Functional analysis of specific promoter elements involved in the control of Candida albicans transcription
To examine the functional significance of these STRE-, GCRE- and YRE-like elements in C. albicans, a basal promoter construct, based on the C. albicans ADH1 promoter, was constructed. The Renilla reniformis luciferase gene (RLUC) was utilised as the reporter of choice in C albicans. Copies of each type of enhancer element were then introduced upstream of the basal ADH1b-RLUC fusion. Transcriptional activation by GCRE, STRE and two YRE was then studied under morphogenetic inducing conditions (serum and pH-inducing), and a range of stress conditions including temperature, amino acid starvation, glucose limitation, and exposure of H2O2, ethanol and heavy metal stresses. The functionality of each element was further tested in C. albans mutants lacking the corresponding transcription factor. The GCRE, STRE, YRE1 and YRE2 elements did not activate transcription in response to morphogenesis induced by serum or Lee’s media. The GCRE was shown to activate transcription in response to amino acid starvation in a Gcn4 dependent and Efg1 independent manner. Surprisingly the well-characterised STRE mediated general stress response of S. cerevisiae did not appear to be conserved in C. albicans. The STRE element did not appear to be functional as a stress responsive element in C. albicans under the range of stress conditions tested. However, Nrg1, a represser of morphogenesis in C. albicans was shown to act through a STRE-related element, the NRE element (MVCCCT). The hypha-specific gene ALS8 contains two Nrg1-responsive NREs (Murad et al. 2001). The fact that the core sequence of the NRE element is closely related to the STRE element raises the possibility of an evolutionary change of function for the STRE sequence in C. albicans. The functionality of the YRE1 element appears to have been conserved in C. albicans. As in S. cerevisiae, YRE1 mediates transcriptional responses to oxidative and heavy metal stresses in a Cap1-dependent manner.