Strains and primers used in this study.

Drug resistance is a critical challenge in treating life-threatening fungal infections. Here, we uncover a mechanism of acquired azole resistance in Candida albicans through mutations in CAP1, encoding a conserved fungal transcription factor that mediates the oxidative stress response. We analyzed 300 clinical isolates and identified 25 distinct CAP1 missense or nonsense mutations, with many occurring within the DNA-binding domain. We identified two nearly identical CAP1 heterozygous nonsense mutations, one in an isolate obtained from a bloodstream infection and one in a population of cells undergoing adaptation to fluconazole in vitro. Both CAP1 nonsense mutations resulted in loss of the C-terminal nuclear export signal, leading to nuclear retention of Cap1 and subsequent activation of genes associated with the oxidative stress response and drug transport. The CAP1 C-terminal truncations conferred significant fitness advantages in the presence of fluconazole, both in vitro and in a murine model of candidiasis. Strikingly, we discovered a therapeutic vulnerability: azole concentrations above the minimal inhibitory concentration were fungicidal to mutants with the CAP1 C-terminal truncation. The fungicidal effect was attributed to both elevated azole-induced reactive oxygen species and a compromised oxidative stress response in Cap1-truncated cells. Our results provide novel characterization of de novo CAP1 point mutations emerging in both laboratory and clinical contexts, elucidate the mechanisms underlying Cap1-regulated stress responses, and reveal a potential therapeutic target for overcoming drug resistance in C. albicans infections.