Regulation of Antifungal Drug Resistance by CZT-1 and TAH-3

Fungal infections kill over a million people every year and lead to dramatic losses in food production. Available treatments are complicated by drug resistance. In the model Neurospora crassa, the Zn2Cys6 transcription factor czt-1, which regulates ATP-binding cassette transporter genes, confers resistance to the cell death inducer staurosporine. Natural genetic variants of czt-1 are associated with the expression of another Zn2Cys6 transcription factor, tah-3, which may also contribute to drug resistance. We tested the susceptibility of Δczt-1, Δtah-3, and Δczt-1Δtah-3 strains to a panel of clinically-relevant drugs. While ∆czt-1 showed a differential phenotype for staurosporine only, ∆tah-3 resulted in increased sensitivity to echinocandins and azoles. The Δczt-1Δtah-3 double mutant showed an identical phenotype to the most sensitive of the two individual parental mutants. A point mutation allele of czt-1 (L680F) has previously been suggested to cause increased drug resistance. CZT-1(L680F) was tagged with GFP and observed to be present in a different localization pattern and in higher amounts intracellularly when compared to CZT-1(wild type), suggesting that the L>F mutation could lead to positive feedback for CZT-1 expression. 3D structure predictions suggested that the L680 residue is at the end of a long α-helix, which may be involved in interactions with elements of the cellular environment. This seemingly important residue is highly conserved amongst CZT-1 orthologs, which are present as expanded sets of paralogs in a number of fungal species. In summary, our work has helped to further understand the role of CZT-1 and TAH-3 during the response to antifungal drugs.