Regulation of ALS8 and HYR1 genes during Candida albicans morphogenesis
Candida albicans is the major fungal pathogen of humans. A number of factors are thought to promote the virulence of C. albicans, including yeast-hypha morphogenesis. It is thought that a variety of environmental signals trigger the dimorphic transition via different signal transduction pathways. HYR1 and ALS8 are hypha-specific genes which must be targeted by the signalling pathways that control hyphal growth. These pathways must regulate the activity of specific transcriptional activators and/or repressors that control hypha-specific genes. Hence, the ALS8 and HYR1 hypha-specific promoters provide a means by which the regulatory mechanisms controlling hyphal development may be investigated. This study involves the in silico and functional dissection of the ALS8 and HYR1 promoters. Transcriptional regulation of a number of ALS8 promoter deletion constructs fused to the Renilla luciferase reporter, were investigated under three hypha-specific growth conditions. These in vivo functional analyses defined a region that is required for hyphal induction under serum-inducing conditions and pH-inducing conditions. However, subtle differences in transcriptional activation mediated by these regions were observed. A different pattern of regulation was observed from the same constructs during GlcNAc-induced morphogenesis. Specific studies on STRE elements in the ALS8 promoter showed that Nrg1, a hypha-specific repressor, does not mediate repression via a single STRE element within the promoter. However, Nrg1 does repress ALS8 expression in yeast growth conditions. Similarly, Efg1 does not activate hypha-specific transcription via an E-box on the ALS8 promoter, although it has been shown by others to activate ALS8 transcription. Initial functional analyses of the HYR1 promoter indicate a specific region of the promoter is required for serum-induced transcriptional regulation. The results of this study support the hypothesis that different morphological regulatory pathways are required under different conditions, and that these pathways make different contributions to the regulation of morphogenetically regulated genes.