CAMPTOTHECIN SENSITIVITY IS MEDIATED BY THE PLEIOTROPIC DRUG-RESISTANCE NETWORK IN YEAST

The antineoplastic alkaloid camptothecin interferes with the catalytic cycle of DNA topoisomerase I rendering it a cellular poison, Camptoth ecin stabilizes a covalent enzyme-DNA intermediate that is converted i nto lethal double strand DNA lesions during S phase of the cell cycle. Yeast SCT1 mutants were isolated in a screen for mutations in genes o ther than TOP1 that result in camptothecin resistance. Here we report SCT1 is allelic to PDR1 and that a Thr-879 to Met substitution in the PDR1-101 transcription factor confers multiple drug resistance. PDR1 r egulates the expression of several gene products including the ATP-bin ding cassette transmembrane transport proteins PDR5, YOR1, and SNQ2. T he PDR1 T879M mutant increased PDR5 transcription compared with wild-t ype PDR1 strains, Deletion of PDR1 or the downstream effector SNQ2 inc reased cell sensitivity to camptothecin, whereas deletion of YOR1 or P DR5 had little effect on camptothecin sensitivity, However, the campto thecin resistance accompanying GAL1-promoted overexpression of PDR5 su ggests some substrate promiscuity among the ATP-binding cassette trans porters. These data underscore the role of the pleiotropic drug resist ance network in regulating camptothecin toxicity and are consistent wi th a model of decreased intracellular concentrations of camptothecin r esulting from the increased expression of the SNQ2 transporter.