Temperature-driven isoform switching reprograms developmental gene expression in a human fungal pathogen

Post-transcriptional regulation is key to development, and yet little is known about how RNA isoform choice contributes to developmental choices in fungi. Here we assemble the first isoform-level transcriptome of Histoplasma, a ubiquitous human fungal pathogen that grows as an infectious environmental form (hyphae) or a pathogenic host form (yeast) in response to temperature. We find extensive morphology-associated longer leader and trailer isoforms, rapid temperature-driven transcription start site remodeling, and inclusion of regulatory elements in longer leaders. We observe isoform-specific patterns of ribosome and polysome association, indicating that isoform switching regulates the proteome. Our results suggest a model in which isoform diversity and rapid isoform switching are central to post-transcriptional regulation during thermal dimorphism, enabling precise and timely translation of factors needed to establish and maintain hyphal and yeast forms. These studies illuminate how eukaryotic systems utilize post- transcriptional regulation to determine developmental states in response to external stimuli.