Induction of Candida albicans Drug Resistance Genes by Hybrid Zinc Cluster Transcription Factors

The pathogenic yeastCandida albicans can develop resistance to azole antifungal drugs by overexpressingERG11 , which encodes the drug target, or the multidrug efflux pumpsMDR1 andCDR1 /CDR2 . The constitutive upregulation of these genes is usually caused by gain-of-function mutations in the zinc cluster transcription factors Upc2, Mrr1, and Tac1, respectively. These transcription factors are also required for the induction of their target genes in drug-susceptible strains in the presence of specific stimuli. By swapping the DNA-binding domains of Mrr1, Tac1, and Upc2 we investigated if the hybrid transcription factors could activate their new target genes in response to the same signals. When Tac1 was targeted to theMDR1 andERG11 promoters, the expression of these genes became inducible by fluphenazine. Similarly,MDR1 andCDR2 were strongly upregulated by fluconazole when Upc2 was fused to the DNA-binding domains of Mrr1 and Tac1, respectively. In contrast, Mrr1 was unable to promote gene expression in response to benomyl when it was targeted to theCDR2 andERG11 promoters instead of theMDR1 promoter. These results suggest that Tac1 and Upc2 themselves are activated by the inducers fluphenazine and fluconazole, respectively, whereas benomyl does not activate Mrr1 itself but a coregulatory factor that is present at the promoters of Mrr1 target genes. Strains in which the expression levels of Mrr1 and Tac1 target genes were controlled by Upc2 exhibited increased fluconazole resistance, demonstrating that the ability to efficiently upregulate the expression of efflux pumps in the presence of the drug results in enhanced intrinsic fluconazole resistance.