Identification of a negative regulatory element which regulates basal transcription of a multidrug resistance gene CDR1 of Candida albicans

Cdr1 in focus: a personal reflection on multidrug transporter research

Drug resistance mechanisms in human pathogenic Candida species are constantly evolving. Over time, these species have developed diverse strategies to counter the effects of various drug classes, making them a significant threat to human health. In addition to well-known mechanisms such as drug target modification, overexpression, and chromosome duplication, Candida species have also developed permeability barriers to antifungal drugs through reduced drug import or increased efflux. The genomes of Candida species contain a multitude of drug resistance genes, many of which encode membrane efflux transporters that actively expel drugs, preventing their toxic accumulation inside the cells and contributing to multidrug resistance. This brief personal retrospective piece for the "Thematic Issue on Celebrating 30 Years of Cdr1 Research: new trends in antifungal therapy and drug resistance" looks back as to how antifungal research has shifted focus since the identification of the first multidrug transporter gene, CDR1 (Candida Drug Resistance 1), leading to new insights into how reduced azole permeability across Candida cell membranes influences antifungal susceptibility.

The challenges of the genome-based identification of antifungal resistance in the clinical routine

The increasing number of chronic and life-threatening infections caused by antimicrobial resistant fungal isolates is of critical concern. Low DNA sequencing cost may facilitate the identification of the genomic profile leading to resistance, the resistome, to rationally optimize the design of antifungal therapies. However, compared to bacteria, initiatives for resistome detection in eukaryotic pathogens are underdeveloped. Firstly, reported mutations in antifungal targets leading to reduced susceptibility must be extensively collected from the literature to generate comprehensive databases. This information should be complemented with specific laboratory screenings to detect the highest number possible of relevant genetic changes in primary targets and associations between resistance and other genomic markers. Strikingly, some drug resistant strains experience high-level genetic changes such as ploidy variation as much as duplications and reorganizations of specific chromosomes. Such variations involve allelic dominance, gene dosage increments and target expression regime effects that should be explicitly parameterized in antifungal resistome prediction algorithms. Clinical data indicate that predictors need to consider the precise pathogen species and drug levels of detail, instead of just genus and drug class. The concomitant needs for mutation accuracy and assembly quality assurance suggest hybrid sequencing approaches involving third-generation methods will be utilized. Moreover, fatal fast infections, like fungemia and meningitis, will further require both sequencing and analysis facilities are available in-house. Altogether, the complex nature of antifungal resistance demands extensive sequencing, data acquisition and processing, bioinformatic analysis pipelines, and standard protocols to be accomplished prior to genome-based protocols are applied in the clinical setting.

Fungal infections in pediatric neurosurgery

INTRODUCTION

Invasive mycosis of the central nervous system represent a diverse group of diseases that have gradually emerged as not only opportunistic infections in patients with immune susceptibility due to congenital and acquired deficiency, immunomodulation, solid organ and stem cell transplantation, hematological malignancies, and chronic steroid use but also in selected risk populations such as low weight preterm infants, patients with shunted hydrocephalus and external ventricular drainages, skull base surgery, and head injury.

OBJECTIVES

The purpose of this review is to familiarize the pediatric neurosurgeon with the most common mycosis and their clinical scenarios which can be encountered in the clinical practice, with special emphasis on clinical, radiological, and laboratory diagnosis beyond classical microorganism cultures as well as options in medical and surgical treatment given the high incidence of morbidity and mortality associated with these challenging entities.

METHODS

We conducted an online database review (Ovid, PubMed) gathering relevant English language literature published in the last 20 years with special emphasis on recent breakthroughs in the diagnosis and treatment of invasive mycosis of the CNS as well as reported cases within the pediatric neurosurgical literature and their surgical management.

RESULTS

Fungal agents capable of invading the CNS can behave as aggressive entities with rapid progression manifesting as overwhelming meningoencephalitis with vascular compromise or can lead to space-occupying lesions with abscess formation which require prompt diagnosis by either laboratory identification of the components of these biological agents and their host response or by obtaining tissue specimens for microbiological identification which may not be straightforward due to prolonged culture time.

