survival and/or proliferation defects, and metabolic alterations. However, the mechanisms underlying how cells cope with freeze-thaw stress are poorly understood. Here, model dough fermentations using two baker's yeast strains, 45 and YF, of Saccharomyces cerevisiae were compared after 2 weeks of cell preservation in a refrigerator or freezer. YF exhibited slow fermentation after exposure to freeze-thaw stress due to low cell viability. A DNA microarray analysis of the YF cells during fermentation revealed that the genes involved in oxidative phosphorylation were relatively strongly expressed, suggesting a decrease in the glycolytic capacity. Furthermore, we found that mRNA levels of the genes that encode the components of the proteasome complex were commonly low, and ubiquitinated proteins were accumulated by freeze-thaw stress in the YF strain. In the cells with a laboratory strain background, treatment with the proteasome inhibitor MG132 or the deletion of each transcriptional activator gene for the proteasome genes (RPN4, PDR1, or PDR3) led to marked impairment of model dough fermentation using the frozen cells. Based on these data, proteasomal degradation of freeze-thaw-damaged proteins may guarantee high cell viability and fermentation performance. We also found that the freeze-thaw stress-sensitive YF strain was heterozygous at the PDR3 locus, and one of the alleles (A148T/A229V/H336R/L541P) was shown to possess a dominant negative phenotype of slow fermentation. Removal of such responsible mutations could improve the freeze-thaw stress tolerance and the fermentation performance of baker's yeast strains, as well as other industrial S. cerevisiae strains.IMPORTANCE The development of freezing technology has enabled the long-term preservation and long-distance transport of foods and other agricultural products. Fresh yeast, however, is usually not frozen because the fermentation performance and/or the viability of individual cells is severely affected after thawing. Here, we demonstrate that proteasomal degradation of ubiquitinated proteins is an essential process in the freeze-thaw stress responses of S. cerevisiae Upstream transcriptional activator genes for the proteasome components are responsible for the fermentation performance after freezing preservation. Thus, this study provides a potential linkage between freeze-thaw stress inputs and the transcriptional regulatory network that might be functionally conserved in higher eukaryotes. Elucidation of the molecular targets of freeze-thaw stress will contribute to advances in cryobiology, such as freezing preservation of human cells, tissues, and embryos for medical purposes and breeding of industrial microorganisms and agricultural crops that adapt well to low temperatures.

Positive autoregulation and repression of transactivation are key regulatory features of the Candida glabrata Pdr1 transcription factor

Resistance to azole drugs, the major clinical antifungal compounds, is most commonly due to gain-of-function (GOF) substitution mutations in a gene called PDR1 in the fungal pathogen Candida glabrata. PDR1 encodes a zinc cluster-containing transcription factor. GOF forms of Pdr1 drive high level expression of downstream target gene expression with accompanying azole resistance. PDR1 has two homologous genes in Saccharomyces cerevisiae, called ScPDR1 and ScPDR3. This study provides evidence that the PDR1 gene in C. glabrata represents a blend of the properties found in the two S. cerevisiae genes. We demonstrated that GOF Pdr1 derivatives are overproduced at the protein level and less stable than the wild-type protein. Overproduction of wild-type Pdr1 increased target gene expression but to a lesser extent than GOF derivatives. Site-directed mutagenesis of Pdr1 binding sites in the PDR1 promoter provided clear demonstration that autoregulation of PDR1 is required for its normal function. An internal deletion mutant of Pdr1 lacking its central regulatory domain behaved as a hyperactive transcription factor that was lethal unless conditionally expressed. A full understanding of the regulation of Pdr1 will provide a new avenue of interfering with azole resistance in C. glabrata.

Insight into the Kluyveromyces lactis Pdr1p regulon

The overexpression of efflux pumps is an important mechanism leading to the development of multidrug resistance phenomenon. The transcription factor KlPdr1p, belonging to the Zn₂Cys₆ family, is a central regulator of efflux pump expression in Kluyveromyces lactis. To better understand how KlPDR1-mediated drug resistance is achieved in K. lactis, we used DNA microarrays to identify genes whose expression was affected by deletion or overexpression of the KlPDR1 gene. Eighty-nine targets of the KlPDR1 were identified. From those the transcription of 16 genes was induced in the transformant overexpressing KlPDR1* and simultaneously repressed in the Klpdr1Δ deletion mutant. Almost all of these genes contain putative binding motifs for the AP-1-like transcription factors in their promoters. Furthermore, we studied the possible interplay between KlPdr1p and KlYap1p transcription factors. Our results show that KlYap1p does not significantly contribute to the regulation of KlPDR1 gene expression in the presence of azoles. However, KlPDR1 expression markedly increased in the presence of hydrogen peroxide and hinged upon the presence of KlYap1p. Our results show that although both KlPdr1p and KlYap1p transcription factors are involved in the control of K. lactis multidrug resistance, further studies will be needed to determine their interplay.

