Control of breakdown of the polyadenylate sequence in mammalian polyribosomes: role of poly(adenylic acid)-protein interactions

Abstract

The poly(A) segment in mouse sarcoma polysomes is not hydrolyzed by snake venome exonuclease under conditions which cause extensive degradation of the poly(A) in deproteinized polysomal RNA. The protecting effect of polysomes is presumably caused by the interaction between the poly(A) sequence and the protein known to be associated with it. This protection is reduced at low KCl concentration, but addition of exogenous RNA restores the protecting effect. The poly(A) segment also becomes susceptible to exonuclease after fragmentation of the polysomes by mild RNase treatment. The latter treatment releases the poly(A) in association with protein. The poly(A) sequence in polysomes is readily degraded by a cytoplasmic extract of S-180 cells [mouse ascites]. Partial purification leads to a preparation active against the poly(A) in polysomes under conditions where no fragmentation of the mRNA is observed. Snake venom exonuclease increases the activity of the cytoplasmic preparation against poly(A) in polysomes. The active cytoplasmic factor appears to interfere with the poly(A)-protein interaction, thus rendering the polynucleotide susceptible to degradation by exonuclease. The poly(A) sequences in polysomes and in free cytoplasmic nucleoprotein particles are hydrolyzed to the same extent. The poly(A) sequence may be normally protected from nucleases by virtue of its association with protein. The slow reduction in poly(A) size in cytoplasmic mRNA can be accounted for by a factor capable of interfering with the poly(A)-protein interaction. The latter interaction seems also dependent on the structural integrity of the polysomes or messenger ribonucleoproteins. A polynucleotide segment adjacent to the poly(A) may be able to modulate the affinity of the protein for the latter sequence, thus permitting control of poly(A) stability in individual mRNA.