We have found that, in the yeast Saccharomyces cerevisiae, overexpression of the DNA helicase Ssl2p confers resistance to adriamycin. Ssl2p is involved, as a subunit of the basic transcription factor TFIIH, in the initiation of transcription and in nucleotide-excision repair (NER), and this helicase is essential for the survival of yeast cells. An examination of the relationship between the known functions of Ssl2p and adriamycin resistance indicated that overexpression of Ssl2p caused little or no increase in the rate of RNA synthesis and in NER. The absence of any involvement of NER in adriamycin resistance was supported by the finding that yeast cells that overexpressed the mutant form of Ssl2p that lacked the carboxy-terminal region, which is necessary for NER, remained resistant to adriamycin. When we examined the effects of overexpression in yeast of other mutant forms of Ssl2p with various deletions, we found that, of the 843 amino acids of Ssl2p, the entire amino acid sequence from position 81 to position 750 was necessary for adriamycin resistance. This region is identical to the region of Ssl2p that is necessary for the survival of yeast cells. Although this region contains helicase motifs, the overexpression of other yeast helicases, such as Rad3 and Sgs1, had little or no effect on adriamycin resistance, indicating that a mere ncrease in the intracellular level of helicases does not result in adriamycin resistance. Our results suggest that the functions of Ssl2p that are essential for yeast survival are also required for protection against adriamycin toxicity.
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