Studies on the structure of rat liver messenger ribonucleoprotein. II. Non-poly(A) RNase resistant fragments and electron microscopic appearance of mRNP.

Partially fragmented 12-21S rat liver messenger ribonucleoprotein (mRNP), labelled either with [3H]-orotic acid or [3H]-adenine was treated with 5 (micrograms/ml or 0.1 microgram/ml pancreatic ribonuclease (EC 3.1.27.5) and the resistant fragments were separated by sucrose gradient centrifugation. Two types of fragments were obtained. Digestion of mRNP with ribonuclease at a concentration of 5 micrograms/ml resulted in 9S poly(A)-protein particles of mRNP as evidenced by their characteristic sedimentation, electrophoretic mobility of the RNA moiety and protein composition. In contrast, ribonuclease at a concentration of 0.1 microgram/ml produced 2-5S pyrimidine labelled fragments carrying polynucleotide sequences consisting of approximately 16-50 residues. Two polypeptides of rat liver mRNP with molecular weights of 38 000 (P38) and 44 000 (P44) were found to be attached to these sequences. The data demonstrate that RNA sequences other than poly(A) interact with protein in mRNP. Electron microscopic studies revealed that the liberation of mRNP from the polysomes by EDTA changes its surface properties since EDTA releases the mRNP mainly in the form of globular particles. However, a small proportion of the mRNP remains in fibril-like form. Among the fibrils several blobs could be seen which were probably the proteins attached to the mRNA. From the available data a model for the structure of an "average" mRNP molecule is proposed.