CONCLUSION

Following a high degree of suspicion with prompt initiation of antifungal agents and reversal of potential immunosuppressant therapies along with neurosurgical evacuation of intracranial collections or removal of infected hardware (CSF shunts) can lead to more optimistic outcomes of these complex clinical scenarios.

The global regulator Ncb2 escapes from the core promoter and impacts transcription in response to drug stress in Candida albicans

Ncb2, the β subunit of NC2 complex, a heterodimeric regulator of transcription was earlier shown to be involved in the activated transcription of CDR1 gene in azole resistant isolate (AR) of Candida albicans. This study examines its genome-wide role by profiling Ncb2 occupancy between genetically matched pair of azole sensitive (AS) and AR clinical isolates. A comparison of Ncb2 recruitment between the two isolates displayed that 29 genes had higher promoter occupancy of Ncb2 in the AR isolate. Additionally, a host of genes exhibited exclusive occupancy of Ncb2 at promoters of either AR or AS isolate. The analysis also divulged new actors of multi-drug resistance, whose transcription was activated owing to the differential occupancy of Ncb2. The conditional, sequence-specific positional escape of Ncb2 from the core promoter in AS isolate and its preferential recruitment to the core promoter of certain genes in AR isolates was most noteworthy means of transcription regulation. Together, we show that positional rearrangement of Ncb2 resulting in either activation or repression of gene expression in response to drug-induced stress, represents a novel regulatory mechanism that opens new opportunities for therapeutic intervention to prevent development of drug tolerance in C. albicans cells.

Chitosan silver nanocomposite (CAgNC) as an antifungal agent against Candida albicans

Due to limited numbers of antifungal drugs and emergence of drug resistance have directed to develop nonconventional therapeutic agents against fungal pathogen Candida albicans. In this study, anticandidal activity of chitosan silver nanocomposite (CAgNC) was tested against C. albicans Minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) of CAgNC were determined as 25 and 100 μg/ml, respectively. Electron microscopic image results confirmed the ultrastructural cell wall deformities and injuries caused by CAgNC. Propidium iodide (PI) penetration into the CAgNC treated cells could be considered as an evidence for loss of cell membrane integrity and cell death at MFC. Level of intracellular reactive oxygen species (ROS) was increased, while cell viability was decreased with the increased of CAgNC concentrations. In our protein profile results, several induced proteins were observed under CAgNC treatment, and they could be related to multidrug and stress resistant proteins such as CDR1 (55 kDa) and CaHSP70 based on the protein band size. CAgNC mediated cell wall damage, loss of cell membrane integrity, elevated ROS level, and associated oxidative stress have been identified as the main causative factors for the anticandidal activity. Overall results from our study indicated that CAgNC could affect negatively on physiological and biochemical functions of C. albicans suggesting CAgNC as a potential alternative for antifungal chemotherapy.

Novel Regulatory Mechanisms of Pathogenicity and Virulence to Combat MDR in Candida albicans

Continuous deployment of antifungals in treating infections caused by dimorphic opportunistic pathogen Candida albicans has led to the emergence of drug resistance resulting in cross-resistance to many unrelated drugs, a phenomenon termed multidrug resistance (MDR). Despite the current understanding of major factors which contribute to MDR mechanisms, there are many lines of evidence suggesting that it is a complex interplay of multiple factors which may be contributed by still unknown mechanisms. Coincidentally with the increased usage of antifungal drugs, the number of reports for antifungal drug resistance has also increased which further highlights the need for understanding novel molecular mechanisms which can be explored to combat MDR, namely, ROS, iron, hypoxia, lipids, morphogenesis, and transcriptional and signaling networks. Considering the worrying evolution of MDR and significance of C. albicans being the most prevalent human fungal pathogen, this review summarizes these new regulatory mechanisms which could be exploited to prevent MDR development in C. albicans as established from recent studies.

