Genetic interaction analysis of Candida glabrata transcription factors CST6 and UPC2A in the regulation of respiration and fluconazole susceptibility.

Candida glabrata is the second most common cause of invasive candidiasis and is widely known to have reduced susceptibility to fluconazole relative to many other Candida spp. Upc2A is a transcription factor that regulates ergosterol biosynthesis gene expression under conditions of sterol stress such as azole drug treatment or hypoxia. Through an in vitro microevolution experiment, we found that loss-of-function mutants of the ATF/CREB transcription factor CST6 suppresses the fluconazole hyper-susceptibility of the upc2A∆ mutant. Here, we confirm that the cst6∆ upc2A∆ mutants are resistant to fluconazole but not to hypoxia relative to the upc2A∆ mutant. Sterol analysis of these mutants indicates that this suppression phenotype is not due to restoration of ergosterol levels in the cst6∆ upc2A∆ mutant. Furthermore, increased expression of CDR1, the efflux pump implicated in the vast majority of azole-resistant C. glabrata strains, does not account for the suppression phenotype. Instead, our data suggest that this effect is due in part to increased expression of the adhesin EPA3, which has been shown by others to reduce fluconazole susceptibility in C. glabrata. In addition, we find that loss of both UPC2A and CST6 reduces the expression of mitochondrial and respiratory genes and that this also contributes to the suppression phenotype as well as to the resistance of cst6∆ to fluconazole. These latter data further emphasize the connection between mitochondrial function and azole susceptibility.