Isolation of a putative Candida albicans transcriptional regulator involved in pleiotropic drug resistance by functional complementation of a pdr1 pdr3 mutation in Saccharomyces cerevisiae

Three Candida albicans genes, designated FCR (for fluconazole resistance), have been isolated by their ability to complement the fluconazole (FCZ) hypersensitivity of a Saccharomyces cerevisiae mutant lacking the transcription factors Pdr1p and Pdr3p. Overexpression of any of the three FCR genes in the pdr1 pdr3 mutant resulted in increased resistance of the cells to FCZ and cycloheximide and in increased expression of PDR5, a gene coding for a drug efflux transporter of the ATP-binding cassette superfamily and whose transcription is under the control of Pdr1p and Pdr3p. Deletion of PDR5 in the pdr1 pdr3 strain completely abrogated the ability of the three FCR genes to confer FCZ resistance, demonstrating that PDR5 is required for FCR-mediated FCZ resistance in S. cerevisiae. The FCR1 gene encodes a putative 517-amino-acid protein with an N-terminal Zn2C6-type zinc finger motif homologous to that found in fungal zinc cluster proteins, including S. cerevisiae Pdr1p and Pdr3p. We have constructed a C. albicans CAI4-derived mutant strain carrying a homozygous deletion of the FCR1 gene and analyzed its ability to grow in the presence of FCZ. We found that the fcr1 Delta/fcr1 Delta mutant displays hyperresistance to FCZ and other antifungal drugs compared to the parental CA14 strain. This hyperresistance could be reversed to wild-type levels by reintroduction of a plasmid-bone copy of FCR1 into the fcr1 Delta/fcr1 Delta mutant. Taken together, our results indicate that the FCR1 gene behaves as a negative regulator of drug resistance in C. albicans and constitute the first evidence that FCZ resistance can result from the inactivation of a regulatory factor such as Fcr1p.