Transcriptional regulatory circuitries in the human pathogen Candida albicans involving sense--antisense interactions

Candida albicans, a major human fungal pathogen, usually contains a diploid genome, but controls adaptation to a toxic alternative carbon source L-sorbose, by the reversible loss of one chromosome 5 (Ch5). We have previously identified multiple unique regions on Ch5 that repress the growth on sorbose. In one of the regions, the CSU51 gene determining the repressive property of the region was identified. We report here the identification of the CSU53 gene from a different region on Ch5. Most importantly, we find that CSU51 and CSU53 are associated with novel regulatory elements, ASUs, which are embedded within CSUs in an antisense configuration. ASUs act opposite to CSUs by enhancing the growth on sorbose. In respect to the CSU transcripts, the ASU long antisense transcripts are in lesser amounts, are completely overlapped, and are inversely related. ASUs interact with CSUs in natural CSU/ASU cis configurations, as well as when extra copies of ASUs are placed in trans to the CSU/ASU configurations. We suggest that ASU long embedded antisense transcripts modulate CSU sense transcripts.

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.

Ncb2 is involved in activated transcription of CDR1 in azole-resistant clinical isolates of Candida albicans

We recently demonstrated that CDR1 overexpression in azole-resistant isolates of Candida albicans is due to its enhanced transcriptional activation and increased mRNA stability. In this study, we provide the first evidence of transcriptional regulation of CDR1 by Ncb2, the β subunit of NC2, a heterodimeric regulator of transcription. Conditional NCB2 null mutants displayed decreased susceptibility toward azole and an enhanced transcription of CDR1. Interestingly, Ncb2 associated with the CDR1 promoter under both repression and activation; however, an increase in recruitment was observed under both transient and constitutive activation states. By chromatin immunoprecipitation (ChIP) assay, we showed the preferential recruitment of Ncb2 to the core TATA region under activation (azole-resistant isolate), while under repression (azole-susceptible isolate) it was present at the TATA upstream region. Further, ChIP analysis revealed that Ncb2 binding was not restricted to the CDR1 gene; instead, it was observed on the promoters of genes coregulated with CDR1 by the transcription activator Tac1. The tac1Δ null mutants, which fail to show the drug-induced transient activation of CDR1, also showed no increase in Ncb2 recruitment at the promoter. Taken together, our results show that Ncb2, in conjunction with Tac1, is involved in the transcriptional activation of CDR1, opening up new therapeutic possibilities to combat multidrug resistance (MDR) in C. albicans.

Molecular determinants of transient and reversible induced up-regulation of CaCDR1 in azole susceptible clinical isolates of Candida albicans

The present study examines the molecular mechanism underlying in vitro-induced resistance to FLC (fluconazole), KTC (ketaonazole), MCZ (miconazole) and CHX (cycloheximide) in AS (azole-susceptible) strains of Candida albicans when exposed to CaCDR1/CaCDR2 inducers like FPZ (fluphenazine) and steroids [PRG (progesterone) and β-EST (β-oestradiol)]. By employing spot and checkerboard titre assays, we provide evidence of an in vitro-induced antagonism between tested drugs and inducers, which was accompanied with a concomitant increase in CaCDR1 and CaCDR2 transcript levels. Notably, unlike AS isolates, parental WT (wild-type) and Δcdr2 null strains, Δcdr1 as well as Δcdr1/Δcdr2 nulls, when challenged with the inducers could not display antagonism. Our results validated by Northern blotting, reporter gene transcription and TRO (transcription run on) assays show that in vitro-induced antagonism between tested drugs and inducer in AS isolates was mainly due to a transient and reversible transcriptional activation of CaCDR1. Notwithstanding our earlier observation that consistent high transcript levels of CaCDR1 in clinical AR (azole-resistant) isolates were maintained due to the combination of its transcriptional activation and enhanced mRNA stability via elongated poly(A) tails, this study shows that transient and reversible transcriptional activation of CaCDR1 was the major determinant of induced antagonism in AS isolates. The distinct strategies between sustained (in AR isolates) and transiently induced resistance mechanisms (in AS isolates) adopted by Candida should become useful in improving therapeutic approaches.

