UCSF

Dimorphic fungi are temperature-sensitive organisms that couple their cell shape with their environment. One of these fungi, Histoplasma capsulatum, exists as both a soil-dwelling hypha and a host-associated yeast. Here I examine the genetic elements required for hyphal growth. Insertional mutagenesis identified a mutant that is disrupted in the promoter region of the previously uncharacterized gene MSB2, whose ortholog in the model yeast Saccharomyces cerevisiae is a known signaling component in the high osmolarity glycerol (HOG) pathway and filamentous growth (FG) pathway. Through genetic analysis I confirmed MSB2 is necessary and sufficient for filamentation in H. capsulatum. To identify elements of the transcriptional program dependent on MSB2, I took both a targeted and global approach. Previous work has established 4 transcription factors, RYP1-4, as regulators of yeast phase growth and probing for their expression here shows that MSB2 is required for appropriate RYP expression at 37°C. Additionally, RNA-seq provided insight on a small number of genes affected by MSB2 expression. This analysis identified STU1, an APSES transcription factor, as dependent on MSB2 expression. It was previously shown that STU1 overexpression is sufficient for filamentation in H. capsulatum at 37°C. To identify signaling elements between the transmembrane protein MSB2 and the transcription factor STU1, I looked at the 4 MAP kinases in H. capsulatum and in the RNA-seq data found that one, called HOG2, is dependent on MSB2 expression. I conclude with showing HOG2 is required for filamentation and for appropriate expression of STU1 at room temperature. This genetic analysis outlines a pathway for regulation of filamentation in H. capsulatum through MSB2, HOG2, and STU1.