Mediator Tail Module Is Required for Tac1-Activated CDR1 Expression and Azole Resistance in Candida albicans

The human fungal pathogen Candida albicans develops drug resistance after long-term exposure to azole drugs in the treatment of chronic candidiasis. Gain-of-function (GOF) mutations in the transcription factor Tac1 and the consequent expression of its targets, drug efflux pumps Cdr1 and Cdr2, are a common mechanism by which C. albicans acquires fluconazole resistance. The mechanism by which GOF mutations hyperactivate Tac1 is currently unknown. Here, we define a transcriptional activation domain (TAD) at the C terminus of Tac1. GOF mutations within the Tac1 TAD, outside the context of full-length Tac1, generally do not enhance its absolute potential as a transcriptional activator. Negative regulation of the Tac1 TAD by the Tac1 middle region is necessary for the activating effect of GOF mutations or fluphenazine to be realized. We have found that full-length Tac1, when hyperactivated by xenobiotics or GOF mutations, facilitates the recruitment of the Mediator coactivator complex to the CDR1 promoter. Azole resistance and the activation of Tac1 target genes, such as CDR1, are dependent on the Tac1 TAD and subunits of the Mediator tail module. The dependence of different Tac1 target promoters on the Mediator tail module, however, varies widely. Lastly, we show that hyperactivation of Tac1 is correlated with its Mediator-dependent phosphorylation, a potentially useful biomarker for Tac1 hyperactivation. The role of Mediator in events downstream of Tac1 hyperactivation in fluconazole-resistant clinical isolates is complex and provides opportunities and challenges for therapeutic intervention.