Phytohormone sensing in the biotrophic fungus Ustilago maydis - the dual role of the transcription factor Rss1

The phenolic compound salicylic acid (SA) is a key signalling molecule regulating local and systemic plant defense responses, mainly against biotrophs. Many microbial organisms, including pathogens, share the ability to degrade SA. However, the mechanism by which they perceive SA is unknown. Here we show that Ustilago maydis, the causal agent of corn smut disease, employs a so far uncharacterized SA sensing mechanism. We identified and characterized the novel SA sensing regulator, Rss1, a binuclear zinc cluster protein with dual functions as putative SA receptor and transcriptional activator regulating genes important for SA and tryptophan degradation. Rss1 represents a major component in the identified SA sensing pathway during the fungus' saprophytic stage. However, Rss1 does not have a detectable impact on virulence. The data presented in this work indicate that alternative or redundant sensing cascades exist that regulate the expression of SA-responsive genes in U. maydis during its pathogenic development.

Coherent feedforward transcriptional regulatory motifs enhance drug resistance

Fluctuations in gene expression give identical cells access to a spectrum of phenotypes that can serve as a transient, nongenetic basis for natural selection by temporarily increasing drug resistance. In this study, we demonstrate using mathematical modeling and simulation that certain gene regulatory network motifs, specifically coherent feedforward loop motifs, can facilitate the development of nongenetic resistance by increasing cell-to-cell variability and the time scale at which beneficial phenotypic states can be maintained. Our results highlight how regulatory network motifs enabling transient, nongenetic inheritance play an important role in defining reproductive fitness in adverse environments and provide a selective advantage subject to evolutionary pressure.

Multidrug resistance in fungi: regulation of transporter-encoding gene expression

A critical risk to the continued success of antifungal chemotherapy is the acquisition of resistance; a risk exacerbated by the few classes of effective antifungal drugs. Predictably, as the use of these drugs increases in the clinic, more resistant organisms can be isolated from patients. A particularly problematic form of drug resistance that routinely emerges in the major fungal pathogens is known as multidrug resistance. Multidrug resistance refers to the simultaneous acquisition of tolerance to a range of drugs via a limited or even single genetic change. This review will focus on recent progress in understanding pathways of multidrug resistance in fungi including those of most medical relevance. Analyses of multidrug resistance in Saccharomyces cerevisiae have provided the most detailed outline of multidrug resistance in a eukaryotic microorganism. Multidrug resistant isolates of S. cerevisiae typically result from changes in the activity of a pair of related transcription factors that in turn elicit overproduction of several target genes. Chief among these is the ATP-binding cassette (ABC)-encoding gene PDR5. Interestingly, in the medically important Candida species, very similar pathways are involved in acquisition of multidrug resistance. In both C. albicans and C. glabrata, changes in the activity of transcriptional activator proteins elicits overproduction of a protein closely related to S. cerevisiae Pdr5 called Cdr1. The major filamentous fungal pathogen, Aspergillus fumigatus, was previously thought to acquire resistance to azole compounds (the principal antifungal drug class) via alterations in the azole drug target-encoding gene cyp51A. More recent data indicate that pathways in addition to changes in the cyp51A gene are important determinants in A. fumigatus azole resistance. We will discuss findings that suggest azole resistance in A. fumigatus and Candida species may share more mechanistic similarities than previously thought.

Yeast ABC proteins involved in multidrug resistance

Pleiotropic drug resistance is a complex phenomenon that involves many proteins that together create a network. One of the common mechanisms of multidrug resistance in eukaryotic cells is the active efflux of a broad range of xenobiotics through ATP-binding cassette (ABC) transporters. Saccharomyces cerevisiae is often used as a model to study such activity because of the functional and structural similarities of its ABC transporters to mammalian ones. Numerous ABC transporters are found in humans and some are associated with the resistance of tumors to chemotherapeutics. Efflux pump modulators that change the activity of ABC proteins are the most promising candidate drugs to overcome such resistance. These modulators can be chemically synthesized or isolated from natural sources (e.g., plant alkaloids) and might also be used in the treatment of fungal infections. There are several generations of synthetic modulators that differ in specificity, toxicity and effectiveness, and are often used for other clinical effects.