PAP1 [poly(A) polymerase 1] homozygosity and hyperadenylation are major determinants of increased mRNA stability of CDR1 in azole-resistant clinical isolates of Candida albicans

Using genetically matched azole-susceptible (AS) and azole-resistant (AR) clinical isolates of Candida albicans, we recently demonstrated that CDR1 overexpression in AR isolates is due to its enhanced transcriptional activation and mRNA stability. This study examines the molecular mechanisms underlying enhanced CDR1 mRNA stability in AR isolates. Mapping of the 3' untranslated region (3' UTR) of CDR1 revealed that it was rich in adenylate/uridylate (AU) elements, possessed heterogeneous polyadenylation sites, and had putative consensus sequences for RNA-binding proteins. Swapping of heterologous and chimeric lacZ-CDR1 3' UTR transcriptional reporter fusion constructs did not alter the reporter activity in AS and AR isolates, indicating that cis-acting sequences within the CDR1 3' UTR itself are not sufficient to confer the observed differential mRNA decay. Interestingly, the poly(A) tail of the CDR1 mRNA of AR isolates was approximately 35-50 % hyperadenylated as compared with AS isolates. C. albicans poly(A) polymerase (PAP1), responsible for mRNA adenylation, resides on chromosome 5 in close proximity to the mating type-like (MTL) locus. Two different PAP1 alleles, PAP1-a/PAP1-alpha, were recovered from AS (MTL-a/MTL-alpha), while a single type of PAP1 allele (PAP1-alpha) was recovered from AR isolates (MTL-alpha/MTL-alpha). Among the heterozygous deletions of PAP1-a (Deltapap1-a/PAP1-alpha) and PAP1-alpha (PAP1-a/Deltapap1-alpha), only the former led to relatively enhanced drug resistance, to polyadenylation and to transcript stability of CDR1 in the AS isolate. This suggests a dominant negative role of PAP1-a in CDR1 transcript polyadenylation and stability. Taken together, our study provides the first evidence, to our knowledge, that loss of heterozygosity at the PAP1 locus is linked to hyperadenylation and subsequent increased stability of CDR1 transcripts, thus contributing to enhanced drug resistance.

Antifungal drug resistance mechanisms in fungal pathogens from the perspective of transcriptional gene regulation

Fungi are primitive eukaryotes and have adapted to a variety of niches during evolution. Some fungal species may interact with other life forms (plants, insects, mammals), but are considered as pathogens when they cause mild to severe diseases. Chemical control strategies have emerged with the development of several drugs with antifungal activity against pathogenic fungi. Antifungal agents have demonstrated their efficacy by improving patient health in medicine. However, fungi have counteracted antifungal agents in several cases by developing resistance mechanisms. These mechanisms rely on drug resistance genes including multidrug transporters and drug targets. Their regulation is crucial for the development of antifungal drug resistance and therefore transcriptional factors critical for their regulation are being characterized. Recent genome-wide studies have revealed complex regulatory circuits involving these genetic and transcriptional regulators. Here, we review the current understanding of the transcriptional regulation of drug resistance genes from several fungal pathogens including Candida and Aspergillus species.

Functional analysis of cis- and trans-acting elements of the Candida albicans CDR2 promoter with a novel promoter reporter system

