Analysis of second-site mutations that suppress the multiple drug resistance phenotype of the yeast PDR1-7 allele

The yeast PDR1 locus encodes a member of the C6 zinc cluster family of transcriptional regulatory proteins. Among the targets of PDR1 is the yeast PDR5 locus. The product of this gene is a member of the ATP-binding cassette (ABC) transmembrane protein family and plays a major role in inhibitor efflux. Mutations in PDR1 affect the relative level of PDR5 transcript and can therefore result in increased or decreased drug resistance. We isolated three second-site suppressors of a PDR1-7 semidominant hyper-resistant mutation. These mutants were drug hypersensitive, as compared with isogenic controls. Two of the three mutations contained alterations in a putative DNA-binding domain. Significantly, the mutant proteins exhibited reduced DNA-binding capacity.