Human mutant cell lines with altered RNA polymerase II

Specific role of manganese and magnesium on RNA synthesis in rabbit bone marrow erythroid cell nuclei

Specific roles of manganese (Mn) and magnesium (Mg) on the activities of DNA-dependent RNA polymerases I and II isolated from rabbit bone marrow erythroid cell nuclei were investigated. Three main polymerases were separated from the cell nuclei. When RNA polymerase I and Mg were added to the RNA synthesis assay mixture containing erythroid cell DNA as template, RNA transcription activity was highest, but when Mg was replaced with Mn, denatured calf thymus DNA formed a better template than erythroid cell DNA. In contrast, nucleoplasmic DNA from erythroid cell and liver DNA were the best templates to stimulate RNA transcription when RNA polymerase II and Mn were added to the assay mixture. However, if Mn was replaced with Mg, RNA synthesis activity was drastically reduced when the template was nucleoplasmic DNA of erythroid cell. RNA polymerase I and Mg synthesized GC rich RNA, whereas RNA polymerase II and Mn synthesized AU rich RNA. Sedimentation analysis showed that the molecular weights of the RNA produced by polymerase I were larger when the enzyme was activated with Mg than with Mn, whereas those of the RNA produced by polymerase II were larger with Mn than with Mg. Furthermore, RNA produced by polymerase I and Mg using chromatin as a template hybridized better with nucleolar DNA than with nucleoplasmic DNA, whereas that produced by polymerase II and Mn hybridized better with nucleoplasmic DNA than with nucleolar DNA. These results suggest that RNA synthesis is dependent on the activity of specific RNA polymerases and the presence of specific divalent cations and templates, and that the cofactor and template for RNA polymerase I are, respectively, Mg and the nucleolar DNA of cell nuclei, whereas those for RNA polymerase II are Mn and nucleoplasmic DNA.

Localization of an alpha-amanitin resistance mutation in the gene encoding the largest subunit of mouse RNA polymerase II

RNA polymerase II is inhibited by the mushroom toxin alpha-amanitin. A mouse BALB/c 3T3 cell line was selected for resistance to alpha-amanitin and characterized in detail. This cell line, designated A21, was heterozygous, possessing both amanitin-sensitive and -resistant forms of RNA polymerase II; the mutant form was 500 times more resistant to alpha-amanitin than the sensitive form. By using the wild-type mouse RNA polymerase II largest subunit (RPII215) gene (J.A. Ahearn, M.S. Bartolomei, M. L. West, and J. L. Corden, submitted for publication) as the probe, RPII215 genes were isolated from an A21 genomic DNA library. The mutant allele was identified by its ability to transfer amanitin resistance in a transfection assay. Genomic reconstructions between mutant and wild-type alleles localized the mutation to a 450-base-pair fragment that included parts of exons 14 and 15. This fragment was sequenced and compared with the wild-type sequence; a single AT-to-GC transition was detected at nucleotide 6819, corresponding to an asparagine-to-aspartate substitution at amino acid 793 of the predicted protein sequence. Knowledge of the position of the A21 mutation should facilitate the study of the mechanism of alpha-amanitin resistance. Furthermore, the A21 gene will be useful for studying the phenotype of site-directed mutations in the RPII215 gene.

Localization of an alpha-amanitin resistance mutation in the gene encoding the largest subunit of mouse RNA polymerase II

RNA polymerase II is inhibited by the mushroom toxin alpha-amanitin. A mouse BALB/c 3T3 cell line was selected for resistance to alpha-amanitin and characterized in detail. This cell line, designated A21, was heterozygous, possessing both amanitin-sensitive and -resistant forms of RNA polymerase II; the mutant form was 500 times more resistant to alpha-amanitin than the sensitive form. By using the wild-type mouse RNA polymerase II largest subunit (RPII215) gene (J.A. Ahearn, M.S. Bartolomei, M. L. West, and J. L. Corden, submitted for publication) as the probe, RPII215 genes were isolated from an A21 genomic DNA library. The mutant allele was identified by its ability to transfer amanitin resistance in a transfection assay. Genomic reconstructions between mutant and wild-type alleles localized the mutation to a 450-base-pair fragment that included parts of exons 14 and 15. This fragment was sequenced and compared with the wild-type sequence; a single AT-to-GC transition was detected at nucleotide 6819, corresponding to an asparagine-to-aspartate substitution at amino acid 793 of the predicted protein sequence. Knowledge of the position of the A21 mutation should facilitate the study of the mechanism of alpha-amanitin resistance. Furthermore, the A21 gene will be useful for studying the phenotype of site-directed mutations in the RPII215 gene.

Mechanism of action of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole. II. A resistant human cell mutant with an altered transcriptional machinery

To determine the role of DRB in transcription, we isolated a resistant (DRBR) HeLa cell mutant. After mutagenesis with N-methyl-N'-nitro-nitrosoguanidine, cell colonies able to grow at 20 micrograms DRB/ml (63 microM) were selected. One of these colonies, DRBR-1, was stable and able to grow at concentrations of DRB three to five times higher than tolerated by normal HeLa cells. The DNA of DRBR-1 was able to confer resistance to DRB to other HeLa cells by transfection. Uridine uptake was reduced by DRB to a similar extent in both wild-type and mutant cells. In contrast, transcription in the mutant cells, as measured by [3H]uridine incorporation into RNA in short pulses, was resistant to DRB. Cell-free extracts prepared from DRBR-1 cells are able to transcribe the epsilon-globin or the adenovirus 2 major late promoter genes at DRB concentrations that eliminate the transcriptional activity of HeLa cell extracts. Thus the transcriptional machinery of the mutant is altered. The presence of both DRB-resistant and DRB-sensitive transcriptional activities in extracts from DRBR-1 cells, grown in the presence of the drug, suggests constitutive expression of this cellular component. Efficient somatic cell hybridization with an alpha-amanitin-resistant RNA polymerase II mouse mutant indicates cross-complementation in vivo. This DRBR mutant provides a useful tool for the biochemical analysis of the mechanism of action of DRB on transcription. It also serves as a genetic handle for selection of the gene responsible for DRB resistance.