Histoplasma capsulatum, which is a respiratory fungal pathogen of humans, is endemic in the United States. Depending on the exposure dose and the immune status of the host, the infection can lead to mild respiratory or life-threatening and systemic disease. H. capsulatum has a dimorphic life cycle, switching from an infectious filamentous form in the soil to a pathogenic yeast form in mammalian hosts. This morphological switch, which requires a dramatic shift in the gene expression profile of the cells, can be easily recapitulated in the laboratory simply by changing the temperature from roomtemperature to 37°C. We previously identified three regulators, Ryp1, Ryp2 and Ryp3, which are required for the yeast-phase growth. ryp1, ryp2 and ryp3 mutants are unable to respond to change in temperature and grow constitutively in the filamentous form even at 37°C. Ryp1 belongs to a conserved family of fungal proteins that regulate cellular differentiation in response to environmental signals. The best- studied member of this family of proteins is Wor1, which is a master regulator of white -to-opaque switching in Candida albicans. Ryp2 and Ryp3 are orthologous to VosA and VelB, respectively, which are developmental regulators in Aspergillus nidulans. In this study, using transcriptional profiling and chromatin immunoprecipitation (ChIP) experiments, we explored complementary and unique roles of Ryp1, Ryp2, and Ryp3 in regulating yeast-phase growth. Our results reveal that Ryp1, Ryp2 and Ryp3 physically interact and associate with DNA throughout the genome. Additionally, we identified a fourth transcription factor, Ryp4, which is a direct target of Ryp1, Ryp2 and Ryp3, as a novel regulator of yeast-phase growth in H. capsulatum. Further transcriptional profiling and ChIP experiments show that Ryp4 regulates and associates with the upstream regions of a subset of Ryp1, Ryp2, and Ryp3 targets, which are involved in morphology and virulence in H. capsulatum. Finally, we identified two distinct cis- regulatory elements that are utilized by Ryp1 or the Ryp2/Ryp3 complex to facilitate gene expression. Our results reveal a tightly regulated and interwoven transcriptional network that controls the ability of a pathogenic fungus to cause disease in response to host temperature.

http://www.fungalgenetics.org/2013/documents/FungalProgramBook2013.pdf