Differential Regulation of Echinocandin Targets Fks1 and Fks2 inCandida glabrataby the Post-Transcriptional Regulator Ssd1

Invasive infections caused by the opportunistic pathogenCandida glabrataare treated with echinocandin antifungals that target beta-1,3-glucan synthase, an enzyme critical for fungal cell wall biosynthesis. Echinocandin resistance develops upon mutation of genes (FKS1orFKS2) that encode the glucan synthase catalytic subunits. We have analyzed cellular factors that influence echinocandin susceptibility and here describe effects of the post-transcriptional regulator Ssd1, which inS. cerevisiae, can bind cell wall related gene transcripts. TheSSD1homolog inC. glabratawas disrupted in isogenic wild type and equivalentFKS1andFKS2mutant strains that demonstrate echinocandin resistance (MICs > 0.5 mu g/mL). A reversal of resistance (8- to 128-fold decrease in MICs) was observed inFKS1mutants, but not inFKS2mutants, followingSSD1deletion. Additionally, this phenotype was complemented upon expression ofSSD1from plasmid (pSSD1). AllSSD1disruptants displayed susceptibility to the calcineurin inhibitor FK506, similar tofks1 increment . Decreases in relative gene expression ratios ofFKS1toFKS2(2.6- to 4.5-fold) and in protein ratios of Fks1 to Fks2 (2.7- and 8.4-fold) were observed inFKSmutants uponSSD1disruption. Additionally, a complementary increase in protein ratio was observed in the pSSD1expressing strain. Overall, we describe a cellular factor that influences Fks1-specific mediated resistance and demonstrates further differential regulation ofFKS1andFKS2inC. glabrata.