Pyrimidine-rich oligonucleotides from rat-liver ribosome surface

The distribution of oligonucleotides which are released from rat liver ribosomes by treatment with pancreatic ribonuclease has been studied. Rat liver monoribosomes lost from 15 to 17% of their nucleotides by treatment with pancreatic ribonuclease. This quantity was highly reproducible and did not depend significantly on the temperature (0-20 degrees C) and time (10-120 min) of incubation or on the concentration of enzyme (1:5000-1:50). Whereas the amounts of oligonucleotides liberated was 16%, it was shown by column chromatography that they consisted of 71% mononucleotides, 16% dinucleotides, 6% trinucleotides, 4% tetranucleotides and 2% pentanucleotides and that these oligonucleotides were enriched in uridine, containing approximately half of the uridine residues present in the high-molecular-weitht ribosomal RNA. The high molecular weight of the RNA from ribonuclease-treated ribosomes was preserved until it was heated; after heating, RNA fragments having sedimentation coefficients of 5 S and less were present. It is inferred that the olignucleotides are derived from pyrimidine-rich clusters located in single-stranded "hairpin" loops on the outside surface of the ribosome.