Azole resistance in Candida albicans can be mediated by the upregulation of the ATP binding cassette transporter genes CDR1 and CDR2. Both genes are regulated by a cis-acting element called the drug-responsive element (DRE), with the consensus sequence 5'-CGGAWATCGGATATTTTTTT-3', and the transcription factor Tac1p. In order to analyze in detail the DRE sequence necessary for the regulation of CDR1 and CDR2 and properties of TAC1 alleles, a one-hybrid system was designed. This system is based on a P((CDR2))-HIS3 reporter system in which complementation of histidine auxotrophy can be monitored by activation of the reporter system by CDR2-inducing drugs such as estradiol. Our results show that most of the modifications within the DRE, but especially at the level of CGG triplets, strongly reduce CDR2 expression. The CDR2 DRE was replaced by putative DREs deduced from promoters of coregulated genes (CDR1, RTA3, and IFU5). Surprisingly, even if Tac1p was able to bind these putative DREs, as shown by chromatin immunoprecipitation, those from RTA3 and IFU5 did not functionally replace the CDR2 DRE. The one-hybrid system was also used for the identification of gain-of-function (GOF) mutations either in TAC1 alleles from clinical C. albicans isolates or inserted in TAC1 wild-type alleles by random mutagenesis. In all, 17 different GOF mutations were identified at 13 distinct positions. Five of them (G980E, N972D, A736V, T225A, and N977D) have already been described in clinical isolates, and four others (G980W, A736T, N972S, and N972I) occurred at already-described positions, thus suggesting that GOF mutations can occur in a limited number of positions in Tac1p. In conclusion, the one-hybrid system developed here is rapid and powerful and can be used for characterization of cis- and trans-acting elements in C. albicans.

Mutations in the multi-drug resistance regulator MRR1, followed by loss of heterozygosity, are the main cause of MDR1 overexpression in fluconazole-resistant Candida albicans strains

Overexpression of the MDR1 gene, encoding a multi-drug efflux pump of the major facilitator superfamily, is a major cause of resistance to the widely used antifungal agent fluconazole and other toxic substances in the fungal pathogen Candida albicans. We found that all tested clinical and in vitro generated C. albicans strains that had become fluconazole-resistant by constitutive MDR1 upregulation contained mutations in the MRR1 gene, which encodes a transcription factor that controls MDR1 expression. Introduction of the mutated alleles into a drug-susceptible C. albicans strain resulted in activation of the MDR1 promoter and multi-drug resistance, confirming that the amino acid substitutions in Mrr1p were gain-of-function mutations that rendered the transcription factor constitutively active. The majority of the MDR1 overexpressing strains had become homozygous for the mutated MRR1 alleles, demonstrating that the increased resistance level conferred by two gain-of-function alleles provides sufficient advantage to select for the loss of heterozygosity in the presence of fluconazole both in vitro and within the human host during therapy. Loss of heterozygosity usually occurred by mitotic recombination between the two chromosome 3 homologues on which MRR1 is located, but evidence for complete loss of one chromosome and duplication of the chromosome containing the mutated MRR1 allele was also obtained in two in vitro generated fluconazole-resistant strains. These results demonstrate that gain-of-function mutations in MRR1 are the major, if not the sole, mechanism of MDR1 overexpression in fluconazole-resistant strains and that this transcription factor plays a central role in the development of drug resistance in C. albicans.

Transcriptional activation and increased mRNA stability contribute to overexpression of CDR1 in azole-resistant Candida albicans

Many azole-resistant (AR) clinical isolates of Candida albicans display increased expression of the drug transporters CDR1 and CDR2. In this study, we evaluate the molecular mechanisms that contribute to the maintenance of constitutively high CDR1 transcript levels in two matched pairs of azole-susceptible (AS) and AR clinical isolates of C. albicans. To address this, we use reporter constructs of GFP and lacZ fused either to the CDR1 promoter (P CDR1-GFP/lacZ; transcriptional fusion) or to the CDR1 open reading frame (P CDR1-CDR1-GFP/lacZ; translational fusion) integrated at the native CDR1 locus. It is observed that expression of the two reporter genes as a transcriptional fusion in the AR isolates is higher than that in matched AS isolates. However, the difference in the reporter activity between the AS and AR isolates is even greater for the translational fusions, indicating that the sequences within the CDR1 coding region also contribute to its increased expression in AR isolates. Further analysis of these observations by transcription run-on assays demonstrated a approximately 5- to 7-fold difference in the transcription initiation rates for the AR isolates from those for their respective matched AS isolates. Measurement of mRNA stability showed that the half-life of CDR1 mRNA in the AR isolates was threefold higher than that in the corresponding AS isolates. Our results demonstrate that both increased CDR1 transcription and enhanced CDR1 mRNA stability contribute to the overexpression of CDR1 in AR C. albicans isolates.

