The production of ribosomal RNA from high molecular weight precursors: II. Demonstration of an exonuclease in isolated nucleoli

Abstract

Isolated nucleoli containing labeled 45-S preribosomal RNA were incubated in vitro under conditions which allowed varying amounts of endogenous nucleolytic activity. The reaction products, analyzed by acrylamide gel electrophoresis and thin-layer chromatography, consisted of large RNA fragments and 5′-mononucleotides; no small RNA fragments (< 4 · 10 5 daltons) or oligonucleotide were detected. The 5′-mononucleotides were produced at a constant rate over the course of the reaction. These results demonstrate that an exonuclease is responsible for a major portion of the nucleolytic activity of isolated nucleoli, and are consistent with the idea that ribosomal RNA processing involves limited endonucleolytic cleavages followed by exonucleolytic trimming.