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[Reduced catalytic effectiveness of RNA polymerase I in hepatocytes of rats treated with cycloheximide]
Abstract:
Rat liver RNA polymerase I solubilized from isolated nuclei and present in a soluble form in the cytoplasmic fraction has been analyzed by phosphocellulose chromatography 3 hours after the administration of cycloheximide. The antibiotic did not induce any change in the chromatographic properties of both nuclear and cytoplasmic RNA polymerase I. They appeared to remain in the IB and IA forms, characteristic of the transcribing (IB) and non-transscribing (IA) enzyme. While the level of the nuclear enzyme was not modified, the level of the cytoplasmic one appeared significantly increased. These results support previous ones indicating that the cycloheximide-induced inhibition of ribosomal RNA synthesis cannot be merely explained by a decrease in the nuclear or cellular level of RNA polymerase I. The cellular level of RNA polymerase I, taking into account the relative proportion of the enzyme found in nuclei and cytoplasm, appeared to be slightly increased. Cycloheximide administration did not seem to result in the appearance, in intact nuclei, of enzyme molecules in a free form or as blocked transcription complexes. It is concluded that the antibiotic affects the catalytic efficiency rather than the number of RNA polymerase I molecules actually engaged in the transcription of ribosomal cistrone.
Insights
Cycloheximide increases cytoplasmic RNA polymerase I levels but does not alter its form or nuclear presence. This suggests the drug impacts enzyme efficiency, not quantity, for ribosomal RNA synthesis inhibition.
Area of Science:
- Molecular Biology
- Biochemistry
- Cell Biology
Background:
- Ribosomal RNA (rRNA) synthesis is crucial for cell function.
- Cycloheximide is known to inhibit rRNA synthesis.
- The precise mechanism of cycloheximide's effect on RNA polymerase I (Pol I) is not fully understood.
Purpose of the Study:
- To investigate the effect of cycloheximide on the cellular levels and properties of rat liver RNA polymerase I.
- To determine if cycloheximide alters the distribution or forms of RNA polymerase I between nuclear and cytoplasmic fractions.
- To elucidate whether cycloheximide affects the quantity or catalytic efficiency of RNA polymerase I involved in rRNA transcription.
Main Methods:
- Analysis of rat liver RNA polymerase I using phosphocellulose chromatography.
- Fractionation of nuclear and cytoplasmic components.
- Administration of cycloheximide and subsequent enzyme analysis at 3 hours post-treatment.
Main Results:
- Cycloheximide did not alter the chromatographic properties (IA and IB forms) of nuclear or cytoplasmic RNA polymerase I.
- The level of nuclear RNA polymerase I remained unchanged.
- The level of cytoplasmic RNA polymerase I significantly increased, leading to a slight overall increase in cellular RNA polymerase I.
- No evidence of free or blocked enzyme forms in nuclei post-cycloheximide treatment.
Conclusions:
- Cycloheximide-induced inhibition of rRNA synthesis is not due to a decrease in RNA polymerase I levels.
- The antibiotic appears to affect the catalytic efficiency of RNA polymerase I rather than the number of enzyme molecules engaged in rRNA gene transcription.