The transcription factor Rpn4 activates its own transcription and induces efflux pump expression to confer fluconazole resistance in Candida auris

The emergence and nosocomial spread of the multidrug-resistant Candida auris pose a severe public health threat. Most C. auris clinical isolates are fluconazole resistant. In this study, we conduct transposon-mediated screens to profile genes whose inactivation causes fluconazole resistance in C. auris. We discover that mutation of genes encoding the Ubr2/Mub1 ubiquitin-ligase complex results in high fluconazole resistance by stabilizing the transcription activator, Rpn4. Global transcriptomic analysis, quantitative PCR, and combinatorial gene deletion reveal that Rpn4 causes fluconazole resistance by upregulating four efflux pump genes, SNQ21, SNQ22, MDR1, and CDR1, increasing efflux activity of the cell. Rpn4 autoactivates its own expression by binding to a PACE element in its promoter, forming a positive autoregulatory loop. Rpn4 also promotes CDR1 expression by binding to a PACE element in the CDR1 promoter. Furthermore, using the published genome sequence data of 304 clinical isolates, we identified a UBR2 A316T mutation in isolates from clades I and III. Although complementation of the ubr2 Delta mutant with the UBR2(A316T) allele resulted in increased susceptibility to fluconazole, the UBR2(A316T) mutant was still more resistant to fluconazole compared to wild type, suggesting that acquisition of the mutation is sufficient to confer an increase in fluconazole resistance. In conclusion, this study identifies Rpn4 as a critical transcription factor that regulates fluconazole resistance via the Rpn4-efflux pump axis in C. auris. IMPORTANCE Candida auris is a recently emerged pathogenic fungus of grave concern globally due to its resistance to conventional antifungals. This study takes a whole-genome approach to explore how C. auris overcomes growth inhibition imposed by the common antifungal drug fluconazole. We focused on gene disruptions caused by a "jumping genetic element" called transposon, leading to fluconazole resistance. We identified mutations in two genes, each encoding a component of the Ubr2/Mub1 ubiquitin-ligase complex, which marks the transcription regulator Rpn4 for degradation. When either protein is absent, stable Rpn4 accumulates in the cell. We found that Rpn4 activates the expression of itself as well as the main drug efflux pump gene CDR1 by binding to a PACE element in the promoter. Furthermore, we identified an amino acid change in Ubr2 in many resistant clinical isolates, contributing to Rpn4 stabilization and increased fluconazole resistance.