Abstract
The pathogenic yeast Candida albicans can develop resistance to the widely used antifungal agent fluconazole, which inhibits ergosterol biosynthesis, by the overexpression of genes encoding multidrug efflux pumps or ergosterol biosynthesis enzymes. Zinc cluster transcription factors play a central role in the transcriptional regulation of drug resistance. Mrr1 regulates the expression of the major facilitator MDR1, Tac1 controls the expression of the ABC transporters CDR1 and CDR2, and Upc2 regulates ergosterol biosynthesis (ERG) genes. Gain-of-function mutations in these transcription factors result in constitutive overexpression of their target genes and are responsible for fluconazole resistance in many clinical C. albicans isolates. The transcription factor Ndt80 contributes to the drug-induced upregulation of CDR1 and ERG genes and also binds to the MDR1 and CDR2 promoters, suggesting that it is an important component of all major transcriptional mechanisms of fluconazole resistance. However, we found that Ndt80 is not required for the induction of MDR1 and CDR2 expression by inducing chemicals. CDR2 was even partially derepressed in ndt80Δ mutants, indicating that Ndt80 is a repressor of CDR2 expression. Hyperactive forms of Mrr1, Tac1, and Upc2 promoted overexpression of MDR1, CDR1/CDR2, and ERG11, respectively, with the same efficiency in the presence and absence of Ndt80. Mrr1- and Tac1-mediated fluconazole resistance was even slightly enhanced in ndt80Δ mutants compared to wild-type cells. These results demonstrate that Ndt80 is dispensable for the constitutive overexpression of Mrr1, Tac1, and Upc2 target genes and the increased fluconazole resistance of strains that have acquired activating mutations in these transcription factors.
Highlights
Infections by the pathogenic yeast Candida albicans are commonly treated with the antifungal agent fluconazole, which blocks ergosterol biosynthesis by inhibiting sterol 14a-demethylase, a key enzyme in the ergosterol biosynthetic pathway
Many details of the molecular basis of drug resistance in C. albicans have been elucidated in recent years, especially with the identification of transcription factors that regulate the expression of ergosterol biosynthesis genes and multidrug efflux pumps
fluorescence-activated cell sorter (FACS) analysis of the cells showed that Ndt80 was not required for the induction of the MDR1 promoter by benomyl, whereas MDR1 induction was abolished in cells lacking Mrr1 (Figure 1A)
Summary
Infections by the pathogenic yeast Candida albicans are commonly treated with the antifungal agent fluconazole, which blocks ergosterol biosynthesis by inhibiting sterol 14a-demethylase, a key enzyme in the ergosterol biosynthetic pathway. C. albicans can develop resistance to fluconazole by various mechanisms, including mutations in the target enzyme that decrease its affinity to the drug, increased expression of the ERG11 gene encoding sterol 14a-demethylase, or overexpression of multidrug efflux pumps of the ABC transporter and major facilitator superfamilies [1]. Upc regulates the expression of ERG11 and other ERG genes [2,3], Tac controls the expression of the ABC transporters CDR1 and CDR2 [4], and Mrr regulates the expression of the major facilitator MDR1 [5] Mutants lacking these transcription factors cannot upregulate their target genes in response to inducing stimuli. Fluconazole-resistant, clinical C. albicans isolates that overexpress CDR1 and CDR2, MDR1, or ERG11 contain gain-of-function mutations in Tac, Mrr, and Upc, respectively, which render the transcription factors constitutively active even under noninducing conditions and are responsible for the increased drug resistance of these strains [4,5,6,7,8,9,10,11,12,13,14]
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