The RNA2 5' leader of Blackcurrant reversion virus mediates efficient in vivo translation through an internal ribosomal entry site mechanism

The 5' and 3' non-translated regions (NTRs) of mRNAs of eukaryotes and their viruses often contain translational enhancers, including internal ribosomal entry sites (IRESs) comprised in the 5' leaders of many uncapped viral mRNAs. Blackcurrant reversion virus (BRV) has a genome composed of two uncapped, polyadenylated RNAs with relatively short 5' NTRs, almost devoid of secondary structure. In this work, a role of the RNA2 5' NTR in translation was studied by using mono- and dicistronic Photinus pyralis and Renilla reniformis luciferase reporter mRNAs in protoplasts of Nicotiana benthamiana. The RNA2 5' leader was found to confer efficient in vivo translation compared with the control 5' NTR, and each half of the BRV leader was essential for stimulatory function. Such efficient translational enhancement was mediated, at least in part, through an IRES mechanism. Multiple RNA2 5' NTR regions, complementary to a fragment of plant 18S rRNA demonstrated previously to be accessible for intermolecular mRNA-rRNA interactions and conserved between eukaryotes, were shown to be important for efficient translation. Similar mRNA-rRNA base-pairing potential was also predicted for the 5' leaders of other nepoviruses.

A mutation in Tac1p, a transcription factor regulating CDR1 and CDR2, is coupled with loss of heterozygosity at chromosome 5 to mediate antifungal resistance in Candida albicans

TAC1, a Candida albicans transcription factor situated near the mating-type locus on chromosome 5, is necessary for the upregulation of the ABC-transporter genes CDR1 and CDR2, which mediate azole resistance. We showed previously the existence of both wild-type and hyperactive TAC1 alleles. Wild-type alleles mediate upregulation of CDR1 and CDR2 upon exposure to inducers such as fluphenazine, while hyperactive alleles result in constitutive high expression of CDR1 and CDR2. Here we recovered TAC1 alleles from two pairs of matched azole-susceptible (DSY294; FH1: heterozygous at mating-type locus) and azole-resistant isolates (DSY296; FH3: homozygous at mating-type locus). Two different TAC1 wild-type alleles were recovered from DSY294 (TAC1-3 and TAC1-4) while a single hyperactive allele (TAC1-5) was isolated from DSY296. A single amino acid (aa) difference between TAC1-4 and TAC1-5 (Asn977 to Asp or N977D) was observed in a region corresponding to the predicted activation domain of Tac1p. Two TAC1 alleles were recovered from FH1 (TAC1-6 and TAC1-7) and a single hyperactive allele (TAC1-7) was recovered from FH3. The N977D change was seen in TAC1-7 in addition to several other aa differences. The importance of N977D in conferring hyperactivity to TAC1 was confirmed by site-directed mutagenesis. Both hyperactive alleles TAC1-5 and TAC1-7 were codominant with wild-type alleles and conferred hyperactive phenotypes only when homozygous. The mechanisms by which hyperactive alleles become homozygous was addressed by comparative genome hybridization and single nucleotide polymorphism arrays and indicated that loss of TAC1 heterozygosity can occur by recombination between portions of chromosome 5 or by chromosome 5 duplication.

