Human diploid fibroblast mutants with altered RNA polymerase II

Abstract
Clones resistant to the cytotoxic action of α-amanitin have been isolated from a strain of fetal human lung diploid fibroblasts. Resistant clones were recovered at a frequency of 5×10−8 after single-step selections following mutagenesis with the mutagen ethyl methane sulfonate. Following propagation in drug-free medium, the clones retained the selected phenotype and in both growth and plating experiments showed a 10–50-fold higher resistance than wild-type cells to the cytotoxicity of 0.25 μg/ml α-amanitin. The α-amanitin sensitivity of RNA polymerase II purified from the mutant cells suggests the presence of two forms of the enzyme, one similar to that found in wild-type cells and a second form with increased resistance to α-amanitin inhibition. These results are consistent with previous evidence that α-amanitin resistance behaves as a codominant marker in mammalian cells.