Kinetics of Synthesis of Cytoplasmic Messenger‐Like RNA not Associated with Ribosomes in HeLa Cells

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Abstract
The turn-over of cytoplasmic messenger-like RNA (mlRNA) not associated with polyribosomes as well as that of polyribosomal mRNA was investigated by labeling with [3H]uridine in conditions of arrested rlRNA and mitochondrial RNA (mtRNA) synthesis. The synthesis of rlRNA was inhibited with toyokamycin and that of mtRNA with ethidium bromide. In both accumulation kinetics and actinomycin-D-chase experiments, cytoplasmic messenger-like ribonucleoprotein particles and polyribosomes were fractionated by buoyant density centrifugation in CsCl gradients. The half-life of free mlRNA was 1-2 h whereas the bulk of polyribosomal mRNA was stable over the time period considered (up to 8 h) but with a minor short-lived component. Purification of RNA from polyribosomes labeled under the same conditions and fractionation of it into polyadenylated and non-polyadenylated fractions showed that this short-lived minor component of half-life < 1 h is non-polyadenylated.