MULTIPLE-DRUG RESISTANCE IN YEAST AND ITS EFFECT ON TRANSFORMANT STABILITY AND KARYOTYPE

Saccharomyces cerevisiae grown in a medium with a subcritical concentr ation of one antibiotic (chloramphenicol) acquired multiple drug resis tance. The latter could be lost on nonselective medium, or become irre versible after several passages on the antibiotic-containing medium. T his trait, called Mdt (medium-dependent tolerance), coincided with the phenotype caused by mutations in the PDR1 gene. Genetic data testifie d to the linkage of MDT with PRD1. DNA-mRNA hybridization revealed enh anced PDR1 expression in Mdt(+) cells. Analysis of the Mdt(+) transfor mants demonstrated specific genomic instability.