Gain-of-function mutation in the KlPDR1 gene encoding multidrug resistance regulator in Kluyveromyces lactis

KlPdr1p is a single Kluyveromyces lactis homologue of Saccharomyces cerevisiae ScPdr1p/ScPdr3p, the main transcriptional regulators of genes involved in S. cerevisiae multidrug resistance. KlPDR1 deletion leads to a sharp increase in K. lactis drug susceptibility. The presence of putative PDRE and YRE regulatory elements in the KlPDR1 gene promoter suggests an autoregulation of its transcription as well as its control by KlYap1p, the transcription factor involved in oxidative stress response. In this study, one plasmid-borne Klpdr1-1 allele that led to amino acid substitution (L273P) in the KlPdr1p was isolated. Overexpression of the Klpdr1-1 allele from a multicopy plasmid in the K. lactis wild-type and Klpdr1Δ mutant strain increased the tolerance of transformants to oligomycin. The plasmid-borne Klpdr1-1 allele increased the activation of the ScPDR5 promoter and complemented the drug hypersensitivity of the S. cerevisiae pdr1Δ pdr3Δ mutant strain. The results indicate that L273P amino acid substitution is the result of a gain-of-function mutation in the KlPDR1 gene that confers KlPdr1p hyperactivity, as revealed by a high expression of the ABC transporter gene KlPDR5, leading to multidrug resistance and rhodamine 6G efflux out of the cells.

Response of pathogenic and non-pathogenic yeasts to steroids

Steroids are known to induce pleiotropic drug resistance states in hemiascomycetes, with tremendous potential consequences on human fungal infections. The proteins capable of binding to steroids such as progesterone binding protein (PBP), estradiol binding proteins (ESP) are found in yeasts, however, the well known receptor mediated signaling present in higher eukaryotic cells is absent in yeasts and fungi. Steroids are perceived as stress by yeast cells which triggers general stress response leading to activation of heat shock proteins, cell cycle regulators, MDR transporters, etc. In this article, we review the response of yeast to human steroid hormones which affects its cell growth, morphology and virulence. We discuss that a fairly conserved response to steroids at the level of transcription and translation exists between pathogenic and non-pathogenic yeasts. Article from a special issue on steroids and microorganisms.

Antifungal resistance and new strategies to control fungal infections

Despite improvement of antifungal therapies over the last 30 years, the phenomenon of antifungal resistance is still of major concern in clinical practice. In the last 10 years the molecular mechanisms underlying this phenomenon were extensively unraveled. In this paper, after a brief overview of currently available antifungals, molecular mechanisms of antifungal resistance will be detailed. It appears that major mechanisms of resistance are essential due to the deregulation of antifungal resistance effector genes. This deregulation is a consequence of point mutations occurring in transcriptional regulators of these effector genes. Resistance can also follow the emergence of point mutations directly in the genes coding antifungal targets. In addition we further describe new strategies currently undertaken to discover alternative therapy targets and antifungals. Identification of new antifungals is essentially achieved by the screening of natural or synthetic chemical compound collections. Discovery of new putative antifungal targets is performed through genome-wide approaches for a better understanding of the human pathogenic fungi biology.

Autoactivated KlPDR1 gene in the control of multidrug resistance in Kluyveromyces lactis

The KlPDR1 gene encodes a zinc finger transcription factor that has recently been shown to be involved in the control of multidrug resistance of Kluyveromyces lactis . In this work, we provide evidence that the K. lactis KlPDR1 gene is under positive autoregulation by KlPdr1p, which plays a role in the activation of the main multidrug resistance transporter gene KlPDR5. Electrophoretic mobility shift assays, as well as the use of gusA reporter constructs, enabled us to identify the 5'-tataTCCGGGTAactt-3' sequence motif in the KlPDR1 promoter (in the position -326 to -319 bp) as the PDRE (pleiotropic drug responsive element) for the binding of KlPdr1p. The drug sensitivity of Klpdr1Δ mutant cells was complemented by introducing the plasmid-born KlPDR1 gene. The KlPdr1p activated the expression of the P(KlPDR1)-gusA fusion gene, and the expression of the KlPDR1 gene was induced by fluconazole. The PDRE was also found in the promoter of KlPDR5, a gene encoding the ATP-dependent efflux pump responsible for the drug resistance phenomenon in K. lactis.

