TAC1 mediated gene expression in Candida albicans tlo mutants
Candida albicans is an opportunistic fungal pathogen commonly found on human mucosal surfaces. Azole antifungal resistance in C. albicans is of clinical concern and both oral and systemic Candida infections can be difficult to treat due to the lack of alternative antifungal drugs. Expression of a hyperactive form of the transcription factor Tac1 is a major contributor to azole resistance in C. albicans isolates resulting in the increased expression of the azole efflux pump Cdr1. In this study we investigated whether the Mediator tail component Med2, encoded by the expanded (n=14) TLO gene family of C. albicans was required for Tac1 activity. A homozygous TAC1-5 gain-of-function point mutation was introduced into wild-type (WT), tlo and med3 strains of C. albicans which enables them to express hyperactive Tac1. qRT-PCR analysis revealed that tlo-TAC1-5 had reduced basal and fluphenazine-induced CDR1 expression relative to WT-TAC1-5 strains and exhibited reduced levels of resistance to fluconazole and terbinafine. Individual copies of alpha, beta and gamma TLO genes were introduced into tlo-TAC1-5 to investigate their ability to restore Tac1-activated resistance. These studies show that alpha and beta TLO genes could restore fluconazole resistance in the tlo-TAC1-5 background whereas gamma family genes did not impart any phenotypic complementation. Transcript profiling showed that TLO1 increased the expression of TAC1-5 activated genes such as CDR1 and ChIP-qPCR revealed that Tlo1 co-localises to the drug response element (DRE) which is the site of Tac1 binding in the CDR1 promoter. These data confirm the requirement for TLO genes in the expression of TAC1-mediated fluconazole resistance.