Phenotypic switching between two opposing cellular states is a fundamental aspect of biology, and fungi provide facile systems to analyze the interactions between regulons that control this type of switch. A long-standing mystery in fungal pathogens of humans is how thermally dimorphic fungi switch their developmental form in response to temperature. These fungi, including the subject of this study, Histoplasma capsulatum, are temperature-responsive organisms that utilize unknown regulatory pathways to couple their cell shape and associated attributes to the temperature of their environment. H. capsulatum grows as a multicellular hypha in the soil that switches to a pathogenic yeast form in response to the temperature of a mammalian host. These states can be triggered in the laboratory simply by growing the fungus either at room temperature (where it grows as hyphae) or at 37ºC (where it grows as yeast). Prior worked revealed that 15-20% of transcripts are differentially expressed in response to temperature, but it is unclear which transcripts are linked to specific phenotypic changes such as cell morphology or virulence. To elucidate temperature-responsive regulons, we previously identified four transcription factors (Ryp1-4) that are required for yeast-phase growth at 37ºC; in each ryp mutant, the fungus grows constitutively as hyphae regardless of temperature and the cells fail to express genes that are normally induced in response to growth at 37ºC. Here we perform the first genetic screen to identify genes required for hyphal growth of H. capsulatum at room temperature and find that disruption of the signaling mucin MSB2 results in a yeast-locked phenotype. RNAseq experiments reveal that MSB2 is not required for the majority of gene expression changes that occur when cells are shifted to room temperature. However, a small subset of temperature-responsive genes is dependent on MSB2 for its expression, thereby implicating these genes in the process of filamentation. Disruption or knockdown of an Msb2-dependent MAP kinase (HOG2) and an APSES transcription factor (STU1) prevents hyphal growth at room temperature, validating that the Msb2 regulon contains genes that control filamentation. Notably, the Msb2 regulon shows conserved hyphal-specific expression in other dimorphic fungi, suggesting that this work defines a small set of genes that are likely to be conserved regulators and effectors of filamentation in multiple fungi. In contrast, a few yeast-specific transcripts, including virulence factors that are normally expressed only at 37ºC, are inappropriately expressed at room temperature in the msb2 mutant, suggesting that expression of these genes is coupled to growth in the yeast form rather than to temperature. Finally, we find that the yeast-promoting transcription factor Ryp3 associates with the MSB2 promoter and inhibits MSB2 transcript expression at 37ºC, whereas Msb2 inhibits accumulation of Ryp transcripts and proteins at room temperature. These findings indicate that the Ryp and Msb2 circuits antagonize each other in a temperature-dependent manner, thereby allowing temperature to govern cell shape and gene expression in this ubiquitous fungal pathogen of humans.
