Chemical stimuli override a temperature-dependent morphological program by reprogramming the transcriptome of a fungal pathogen

The human fungal pathogen Histoplasma changes its morphology in response to temperature. At 37C it grows as a budding yeast whereas at room temperature it transitions to hyphal growth. Prior work has demonstrated that 15-20% of transcripts are temperature-regulated, and that transcription factors (TFs) Ryp1-4 are necessary to establish yeast growth. However, little is known about transcriptional regulators of the hyphal program. To identify TFs that regulate filamentation, we utilize chemical inducers of hyphal growth. We show that addition of cAMP analogs or an inhibitor of cAMP breakdown overrides yeast morphology, yielding inappropriate hyphal growth at 37C. Additionally, butyrate supplementation triggers hyphal growth at 37C. Transcriptional profiling of cultures filamenting in response to cAMP or butyrate reveals that a limited set of genes respond to cAMP while butyrate dysregulates a larger set. Comparison of these profiles to previous temperature- or morphology-regulated gene sets identifies a small set of morphology-specific transcripts, including 9 TFs. We characterized three TFs, STU1, FBC1, and PAC2, whose orthologs regulate development in other fungi. We found that while only FBC1 is necessary for room-temperature (RT) induced filamentation, each of these TFs is required for other aspects of RT development. Additionally, FBC1 and PAC2, but not STU1, are necessary for filamentation in response to cAMP at 37C. Ectopic expression of each of these TFs is sufficient to induce filamentation at 37C and PAC2 induction of filamentation at 37C is dependent on STU1. Taken together, this work identifies filamentation-promoting TFs that form a regulatory circuit to promote the hyphal program.