Inhibition of Cell‐Free Protein Synthesis by Low‐Molecular‐Weight RNAs from Free Cytoplasmic Ribonucleoprotein Particles

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Abstract
Free cytoplasmic messenger ribonucleoprotein (mRNP) particles from rat liver were treated with EDTA and separated into two populations of RNP particles with sedimentation maxima of 20 S and 35 S respectively. The 20‐S and 35‐S RNP particles, treated with 0.5 M KCl, have protein‐to‐RNA ratios of 0.31:1 and 5.7:1 respectively. Whereas 20‐S and 35‐S RNP particles exhibit a similar protein complement of seven major polypeptides, the low‐molecular‐weight RNA components of the two particle populations are different. A characteristic set of distinct low‐molecular‐weight RNAs is found for 20‐S and 35‐S RNP particles. When the individual low‐molecular‐weight RNAs of 20‐S and 35‐S RNP particles isolated from preparative polyacrylamide gels were assayed for their capability to inhibit protein synthesis in vitro, several potent translational inhibitory RNAs were detected. In particular, the low‐molecular‐weight RNAs of 147, 203 and 263 nucleotides in length associated with the 35‐S RNP particles turned out to be strong inhibitors of protein synthesis.