SCT1 MUTANTS SUPPRESS THE CAMPTOTHECIN SENSITIVITY OF YEAST-CELLS EXPRESSING WILD-TYPE DNA TOPOISOMERASE-I

Camptothecin is a potent antineoplastic agent that interferes with the action of eukaryotic DNA topoisomerase I; the covalent enzyme-DNA int ermediate is reversibly stabilized, leading to G(2) arrest and cell de ath, We used a genetic screen to identify cellular factors, other than DNA topoisomerase I, that participate in the process of camptothecin- induced cell death, Following ethyl methanesulfonate mutagenesis of to p 1 Delta yeast cells expressing plasmid-borne wild-type DNA topoisome rase I, six dominant suppressors of camptothecin toxicity were isolate d that define a single genetic locus, sct1. Mutant SCT1 cells expresse d DNA topoisomerase I protein of similar specific activity and camptot hecin sensitivity to that of congenic, drug-sensitive sct1 cells, yet were resistant to camptothecin-mediated lethality. Moreover, camptothe cin-treated SCT1 cells did not exhibit the G(2)-arrested, terminal phe notype characteristic of drug-treated wild-type cells, SCT1 cell sensi tivity to other DNA-damaging agents suggests that alterations in SCT1 function suppress camptothecin-induced DNA damage produced in the pres ence of yeast DNA topoisomerase I.