An update on antifungal targets and mechanisms of resistance in Candida albicans

Much progress has been made in the last decade in identifying genes responsible for antifungal resistance in Candida albicans. Attention has focused on five major C. albicans genes: ABC transporter genes CDR1 and CDR2, major facilitator efflux gene MDR1, and ergosterol biosynthesis genes ERG11 and ERG3. Resistance involves mutations in 14C-lanosterol demethylase, targeted by fluconazole (FLZ) and encoded by ERG11, and mutations that up-regulate efflux genes that probably efflux the antifungals. Mutations that affect ERG3 mutations have been understudied as mechanism resistance among clinical isolates. In vitro resistance in clinical isolates typically involves step-wise mutations affecting more than one of these genes, and often unidentified genes. Different approaches are needed to identify these other genes. Very little is understood about reversible adaptive resistance of C. albicans despite its potential clinical significance; most clinical failures to control infections other than oropharyngeal candidiasis (OPC) occur with in vitro susceptible strains. Tolerance of C. albicans to azoles has been attributed to the calcineurin stress-response pathway, offering new potential targets for next generation antifungals. Recent studies have identified genes that regulate CDR1 or ERG genes. The focus of this review is C. albicans, although information on Saccharomyces cerevisiae or Candida glabrata is provided in areas in where Candida research is underdeveloped. With the completion of the C. albicans genomic sequence, and new methods for high throughput gene overexpression and disruption, rapid progress towards understanding the regulation of resistance, novel resistance mechanisms, and adaptive resistance is expected in the near future.

TAC1, transcriptional activator of CDR genes, is a new transcription factor involved in the regulation of Candida albicans ABC transporters CDR1 and CDR2

The ABC transporter genes CDR1 and CDR2 can be upregulated in Candida albicans developing resistance to azoles or can be upregulated by exposing cells transiently to drugs such as fluphenazine. The cis-acting drug-responsive element (DRE) present in the promoters of both genes and necessary for their upregulation contains 5'-CGG-3' triplets that are often recognized by transcriptional activators with Zn(2)-Cys(6) fingers. In order to isolate regulators of CDR1 and CDR2, the C. albicans genome was searched for genes encoding proteins with Zn(2)-Cys(6) fingers. Interestingly, three of these genes were tandemly arranged near the mating locus. Their involvement in CDR1 and CDR2 upregulation was addressed because a previous study demonstrated a link between mating locus homozygosity and azole resistance. The deletion of only one of these genes (orf19.3188) was sufficient to result in a loss of transient CDR1 and CDR2 upregulation by fluphenazine and was therefore named TAC1 (transcriptional activator of CDR genes). Tac1p has a nuclear localization, and a fusion of Tac1p with glutathione S-transferase could bind the cis-acting regulatory DRE in both the CDR1 and the CDR2 promoters. TAC1 is also relevant for azole resistance, since a TAC1 allele (TAC1-2) recovered from an azole-resistant strain could trigger constitutive upregulation of CDR1 and CDR2 in an azole-susceptible laboratory strain. Transcript profiling experiments performed with a TAC1 mutant and a revertant containing TAC1-2 revealed not only CDR1 and CDR2 as targets of TAC1 regulation but also other genes (RTA3, IFU5, and HSP12) that interestingly contained a DRE-like element in their promoters. In conclusion, TAC1 appears to be the first C. albicans transcription factor involved in the control of genes mediating antifungal resistance.

Expression of the CDR1 efflux pump in clinical Candida albicans isolates is controlled by a negative regulatory element

Resistance to azole antifungal drugs in clinical isolates of the human fungal pathogen Candida albicans is often caused by constitutive overexpression of the CDR1 gene, which encodes a multidrug efflux pump of the ABC transporter superfamily. To understand the relevance of a recently identified negative regulatory element (NRE) in the CDR1 promoter for the control of CDR1 expression in the clinical scenario, we investigated the effect of mutation or deletion of the NRE on CDR1 expression in two matched pairs of azole-sensitive and resistant clinical isolates of C. albicans. Expression of GFP or lacZ reporter genes from the wild type CDR1 promoter was much higher in the azole-resistant C. albicans isolates than in the azole-susceptible isolates, reflecting the known differences in CDR1 expression in these strains. Deletion or mutation of the NRE resulted in enhanced reporter gene expression in azole-sensitive strains, but did not further increase the already high CDR1 promoter activity in the azole-resistant strains. In agreement with these findings, electrophoretic mobility shift assays showed a reduced binding to the NRE of nuclear extracts from the resistant C. albicans isolates as compared with extracts from the sensitive isolates. These results demonstrate that the NRE is involved in maintaining CDR1 expression at basal levels and that this repression is overcome in azole-resistant clinical C. albicans isolates, resulting in constitutive CDR1 overexpression and concomitant drug resistance.