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The yeast transcription factor Pdr1p regulates the expression of a number of genes, several of which encode ATP-driven transport proteins involved in multiple drug resistance. Among 20 genes containing binding consensus sequences for the transcription factor Pdr1p in their promoter, we studied more particularly the regulation and function of PDR16 (involved in phospholipid synthesis), TPO1 (involved in vacuolar transport of polyamines), YAL061W (homologous to polyol dehydrogenases) and YLR346C (unknown function). We found that the regulation of these four genes depends on Pdr1p, since promoter activities studied by lacZ fusion analysis and mRNA levels studied by Northern blotting analysis changed upon deletion or hyperactivation by the pdr1-3 mutant of this transcription factor. The drug sensitivity of the strains deleted for these genes revealed that TPO1, a gene previously found to be involved in spermidine resistance and vacuolar polyamine transport, is a determinant of multidrug transporter since it also mediates growth resistance to cycloheximide and quinidine. This resistance pattern overlapped with that of YOR273C, a homolog of TPO1. These two homologous transporters are thus bona fide members of the phylogenetic subfamily DHA1 (drug/proton antiport TC 2.A.1. 2) of the major facilitator superfamily. Both YOR273C and TPO1 as well as at least one other determinant involved in the yeast pleiotropic drug resistance network contribute to resistance to a quinoline-containing antimalarial drug.

The Saccharomyces cerevisiae open reading frame YNL231C was recently found to be controlled by the multiple drug resistance regulator Pdr1p. Here we characterize YNL231C (PDR16) and its homologue YNL264C (PDR17). Deletion of PDR16 resulted in hypersensitivity of yeast to azole inhibitors of ergosterol biosynthesis. While no increase in drug sensitivity was found upon deletion of PDR17 alone, a Deltapdr16,Deltapdr17 double mutant was hypersensitive to a broad range of drugs. Both mutations caused significant changes of the lipid composition of plasma membrane and total cell extracts. Deletion of PDR16 had pronounced effects on the sterol composition, whereas PDR17 deletion mainly affected the phospholipid composition. Thus, Pdr16p and Pdr17p may regulate yeast lipid synthesis like their distant homologue, Sec14p. The azole sensitivity of the PDR16-deleted strain may be the result of imbalanced ergosterol synthesis. Impaired plasma membrane barrier function resulting from a change in the lipid composition appears to cause the increased drug sensitivity of the double mutant strain Deltapdr16,Deltapdr17. The uptake rate of rhodamine-6-G into de-energized cells was shown to be almost 2-fold increased in a Deltapdr16,Deltapdr17 strain as compared with wild-type and Deltapdr5 strains. Collectively, our results indicate that PDR16 and PDR17 control levels of various lipids in various compartments of the cell and thereby provide a mechanism for multidrug resistance unrecognized so far.