Regulation of the CgPdr1 transcription factor from the pathogen Candida glabrata

Candida glabrata is an opportunistic human pathogen that is increasingly associated with candidemia, owing in part to the intrinsic and acquired high tolerance the organism exhibits for the important clinical antifungal drug fluconazole. This elevated fluconazole resistance often develops through gain-of-function mutations in the zinc cluster-containing transcriptional regulator C. glabrata Pdr1 (CgPdr1). CgPdr1 induces the expression of an ATP-binding cassette (ABC) transporter-encoding gene, CgCDR1. Saccharomyces cerevisiae has two CgPdr1 homologues called ScPdr1 and ScPdr3. These factors control the expression of an ABC transporter-encoding gene called ScPDR5, which encodes a homologue of CgCDR1. Loss of the mitochondrial genome (ρ(0) cell) or overexpression of the mitochondrial enzyme ScPsd1 induces ScPDR5 expression in a strictly ScPdr3-dependent fashion. ScPdr3 requires the presence of a transcriptional Mediator subunit called Gal11 (Med15) to fully induce ScPDR5 transcription in response to ρ(0) signaling. ScPdr1 does not respond to either ρ(0) signals or ScPsd1 overproduction. In this study, we employed transcriptional fusions between CgPdr1 target promoters, like CgCDR1, to demonstrate that CgPdr1 stimulates gene expression via binding to elements called pleiotropic drug response elements (PDREs). Deletion mapping and electrophoretic mobility shift assays demonstrated that a single PDRE in the CgCDR1 promoter was capable of supporting ρ(0)-induced gene expression. Removal of one of the two ScGal11 homologues from C. glabrata caused a major defect in drug-induced expression of CgCDR1 but had a quantitatively minor effect on ρ(0)-stimulated transcription. These data demonstrate that CgPdr1 appears to combine features of ScPdr1 and ScPdr3 to produce a transcription factor with chimeric regulatory properties.

Comparative transcriptome profiling analyses during the lag phase uncover YAP1, PDR1, PDR3, RPN4, and HSF1 as key regulatory genes in genomic adaptation to the lignocellulose derived inhibitor HMF for Saccharomyces cerevisiae

BACKGROUND

The yeast Saccharomyces cerevisiae is able to adapt and in situ detoxify lignocellulose derived inhibitors such as furfural and HMF. The length of lag phase for cell growth in response to the inhibitor challenge has been used to measure tolerance of strain performance. Mechanisms of yeast tolerance at the genome level remain unknown. Using systems biology approach, this study investigated comparative transcriptome profiling, metabolic profiling, cell growth response, and gene regulatory interactions of yeast strains and selective gene deletion mutations in response to HMF challenges during the lag phase of growth.

RESULTS

We identified 365 candidate genes and found at least 3 significant components involving some of these genes that enable yeast adaptation and tolerance to HMF in yeast. First, functional enzyme coding genes such as ARI1, ADH6, ADH7, and OYE3, as well as gene interactions involved in the biotransformation and inhibitor detoxification were the direct driving force to reduce HMF damages in cells. Expressions of these genes were regulated by YAP1 and its closely related regulons. Second, a large number of PDR genes, mainly regulated by PDR1 and PDR3, were induced during the lag phase and the PDR gene family-centered functions, including specific and multiple functions involving cellular transport such as TPO1, TPO4, RSB1, PDR5, PDR15, YOR1, and SNQ2, promoted cellular adaptation and survival in order to cope with the inhibitor stress. Third, expressed genes involving degradation of damaged proteins and protein modifications such as SHP1 and SSA4, regulated by RPN4, HSF1, and other co-regulators, were necessary for yeast cells to survive and adapt the HMF stress. A deletion mutation strain Δrpn4 was unable to recover the growth in the presence of HMF.

CONCLUSIONS

Complex gene interactions and regulatory networks as well as co-regulations exist in yeast adaptation and tolerance to the lignocellulose derived inhibitor HMF. Both induced and repressed genes involving diversified functional categories are accountable for adaptation and energy rebalancing in yeast to survive and adapt the HMF stress during the lag phase of growth. Transcription factor genes YAP1, PDR1, PDR3, RPN4, and HSF1 appeared to play key regulatory rules for global adaptation in the yeast S. cerevisiae.

The serine/threonine protein phosphatase Sit4p activates multidrug resistance in Saccharomyces cerevisiae

Multidrug resistance in Saccharomyces cerevisiae is frequently associated with gain-of-function mutations in zinc finger-containing transcription factors Pdr1p and Pdr3p. These regulatory proteins activate the expression of several ATP-binding cassette transporter genes, leading to elevated drug resistance. Here, we report that loss of the type 2A-related serine/threonine protein phosphatase Sit4p renders yeast cells sensitive to cycloheximide, azoles, daunorubicin and rhodamine 6G. This effect is a consequence of the decreased transcriptional levels of mainly PDR3 and its target genes, PDR5, SNQ2 and YOR1, which encode multidrug efflux pumps. The multidrug sensitivity of sit4 mutant cells is suppressed by the PDR1-3 mutant allele, which encodes a hyperactive form of Pdr1p. Sit4p is known to associate with regulatory proteins Sap155p, Sap4p, Sap185p and Sap190p. We found that the sap155 mutant strain is sensitive to azoles, but not to cycloheximide, while the sap155sap4 and sap185sap190 mutant strains are sensitive to both drugs. This finding indicates that the Sit4p-Sap protein complex subtly modulates the expression of drug efflux pumps. Drug resistance conferred by the expression of the Candida albicans CDR1 gene, an ortholog of PDR5 in S. cerevisiae, is also positively modulated by Sit4p. These data uncover a new regulatory pathway that connects multidrug resistance to Sit4p function.

