Functional characterization of the transcription programs underlying phase transition in the BSL3 pathogen Coccidioides immitis

Coccidioidomycosis (Valley Fever) is an emerging endemic fungal infection with a rising incidence and an expanding geographic range. It is caused by Coccidiodes, which are thermally dimorphic fungi that grow as mycelia in soil but transition in the lung to form pathogenic spherules. The regulatory mechanisms underlying this transition are not understood. Exploiting capped small (cs)RNA-seq, which identifies actively initiated stable and unstable transcripts and thereby detects acute changes in gene regulation with remarkable sensitivity, here we report the changes in architectural organization and key sequence features underlying phase transition of this highly pathogenic fungus. Spherule transition was accompanied by large-scale transcriptional reprogramming, functional changes in transcript isoforms, and a massive increase in promoter-distal transcription of ncRNAs. Analysis of spherule-activated regulatory elements revealed a motif predicted to recruit a WOPR family transcription factor, which are known regulators of virulence in other fungi. We identify CIMG_02671 as a C. immitis WOPR homologue and show that it activates transcription in a WOPR motif-dependent manner, suggesting it is an important regulator of pathogenic phase transition. Collectively, this also highlights csRNA-seq as a powerful means to identify transcriptional mechanisms that control pathogenesis. Decoding Gene Regulatory Features of C. immitis phase transition