Expression of 1,3-β-glucan synthase subunits in <i>Candida glabrata</i> 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 <i>Candida glabrata</i>, where GS is encoded by two homologous genes, <i>FKS1</i> and <i>FKS2</i>. 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 <i>C. glabrata</i> GS expression. We determined that <i>FKS1</i> and <i>FKS2</i> promoter activities peak during S-phase and that during exponential growth in culture, <i>FKS1</i> is expressed at higher levels than <i>FKS2</i>. Interestingly, although <i>FKS</i>2 mRNA expression appeared to be strongly induced in an <i>fks1∆</i> mutant, this calcineurin-mediated induction was not accompanied by increased <i>FKS2</i> promoter activity, suggesting post-transcriptional regulation. Examination of <i>FKS2</i> 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, <i>FKS2</i> is expressed at equivalent or higher levels than <i>FKS1</i>. Together, these experiments revealed that in <i>C. glabrata,</i> the expression of both GS subunits is regulated transcriptionally by the cell cycle, that <i>FKS2</i> expression is regulated post-transcriptionally by calcineurin and <i>fks1∆</i>, and that in the host, <i>FKS2</i> may be the more abundant subunit.