Candida glabrata PDR1 , a Transcriptional Regulator of a Pleiotropic Drug Resistance Network, Mediates Azole Resistance in Clinical Isolates and Petite Mutants

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Abstract
Candida glabrata , a yeast with intrinsically low susceptibility to azoles, frequently develops increased azole resistance during prolonged treatment. Transposon mutagenesis revealed that disruption of Cg PDR1 resulted in an 8- to 16-fold increase in fluconazole susceptibility of C. glabrata . Cg PDR1 is a homolog of Saccharomyces cerevisiae PDR1 , which encodes a transcriptional regulator of multidrug transporters. Northern blot analyses indicated that Cg PDR1 regulated both constitutive and drug-induced expression of Cg CDR1 , a multidrug transporter gene. In agreement with the Northern analysis, the Cg pdr1 mutant had increased rhodamine accumulation, in contrast to the decreased accumulation in the Cg PDR1 -overexpressing strain. Northern analyses also indicated the importance of Cg PDR1 in fluconazole resistance arising during therapy. Two clinically resistant isolates had higher expression of Cg PDR1 and Cg CDR1 compared to their paired susceptible isolates. Integrative transformation of Cg PDR1 from the two resistant isolates converted the Cg pdr1 mutant into azole-resistant strains with upregulated Cg PDR1 expression. Two different amino acid substitutions, W297S in one isolate and F575L in the other, accounted for the upregulated Cg PDR1 expression and the resistance. Finally, Cg PDR1 was shown to be required for the azole resistance due to mitochondrial deficiency. Thus, Cg PDR1 encodes a transcriptional regulator of a pleiotropic drug resistance network and contributes to the azole resistance of clinical isolates and petite mutants.

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