α -Amanitin-resistant Mutants of Mammalian Cells and the Regulation of RNA Polymerase II Activity

INTRODUCTION Eukaryotic cells are characterized by the presence of multiple forms of DNA-dependent RNA polymerase, which differ in their chromotographic and catalytic properties (Roeder and Rutter 1969Roeder and Rutter 1970a) as well as in their subunit structures (Gissinger and Chambon 1972; Kedinger and Chambon 1972; Sklar, Schwartz and Roeder 1975) and intracellular localizations (Roeder and Rutter 1970b). The various forms of the enzyme appear to be responsible for the synthesis of different classes of cellular RNA, form I catalyzing the synthesis of ribosomal RNA (Blatti et al. 1971; Reeder and Roeder 1972), form II, heterogeneous nuclear RNA, the precursor to cytoplasmic messenger RNA (Zylber and Penman 1971), and form III, 5S ribosomal and transfer RNA (Weinmann and Roeder 1974). The multiple forms are most easily distinguished by their sensitivity to inhibition by the Amanita phalloides toxin, α -amanitin. RNA polymerase I is completely resistant to inhibition by this bicyclic octapeptide, whereas polymerases II (Lindell et al. 1970; Kedinger et al. 1970) and III (Weinmann and Roeder 1974) are inhibited by low (≥0.1 μ g/ml) and high (≥ 100 μ g/ml) concentrations of α -amanitin, respectively. Elucidation of the role that these RNA polymerases play in the regulation of gene transcription in vivo would be facilitated considerably if strains of cells with modified enzymes could be obtained. Since α -amanitin in low concentrations specifically inhibits the activity of RNA polymerase II and since it is toxic to eukaryotic cells, the selection of strains resistant to the drug theoretically provides an opportunity to obtain cell lines with altered polymerase...