Endoribonuclease activity associated with nucleolar ribonucleoprotein particles from Novikoff hepatoma

Abstract

Ribonucleoprotein particles have been extracted from isolated nucleoli from Novikoff hepatoma ascites cells. Electron microscopic examination of these ribonucleoprotein particles showed that they are 200–300 Å in diameter and contain electron dense filaments. Incubation of these particles with nucleolar 45-S RNA resulted in degradation of the RNA into fragments that sedimented in the region of a sucrose gradient corresponding to 10–16 S. No exonuclease activity was detected in these preparations. Incubation of ribonucleoprotein particles with 28-S or 18-S RNAs resulted in a less complete degradation suggesting a greater affinity of the enzyme for 45-S RNA. The endoribonuclease activity was not removed from the ribonucleoprotein particles by treatment with 8 mM EDTA, but was extracted with 2 M LiCl into a soluble fraction which was separated into 8 bands by electrophoresis on polyacrylamide gels.