Dihydrorifampicin, a rifampicin derivative hydrogenated at the 18-19 carbon atoms of the aliphatic ansa chain of the rifampicin molecule, inhibits the enzymatic activity of RNA polymerases I and II, isolated from the nuclei of avian tumor cells (Rous sarcoma) and from the human tumor cell line HEp-2. The RNA polymerases from these tumors have been separated and partially purified by chromatography on DEAE Sephadex A-25 and characterized by the sensitivity to α-amanitin. The [3H]UMP-labeled ribonucleic acids synthesized in the isolated nuclei of Rous sarcoma cells in the presence and absence of DHR were analyzed by sedimentation analysis in sucrose density gradients. It was found that the synthesis of rRNAs and mRNAs is very significantly inhibited by dihydrorifampicin, whereas the synthesis of tRNAs is much less inhibited at the same DHR concentration (100μg/ml). The observed cytostatic effect of DHR on the growth of human tumor cells HEp-2 and embryonic cells in culture is apparently mediated by the selective inhibition of RNA polymerases I and II in human and avian cells. The relationship between the molecular structure of DHR and its affinity to RNA polymerases of eukaryotic cells is discussed.