Differential roles of transcriptional mediator subunits in regulation of multidrug resistance gene expression in Saccharomyces cerevisiae

Med12 is a transcriptional Mediator subunit most typically associated with negative control of gene expression. Here Med12 is demonstrated to serve as a positive regulator required for activation of multidrug resistance gene expression in yeast cells lacking their mitochondrial genome.

The multiprotein transcriptional Mediator complex provides a key link between RNA polymerase II and upstream transcriptional activator proteins. Previous work has established that the multidrug resistance transcription factors Pdr1 and Pdr3 interact with the Mediator component Med15/Gal11 to drive normal levels of expression of the ATP-binding cassette transporter-encoding gene PDR5 in Saccharomyces cerevisiae. PDR5 transcription is induced upon loss of the mitochondrial genome (ρ⁰ cells) and here we provide evidence that this ρ⁰ induction is Med15 independent. A search through other known Mediator components determined that Med12/Srb8, a member of the CDK8 Mediator submodule, is required for ρ⁰ activation of PDR5 transcription. The CDK8 submodule contains the cyclin C homologue (CycC/Srb11), cyclin-dependent kinase Cdk8/Srb10, and the large Med13/Srb9 protein. Loss of these other proteins did not lead to the same block in PDR5 induction. Chromatin immunoprecipitation analyses demonstrated that Med15 is associated with the PDR5 promoter in both ρ⁺ and ρ⁰, whereas Med12 recruitment to this target promoter is highly responsive to loss of the mitochondrial genome. Coimmunoprecipitation experiments revealed that association of Pdr3 with Med12 can only be detected in ρ⁰ cells. These experiments uncover the unique importance of Med12 in activated transcription of PDR5 seen in ρ⁰ cells.

Site-directed mutagenesis of Asp853 in Pdr3p transcriptional activator from Saccharomyces cerevisiae

The PDR3 gene encodes one of the main transcriptional activators involved in the control of multidrug resistance in the yeast Saccharomyces cerevisiae. Recently, it has been demonstrated that a specific D853Y mutation results in the loss of transactivation activity of Pdr3p and its conversion to multicopy suppressor of multidrug resistance. In this study, the Asp853 in Pdr3p was replaced by eight different amino acids and the function of mutated proteins was analysed. Different levels of complementation of cycloheximide hypersensitivity and expression of autoregulated PDR3 and its PDR5 target in the pdr1Deltapdr3Delta mutant strain, ranging from that of the wild-type to loss-of-function alleles, were observed in pdr3 mutants containing Pro, Glu, Arg, Asn, Ser, Leu, Phe, Ile or Tyr instead of Asp853 in Pdr3p. The introduction of the D853Y mutation into gain-of-function Pdr3p suppressed the transcription of the PDR3 and PDR5 genes and reduced both the rhodamine 6G efflux rate and the drug resistance level in corresponding double mutants. The results indicate that, while Pdr3p can tolerate several substitutions of Asp853, the occurrence of a hydrophobic amino acid at this position has an adverse effect on its function.

Gain of function mutations in CgPDR1 of Candida glabrata not only mediate antifungal resistance but also enhance virulence

CgPdr1p is a Candida glabrata Zn(2)-Cys(6) transcription factor involved in the regulation of the ABC-transporter genes CgCDR1, CgCDR2, and CgSNQ2, which are mediators of azole resistance. Single-point mutations in CgPDR1 are known to increase the expression of at least CgCDR1 and CgCDR2 and thus to contribute to azole resistance of clinical isolates. In this study, we investigated the incidence of CgPDR1 mutations in a large collection of clinical isolates and tested their relevance, not only to azole resistance in vitro and in vivo, but also to virulence. The comparison of CgPDR1 alleles from azole-susceptible and azole-resistant matched isolates enabled the identification of 57 amino acid substitutions, each positioned in distinct CgPDR1 alleles. These substitutions, which could be grouped into three different “hot spots,” were gain of function (GOF) mutations since they conferred hyperactivity to CgPdr1p revealed by constitutive high expression of ABC-transporter genes. Interestingly, the major transporters involved in azole resistance (CgCDR1, CgCDR2, and CgSNQ2) were not always coordinately expressed in presence of specific CgPDR1 GOF mutations, thus suggesting that these are rather trans-acting elements (GOF in CgPDR1) than cis-acting elements (promoters) that lead to azole resistance by upregulating specific combinations of ABC-transporter genes. Moreover, C. glabrata isolates complemented with CgPDR1 hyperactive alleles were not only more virulent in mice than those with wild type alleles, but they also gained fitness in the same animal model. The presence of CgPDR1 hyperactive alleles also contributed to fluconazole treatment failure in the mouse model. In conclusion, this study shows for the first time that CgPDR1 mutations are not only responsible for in vitro/in vivo azole resistance but that they can also confer a selective advantage under host conditions.

