DEGRADATION OF POLYURIDYLIC ACID BY RIBONUCLEASE .2. PROTECTION BY RIBOSOMES

It has been suggested that both polynucleotide phosphorylase and the potassium-activated phosphodiesterase (RNase II) are involved in the degradation of messenger RNA in Escherichia coli. If this suggestion holds, it follows that a control mechanism which protects messenger RNA from degradation long enough for it to be translated into protein must also exist. One common proposal is that ribosomes may protect messenger RNA from degradation, and direct evidence for this notion is provided in this report. Polyuridylic acid is used as a model for messenger RNA. The experiments indicate that when polyuridylic acid is bound to the ribosomes in such a fashion that it does not readily dissociate, protection from degradation is observed. The requisite stability of the polyuridylic acid-ribosome complex can be achieved by low temperature (10 °C), by treating the complex with formaldehyde, or by the addition of transfer RNA to give a transfer RNA-poly U-ribosome complex. The experiments reported here indicate that RNase II and polynucleotide phosphorylase degrade poly U chains from the 3′-hydroxyl end of the chain to the point of ribosome attachment; the remainder of the polyribonucleotide is protected. Using these data along with current theories and knowledge on transcription and translation, a model for the control of messenger RNA degradation in E. coli is proposed. © 1969.