Mechanistic Basis of pH-Dependent 5-Flucytosine Resistance in Aspergillus fumigatus

Abstract

The antifungal drug 5-flucytosine (5FC), a derivative of the nucleobase cytosine, is licensed for the treatment of fungal diseases; however, it is rarely used as a monotherapeutic to treat <i>Aspergillus</i> infection. Despite being potent against other fungal pathogens, 5FC has limited activity against <i>Aspergillus fumigatus</i> when standard <i>in vitro</i> assays are used to determine susceptibility. However, in modified <i>in vitro</i> assays where the pH is set to pH 5, the activity of 5FC increases significantly. Here we provide evidence that <i>fcyB</i>, a gene that encodes a purine-cytosine permease orthologous to known 5FC importers, is downregulated at pH 7 and is the primary factor responsible for the low efficacy of 5FC at pH 7. We also uncover two transcriptional regulators that are responsible for the repression of <i>fcyB</i> and, consequently, mediators of 5FC resistance, the CCAAT binding complex (CBC) and the pH regulatory protein PacC. We propose that the activity of 5FC might be enhanced by the perturbation of factors that repress <i>fcyB</i> expression, such as PacC or other components of the pH-sensing machinery.