Screening for effectors that modify multidrug resistance in yeast.

The yeast transcription factors Pdr1p and Pdr3p regulate the expression of several genes that encode energy-dependent efflux pumps involved in multidrug resistance. They recognize specific pleiotropic drug resistance elements in the promoters of the target gene such as PDR5 coding for a major multidrug transporter. Gain-of-function mutations in Pdr1p/Pdr3p result in over-expression of transporter genes and establishment of multidrug resistance. We developed a novel yeast-based screening procedure designed to detect compounds that specifically modify multidrug resistance due to an interference with the expression of drug efflux transporter genes. The screening is based on the ability to abrogate the growth defect of cells suffering from the galactose induced Pdr3p driven over-expression of a dominant-lethal allele of the PMA1 gene placed under the control of the PDR5 promoter. Validation of the assay was achieved by showing that growth inhibition was relieved by mutant Pdr3p devoid of activation domain. This screening system may also be used to select the loss-of-function pdr3 (or pdr1) mutants and to identify specific gene(s) whose over-expression or deletion will suppress the expression of multidrug transporters and increase the susceptibility of yeast cells to antifungals.