Candida glabrata is a yeast causing several diseases in humans and especially in immuno-compromised people. C. glabrata infections are treated with antifungal agents, however the use of some agents, for example azoles, is associated with the development of resistance. In this yeast species, azole resistance is mediated almost exclusively by ATP Binding Cassette (ABC) multidrug transporters. Their overexpression results in enhanced efflux of azoles and thus generates resistance. Regulation of ABC transporters is therefore of pivotal importance to understanding azole resistance. In C. glabrata, the expression of ABC transporters is mediated by a zinc finger transcription factor called CgPDR1. Gain of function (GOF) mutations in CgPDR1 result in high ABC transporter expression. In this study, we investigated the occurrence of GOF mutations in a large collection of azole-resistant isolates and found a high variety of mutations localized in three distinct domains of CgPDR1. We found that these mutations are not only associated with resistance but also enhanced virulence and fitness of C. glabrata in animal models. Our study provides for the first time evidence that mutations causing antifungal resistance can also provide a selective advantage under host conditions and thus highlights the need of carefully monitoring resistance in this pathogen.

Coordinate control of lipid composition and drug transport activities is required for normal multidrug resistance in fungi

Pathogenic fungi present a special problem in the clinic as the range of drugs that can be used to treat these types of infections is limited. This situation is further complicated by the presence of robust inducible gene networks encoding different proteins that confer tolerance to many available antifungal drugs. The transcriptional control of these multidrug resistance systems in several key fungi will be discussed. Experiments in the non-pathogenic Saccharomyces cerevisiae have provided much of our current understanding of the molecular framework on which fungal multidrug resistance is built. More recent studies on the important pathogenic Candida species, Candida albicans and Candida glabrata, have provided new insights into the organization of the multidrug resistance systems in these organisms. We will compare the circuitry of multidrug resistance networks in these three organisms and suggest that, in addition to the well-accepted drug efflux activities, the regulation of membrane composition by multidrug resistance proteins provides an important contribution to the resistant phenotypes observed.

Decreased susceptibility to antifungals in respiratory-deficient Kluyveromyces lactis mutants

Decreased susceptibility of K. lactis mutants impaired in the function of cytochrome c, cytochrome c1 and cytochrome-c oxidase to fluconazole, bifonazole and amphotericin B in comparison with the isogenic wild-type strain was observed. Flow cytometry with rhodamine 6G did not show any changes in the accumulation of the dye in the mutant cells compared with the corresponding wild-type strain. Sterol analysis showed similar overall amount of sterols in both wild-type and mutant cells. Taking into account the increased amphotericin B resistance and significantly diminished susceptibility of mutant cells to lyticase digestion, the cell wall structure and/or composition may probably be responsible for the observed changes in the susceptibility of mutants to the antifungal compounds used.

Compensatory activation of the multidrug transporters Pdr5p, Snq2p, and Yor1p by Pdr1p in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the transcription factors Pdr1p and Pdr3p activate the expression of several genes, including PDR5, SNQ2, and YOR1, which encode ATP-binding cassette transporters that extrude dozens of antifungals with overlapping but distinct specificity. In this study, it was observed that growth resistance to specific Pdr5p substrates rose upon disruption of the YOR1 or SNQ2 coding region and was accompanied by increased efflux. Similarly, resistance to Yor1p- and Snq2p-specific substrates increased upon deletion of PDR5. The mRNA and protein levels of the respective transporters increased in parallel to drug resistance. beta-Galactosidase activity fused to the PDR5 or YOR1 promoter required the presence of Pdr1p and its specific binding sites for the compensatory induction, whereas Pdr3p had an inhibitory effect.

