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A group of rifamycin derivatives were tested against the nucleolar and nucleoplasmic RNA polymerases from three types of animal cells. It was found that several compounds effectively inhibited such enzymes without substantial discrimination. In addition, the same derivatives inhibited polyphenylalanine synthesis in an animal cell-free system.Experiments with intact Hela cells showed that the synthesis of RNA, the processing ofrRNA, the synthesis of DNA and the uptake of nucleosides were inhibited by rifamycins.It is concluded that the specificity of rifamycin effects obtained a t the concentrations that appear to be necessary to interfere with normal (or virus-directed) processes in animal cells can be seriously questioned.Rifamycins are a large group of chemical derivatives of a natural antibiotic produced by Xtreptomycea Mediterranei. The best known among these compounds, rifampicin, has proved to be both an important tool in studying bacterial transcription and a drug with a wide range of clinical applicability. Its mechanism of action is fairly specsc as indicated by genetic and biochemical evidence. The binding of one rifampicin molecule per molecule of bacterial RNA polymerase, before a stable enzyme -DNA complex is formed, is sufficient to block RNA synthesis. Once transcription has started, the subsequent chain elongation is insensitive to the drug. Rifampicin is inactive on RNA polymerases from eukaryotes, with the possible exception of mitochondrial RNA polymerases [l, 21. Several rifamycins, however, a t concentrations considerably higher than those employed with rifampicin on bacteria, have been shown to inhibit animal RNA polymerases and to prevent the growth of certain animal viruses [3-71. Obviously, one major point of interest concerning rifamycins is the potential exploitability of their properties to investigate normal and virus-directed transcriptional mechanisms also in animal cells. Even more important appears to be the capacity of some of these drugs to inhibit reverse transcription [S-lo], an essential step in cell transformation by RNA oncogenic viruses.For the sake of simplicity Lepetit code names have been used throughout this paper instead of the complex chemical nomenclature.Enzymes. RNA polymerase or RNA nucleotidyl transferase (EC 2.7.7.6); DNA polymerase or DNA nucleotidyl transferase (EC 2.7.7.7). 17'On the other hand, although it may be correct in the first approximation to state that polynucleotide synthesis is the primary target of rifamycins in general, it cannot be safely assumed that this target is definitely specitic. The relatively high dosage required by most rifamycins to inhibit non-bacterial enzymes may conceivably interfere with several metabolic functions when applied to intact cells and even cause some difficulty in interpreting results obtained in vitro.We tested 15 rifamycins on nucleolar and nucleoplasmic RNA polymerases from Hela cells, rat liver and mouse liver. A reticulocyte cell-free system was used to determine possible interference with polyphenylalanin...

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