MESSENGER RNA IN RAT LIVER POLYRIBOSOMES - EVIDENCE THAT IT EXISTS AS RIBONUCLEOPROTEIN PARTICLES

After a 40-minute labeling period with a radioactive RNA precursor, the majority of the labeled RNA of a polyribosome preparation from rat liver cytoplasm is heterogeneous in size, non-ribosomal in base composition, and is directly associated with polyribosomes. On the basis of these and other criteria it can tentatively be identified as mRNA. The messenger RNA molecules can be released from polyribosomes by treatment with EDTA, and they are released not as naked strands but in association with less dense material, presumably protein, as ribonucleoprotein particles of heterogeneous sedimentation coefficient from 20 s to greater than 100 s and of heterogeneous buoyant density in CsCl from about 1.35 g/cm3 to 1.55 g/cm3. The mRNA-containing particles released from polyribosomes are in many ways similar to newly synthesized mRNA-containing particles found free (unassociated with polyribosomes) in the nucleus and in the cytoplasm, and it is postulated that mRNA is transferred from the nucleus as ribonucleoprotein particles, which are incorporated into polyribosomes. We speculate that the protein the mRNA carries with it from the nucleus into the polyribosome may serve to modulate protein synthesis at the translational level. © 1968.