Ahr1-dependent regulation of ECE1 and other core filamentation response genes in Candida albicans

Candida albicans is a normally harmless commensal fungus and part of the human microbiota. However, in immunocompromised patients, C. albicans can cause diseases ranging from superficial infections to life threatening systemic infections. A major virulence trait is the fungus ability to undergo reversible yeast-to-hyphae transition depending on environment. The up-regulation of hyphae-specific genes is tightly linked to this morphological transition. Several of these genes are also virulence-associated, like ALS3 and ECE1. ECE1 is one of the most highly expressed genes in hyphae and encodes for a precursor protein of the peptide toxin Candidalysin which is crucial for host cell cytolysis during infection. During a screen of transcription factor deletion mutants, Ahr1 was identified to be important for the high-level expression of ECE1 in C. albicans hyphae. A mutant lacking Ahr1 is characterized by intermediate ECE1 transcription, low Cansisalysin secretion and attenuated virulence in an oral epithelial cell infection model. In contrast, a hyperactive AHR1 allele induces high levels of ECE1 expression even under yeast growth conditions. A similar regulatory pattern could be observed for the hypha-specific ALS3 gene, which encodes for an important adhesin and invasin. Chromatin immunoprecipitation DNA sequencing proved that a hyperactive Ahr1 binds upstream of ECE1, ALS3, and other hyphae-associated genes. In a partially Ahr1-dependent process, overexpression of MCM1, which encodes an essential transcription factor and binding partner of Ahr1, led to the upregulation of ECE1 and ALS3. Interestingly, this study found that Tup1, which is normally a global repressor of hyphal development and associated gene expression, is required for the beneficial effects of the hyperactive AHR1 allele as well as of MCM1 overexpression. This indicates that Tup1 does not only work as a repressor but is also required for high-level expression of ECE1 and ALS3 mediated by Ahr1 and Mcm1.