Two Functionally Redundant FK506-Binding Proteins Regulate Multidrug Resistance Gene Expression and Govern Azole Antifungal Resistance

Increasing resistance to antifungal therapy is an impediment to the effective treatment of fungal infections. is an opportunistic human fungal pathogen that is inherently less susceptible to cost-effective azole antifungals. Gain-of-function mutations in the Zn-finger pleiotropic drug resistance transcriptional activator-encoding gene are the most prevalent causes of azole resistance in clinical settings. is also transcriptionally activated upon azole exposure; however, factors governing gene expression are not yet fully understood. Here, we have uncovered a novel role for two FK506-binding proteins, CgFpr3 and CgFpr4, in the regulation of the regulon. We show that CgFpr3 and CgFpr4 possess a peptidyl-prolyl isomerase domain and act redundantly to control expression, as a ΔΔ mutant displayed elevated expression of the gene along with overexpression of its target genes, , , and , which code for ATP-binding cassette multidrug transporters. Furthermore, CgFpr3 and CgFpr4 are required for the maintenance of histone H3 and H4 protein levels, and fluconazole exposure leads to elevated H3 and H4 protein levels. Consistent with the role of histone proteins in azole resistance, disruption of genes coding for the histone demethylase CgRph1 and the histone H3K36-specific methyltransferase CgSet2 leads to increased and decreased susceptibility to fluconazole, respectively, with the Δ mutant displaying significantly lower basal expression levels of the and genes. These data underscore a hitherto unknown role of histone methylation in modulating the most common azole antifungal resistance mechanism. Altogether, our findings establish a link between CgFpr-mediated histone homeostasis and gene expression and implicate CgFpr in the virulence of .