Methylation of newly synthesized ribonucleic acid by isolated rat liver nuclei. Characterization of the ribonucleic acid synthesized by nuclei from starved animals

摘要

p1. Nuclei from rat liver incubated with iS/i-adenosyl[imethyl/i-sup14/supC]methionine incorporated radioactivity into RNA and into lipid and protein. 2. All of the labelled RNA was extracted from the nuclei with trichloroacetic acid at 90°C. 3. The [sup14/supC]methyl-group incorporation into the hot-trichloroacetic acid extract was 30% inhibited by the addition of actinomycin D (100μg/mg of DNA) or by the omission of CTP, GTP and UTP. 4. Assuming that the main substrate for this triphosphate-dependent methylation was newly synthesized precursor rRNA containing one methyl group/30 uridylate residues, it was calculated that approx. 60% of the [sup14/supC]UMP incorporated under similar conditions represented precursor rRNA synthesis. 5. In agreement with this, low concentrations of actinomycin D (approx. 1μg/mg of DNA) sufficient to abolish the triphosphate-dependent incorporation of [sup14/supC]methyl group inhibited 68% of the [sup14/supC]UMP incorporation. 6. The incorporation of [sup14/supC]UMP by nuclei from starved animals decreased progressively with increasing periods of starvation, whereas the triphosphate-dependent [sup14/supC]methyl-group incorporation was not further decreased after 1 day of starvation. 7. This suggests that precursor rRNA synthesis decreased within 1 day whereas other species of RNA were affected only after longer periods of starvation./p