Mechanisms of self-resistance in the echinocandin-producing fungus Pezicula radicicola NRRL12192

Development of antibiotic resistance is a growing problem. Understanding intrinsic resistance mechanisms of antibiotic-producing microbes to their own products can help predict paths to clinical resistance. The echinocandins are a class of antifungal lipopeptides targeting fungi via noncompetitive inhibition of the β-1,3-D-glucan synthase FKS1 subunit which causes fungal cell lysis. Reports of echinocandin resistance in pathogenic fungi have been associated with the drug’s use and have been linked to FKS1 mutations at the hypothetical binding target (hotspots) of echinocandins. While surveying echinocandin gene clusters, we found that the sporiofungin-producing fungus P. radicicola contained two FKS1 genes (psfks1n and psfks1a), rather than a typical single copy of FKS1 gene. psfks1a sits adjacent to the sporiofungin gene cluster and predicts a protein of 1,999 amino acids with 71% identity to the FKS1 of the echinocandin-producing fungus Glarea lozoyensis. Phylogenetic analysis of ascomycete FKS1 shows the FKS1 tree coincides with species phylogeny except for PSFKS1a which is only distantly related to Leotiomycete FKS1s. Wild type P. radicicola is resistant to echinocandins, but inactivation of psfks1a greatly increased echinocandin sensitivity suggesting that psfks1a is involved in echinocandin self-resistance during sporiofungin biosynthesis. Alignment of FKS1 proteins shows that the hotspot regions in PSFKS1a differ significantly from those in Candida glabrata and Saccharomyces cerevisiae, including the amino acids known to be related to echinocandin susceptibility. This is the first report of a filamentous fungus with an auxiliary FKS1 gene and the first report of a naturally occurring self-resistance gene for the echinocandins.