Responses of pathogenic and nonpathogenic yeast species to steroids reveal the functioning and evolution of multidrug resistance transcriptional networks

Steroids are known to induce pleiotropic drug resistance states in hemiascomycetes, with tremendous potential consequences for human fungal infections. Our analysis of gene expression in Saccharomyces cerevisiae and Candida albicans cells subjected to three different concentrations of progesterone revealed that their pleiotropic drug resistance (PDR) networks were strikingly sensitive to steroids. In S. cerevisiae, 20 of the Pdr1p/Pdr3p target genes, including PDR3 itself, were rapidly induced by progesterone, which mimics the effects of PDR1 gain-of-function alleles. This unique property allowed us to decipher the respective roles of Pdr1p and Pdr3p in PDR induction and to define functional modules among their target genes. Although the expression profiles of the major PDR transporters encoding genes ScPDR5 and CaCDR1 were similar, the S. cerevisiae global PDR response to progesterone was only partly conserved in C. albicans. In particular, the role of Tac1p, the main C. albicans PDR regulator, in the progesterone response was apparently restricted to five genes. These results suggest that the C. albicans and S. cerevisiae PDR networks, although sharing a conserved core regarding the regulation of membrane properties, have different structures and properties. Additionally, our data indicate that other as yet undiscovered regulators may second Tac1p in the C. albicans drug response.

The zinc cluster transcription factor Tac1p regulates PDR16 expression in Candida albicans

The Candida albicans PDR16 gene, encoding a putative phosphatidylinositol transfer protein, is co-induced with the multidrug transporter genes CDR1 and CDR2 in azole-resistant (A(R)) clinical isolates and upon fluphenazine exposure of azole-susceptible (A(S)) cells, suggesting that it is regulated by Tac1p, the transcriptional activator of CDR genes. Deleting TAC1 in an A(R) isolate (5674) overexpressing PDR16, CDR1 and CDR2 decreased the expression of the three genes and fluconazole resistance to levels similar to those detected in the matched A(S) isolate (5457), demonstrating that Tac1p is responsible for PDR16 upregulation in that strain. Deleting TAC1 in the A(S) strain SC5314 abolished CDR2 induction by fluphenazine and decreased that of PDR16 and CDR1, uncovering the participation of an additional factor in the regulation of PDR16 and CDR1 expression. Sequencing of the TAC1 alleles identified one homozygous mutation in strain 5674, an Asn to Asp substitution at position 972 in the C-terminus of Tac1p. Introduction of the Asp(972) allele in a tac1Delta/Delta mutant caused high levels of fluconazole resistance and TAC1, PDR16, CDR1 and CDR2 constitutive induction. These results demonstrate that: (i) Tac1p controls PDR16 expression; (ii) Asn(972) to Asp(972) is a gain-of-function mutation; and (iii) Tac1p is positively autoregulated, directly or indirectly.

Negative Transcriptional Regulation of Multidrug Resistance Gene Expression by an Hsp70 Protein

One of the most common origins of multidrug resistance occurs via the overproduction of ATP-binding cassette (ABC) transporter proteins. These ABC transporters then act as broad specificity drug pumps and efflux a wide range of toxic agents out of the cell. The yeast Saccharomyces cerevisiae exhibits multiple or pleiotropic drug resistance (Pdr) often through the over-production of a plasma membrane-localized ABC transporter protein called Pdr5p. Expression of the PDR5 gene is controlled by two zinc cluster-containing transcription factors called Pdr1p and Pdr3p. Cells that lack their mitochondrial genome (rho(0) cells) strongly induce PDR5 transcription in a Pdr3p-dependent fashion. To identify proteins associated with Pdr3p that might act to regulate this factor, a tandem affinity purification (TAP) moiety was fused to Pdr3p, and this recombinant protein was purified from yeast cells. The cytosolic Hsp70 chaperone Ssa1p co-purified with TAP-Pdr3p. Overexpression of Ssa1p repressed expression of PDR5 but had no effect on expression of other genes involved in the Pdr phenotype. This Ssa1p-mediated repression required the presence of Pdr3p and did not influence Pdr1p-dependent gene expression. Loss of the nucleotide exchange factor Fes1p mimicked Ssa1p-mediated repression of PDR5. Co-immunoprecipitation experiments indicated that Ssa1p was associated with Pdr3p but not Pdr1p in yeast cells. Finally, rho(0) cells had less Ssa1p bound to Pdr3p than rho(+) cells, consistent with Ssa1p-mediated repression of Pdr3p activity serving as a key regulatory step in control of multidrug resistance in yeast.

