[Autodegradation of rat liver polyribosomes in vitro].

In vihro decay of rat liver polyribosomes is studied under conditions of occuring phosphorolytic activity of polynucleotidephosphorylase (PNPase) and, in contrast, excluding PNPase activity (in the absence of orthophosphate). The rate of polyribosome degradation is found to be higher in the presence of orthophosphate, than in its absence. Sedimentation coefficient of ribosomes formed from degraded polyribosomes in the absence of orthophosphate is 70S, while that of ribosomes formed under phosphorolysis is 67S. It is suggested that polyribosome degradation in the presence of orthophosphate is carried out by the combined action of 5'-RNase and PNPase. 5'-RNase degrades mRNA and rRNA without disturbing the integrity of ribosomal complexes, while PNPase phosphorolyzes the fragments formed from the 3'-OH terminal, successively releasing ribosomes. The rate of polysome inactivation in the presence of orthophosphate (half-life period being approximately 17 min) is considerably higher than that in the absence of orthophosphate (half-life period being 37 min). The amount of active polysomes in samples incubated for 3 hours both in the presence and in the absence of orthophosphate is 16% and 24% of the total amount of active polysomes, respectively. It is suggested that there are a certain type of polyribosomes, which comprise about 8% of polyribosomes that can be inactivated only by PNPase.