Kim Setiawan    email
Institute for Molecular Infection Biology

Supervisor:
Prof. Dr. Joachim Morschhäuser (Würzburg)

Promotion Committee:
Prof. Dr. Joachim Morschhäuser (Würzburg)
Dr. J. Christian Pérez (Würzburg)
PD Dr. Brooke Morriswood (Würzburg)
Prof. Dr. Christian Janzen (Würzburg)

 

 

Generation of novel gene regulatory networks in the pathogenic yeast Candida albicans by hybrid transcription factors

The regulation of gene expression is crucial for the ability of cells to adapt to changes in their environment. Transcription factors that activate or repress gene expression in response to specific signals play a central role in coordinating appropriate cellular behaviors. The target genes that are controlled by transcription factors and the signals to which they react can change over evolutionary timescales. Such changes in transcriptional networks alter the properties of organisms and are an intrinsic part of species development. The zinc cluster transcription factors are a family of transcriptional regulators that are unique to the fungal kingdom. The pathogenic yeast Candida albicans possesses 82 zinc cluster transcription factors, which regulate diverse processes that are important for the successful establishment of the fungus in different host niches. One of them, Upc2, induces the expression of ergosterol biosynthesis genes in response to ergosterol depletion, which occurs under hypoxic conditions or in the presence of antifungal drugs that inhibit ergosterol biosynthesis. Gain-of-function mutations in Upc2 that result in hyperactivity of the transcription factor are a common cause of increased resistance of clinical C. albicans isolates to the widely used antifungal drug fluconazole. Activating mutations in two other zinc cluster transcription factors, Mrr1 and Tac1, result in the constitutive overexpression of multidrug efflux pumps and are another frequent cause of drug resistance. Mrr1 and Tac1 do not activate their target genes in response to fluconazole, such that the efflux pumps are expressed only at low basal levels in wild-type C. albicans strains. However, substitution of the DNA-binding domain of Mrr1 or Tac1 for the Upc2 DNA-binding domain generated hybrid transcription factors that induced the expression of the respective multidrug efflux pumps in the presence of fluconazole and conferred strongly increased drug resistance upon the cells. This observation demonstrated that the acquisition of a novel set of target genes by a transcription factor and their activation in response to a normally non-inducing signal can confer an advantageous altered phenotype. The aim of my thesis is to investigate in a systematic approach if the gain of specific sets of new target genes by a C. albicans transcription factor confers a selective advantage under certain stress conditions. For this purpose, I will generate a library of strains that in addition to wild-type Upc2 contain hybrid transcription factors in which the Upc2 DNA-binding domain is replaced by those of all other zinc cluster transcription factors. Mixtures of these strains will be propagated under various conditions in which Upc2 is activated (e.g., hypoxic growth conditions in vitro, but also during experimental infections) to isolate clones that have a competitive advantage over the other strains. Transcriptional profiling experiments will reveal the target genes that are additionally activated by the hybrid transcription factor and provide insight into the underlying mechanism. The studies may uncover some of the adaptive potential of rewiring transcriptional networks.