Fcr1p inhibits development of fluconazole resistance in Candida albicans by abolishing CDR1 induction

Overexpression of Candida drug resistance 1 (CDR1) gene in Candida albicans (C. albicans), an efflux pump, is one of the major mechanisms contributing to drug resistance. C. albicans for fluconazole resistance 1 protein (Fcr1p) is a member of the family of zinc cluster proteins homologous to Pdr1p and Pdr3p (pleiotropic drug resistance protein) mediating azole resistance in Saccharomyces cerevisiae (S. cerevisiae) by regulating the expression of pleiotropic drug resistance 5 (PDR5) homologous to C. albicans CDR1. A previous study has showed that for fluconazole resistance 1 (FCR1) could also confer azole resistance in S. cerevisiae pdr1 pdr3 mutant by regulating PDR5. Therefore, we investigated the role of FCR1 in the development of C. albicans azole resistance in vitro and in vivo. Our results showed that Fcr1p inhibited fluconazole (FLC) resistance development in C. albicans through abolishing the induction of CDR1 expression by FLC, and in contrast FLC resistance development was accelerated resulting from the deletion of FCR1.

Loss-of-functionpdr3mutations convert the Pdr3p transcription activator to a protein suppressing multidrug resistance inSaccharomyces cerevisiae

The PDR1 and PDR3 genes encode the main transcription activators involved in the control of multidrug resistance in Saccharomyces cerevisiae. To identify the amino acids essential for Pdr3p function, the loss-of-function pdr3 mutants were isolated and characterized. Two plasmid-borne pdr3 alleles, pdr3-E902Ter and pdr3-D853Y, which failed to complement drug hypersensitivity in the Deltapdr1Deltapdr3 mutant strain, were isolated. The E902Ter mutation resulted in a truncated protein lacking the C-terminal activation domain. The D853Y mutation allowed the expression of entire Pdr3p, but its transactivation function was lost. When overexpressed from the P(GAL1) promoter, the two mutant alleles increased the sensitivity of wild-type cells to cycloheximide and fluconazole and suppressed drug resistance in gain-of-function pdr1 and pdr3 mutant strains. The drug-sensitizing effect of overexpressed loss-of-function pdr3 mutant alleles correlated with their ability to suppress PDR5 transcription and rhodamine 6G accumulation in transformants of the wild-type and Deltapdr1 mutant strains. These results demonstrate that amino acid residue Asp853 is essential for Pdr3p function, and indicate that specific loss-of-function pdr3 mutations can convert the Pdr3p transcription activator to a multicopy suppressor of multidrug resistance.

The central role of PDR1 in the foundation of yeast drug resistance

The widespread pleiotropic drug resistance (PDR) phenomenon is well described as the long term selection of genetic variants expressing constitutively high levels of membrane transporters involved in drug efflux. However, the transcriptional cascades leading to the PDR phenotype in wild-type cells are largely unknown, and the first steps of this phenomenon are poorly understood. We investigated the transcriptional mechanisms underlying the establishment of an efficient PDR response in budding yeast. We show that within a few minutes of drug sensing yeast elicits an effective PDR response, involving tens of PDR genes. This early PDR response (ePDR) is highly dependent on the Pdr1p transcription factor, which is also one of the major genetic determinants of long term PDR acquisition. The activity of Pdr1p in early drug response is not drug-specific, as two chemically unrelated drugs, benomyl and fluphenazine, elicit identical, Pdr1p-dependent, ePDR patterns. Our data also demonstrate that Pdr1p is an original stress response factor, the DNA binding properties of which do not depend on the presence of drugs. Thus, Pdr1p is a promoter-resident regulator involved in both basal expression and rapid drug-dependent induction of PDR genes.

A fungal family of transcriptional regulators: the zinc cluster proteins

The trace element zinc is required for proper functioning of a large number of proteins, including various enzymes. However, most zinc-containing proteins are transcription factors capable of binding DNA and are named zinc finger proteins. They form one of the largest families of transcriptional regulators and are categorized into various classes according to zinc-binding motifs. This review focuses on one class of zinc finger proteins called zinc cluster (or binuclear) proteins. Members of this family are exclusively fungal and possess the well-conserved motif CysX(2)CysX(6)CysX(5-12)CysX(2)CysX(6-8)Cys. The cysteine residues bind to two zinc atoms, which coordinate folding of the domain involved in DNA recognition. The first- and best-studied zinc cluster protein is Gal4p, a transcriptional activator of genes involved in the catabolism of galactose in the budding yeast Saccharomyces cerevisiae. Since the discovery of Gal4p, many other zinc cluster proteins have been characterized; they function in a wide range of processes, including primary and secondary metabolism and meiosis. Other roles include regulation of genes involved in the stress response as well as pleiotropic drug resistance, as demonstrated in budding yeast and in human fungal pathogens. With the number of characterized zinc cluster proteins growing rapidly, it is becoming more and more apparent that they are important regulators of fungal physiology.