Expression of 1,3-β-glucan synthase subunits in is regulated by the cell cycle and growth conditions and at both transcriptional and post-transcriptional levels.

Fungal cell wall-synthesizing enzyme 1,3-β-glucan synthase (GS) is the target of the echinocandins, a frontline antifungal drug class. However, increasing echinocandin resistance due to mutations in GS has been observed in certain fungal pathogens, notably , where GS is encoded by two homologous genes, and . Despite the importance of GS in the fungal life cycle and as a drug target, the regulation of its expression in culture and in the host is still poorly understood. In this study, we used a fluorescent transcriptional reporter, quantitative reverse-transcriptase PCR, protein analysis, and mining of RNA-seq data sets to examine the regulation of GS expression. We determined that and promoter activities peak during S-phase and that during exponential growth in culture, is expressed at higher levels than . Interestingly, although 2 mRNA expression appeared to be strongly induced in an mutant, this calcineurin-mediated induction was not accompanied by increased promoter activity, suggesting post-transcriptional regulation. Examination of transcript across the ORF as well as Fks2 protein levels was consistent with post-transcriptional regulation. Finally, RNA-seq data mining revealed that, in contrast to vegetative growth, during the stationary phase and under host conditions, is expressed at equivalent or higher levels than . Together, these experiments revealed that in the expression of both GS subunits is regulated transcriptionally by the cell cycle, that expression is regulated post-transcriptionally by calcineurin and , and that in the host, may be the more abundant subunit.