An allele of the yeast RPB7 gene, encoding an essential subunit of RNA polymerase II, reduces cellular resistance to the antitumor drug bleomycin

Bleomycin is an antitumor drug that kills cells by introducing lesions in D NA. Thus, normal cells exposed to bleomycin must rely on efficient DNA repa ir mechanisms to survive. In the yeast Saccharomyces cerevisiae, the transc riptional activator Imp2 is required to fend off the toxic effects of bleom ycin. However, it remains unclear whether Imp2 controls the expression of a protein that either repairs bleomycin-induced DNA lesions, or detoxifies t he drug, and or both. To gain further insight into the mechanisms by which yeast cells mount a response towards bleomycin, we began to sequentially ch aracterize the genetic defect in a collection of bleomycin-sensitive mutant s that were previously isolated by mini-Tn3 transposon mutagenesis. A rescu e plasmid designed to integrate at the site of the mini-Tn3 insertion was u sed to identify the defective gene in one of the mutant strains, HCY53, whi ch was not allelic to IMP2. We showed that in strain HCY53, the mini-Tn3 wa s inserted at the distal end of an essential gene RPB7, which encodes one o f the two subunits, Rpb4-Rbp7, that forms a subcomplex with RNA polymerase II. Since rpb7 null mutants are nonviable, it would appear that the rpb7::m ini-Tn3 allele produces a protein that retains partial biological function thus permitting cell viability, but which is unable to provide bleomycin re sistance to strain HCY53. The defective phenotype of strain HCY53 could be corrected by a plasmid bearing the entire RPB7 gene. Two dimensional gel an alysis revealed that the expression of several proteins were diminished or absent in the rpb7::mini-Tn3 mutant when challenged with bleomycin. These r esults are in accord with our previous report that bleomycin resistance in yeast is controlled at the transcriptional level.