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) hyp ersensitivity of a Saccharomyces cerevisiae mutant lacking the transcriptio n 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 a nd cycloheximide and in increased expression of PDR5, a gene coding for a d rug efflux transporter of the ATP-binding cassette superfamily and whose tr anscription is under the control of Pdr1p and Pdr3p. Deletion of PDR5 in th e pdr1 pdr3 strain completely abrogated the ability of the three FCR genes to confer FCZ resistance, demonstrating that PDR5 is required for FCR-media ted FCZ resistance in S. cerevisiae. The FCR1 gene encodes a putative 517-a mino-acid protein with an N-terminal Zn2C6-type zinc finger motif homologou s to that found in fungal zinc cluster proteins, including S. cerevisiae Pd r1p and Pdr3p. We have constructed a C. albicans CAI4-derived mutant strain carrying a homozygous deletion of the FCR1 gene and analyzed its ability t o grow in the presence of FCZ. We found that the fcr1 Delta/fcr1 Delta muta nt displays hyperresistance to FCZ and other antifungal drugs compared to t he parental CA14 strain. This hyperresistance could be reversed to wild-typ e levels by reintroduction of a plasmid-bone copy of FCR1 into the fcr1 Del ta/fcr1 Delta mutant. Taken together, our results indicate that the FCR1 ge ne behaves as a negative regulator of drug resistance in C. albicans and co nstitute the first evidence that FCZ resistance can result from the inactiv ation of a regulatory factor such as Fcr1p.