Comparison of poly(A)-mRNA prepared from membranes and free polyribosomes of mouse liver.

We have compared poly(A)-mRNA isolated from mouse liver endoplasmic reticulum (ER) membranes (ER mRNA) with that from free polyribosomes (free mRNA) by translation in a cell-free system and by mRNA--cDNA reassociation analysis. The partitioning of certain translation products between the two fractions suggests that the physical contamination of the free mRNA by the ER is 10%, and contamination of the ER by the free is 1%. Reassociation crossreactions suggest that no sequences are totally absent from either fraction. In particular, the rare sequences are shared between the two fractions. Among the more abundant sequences, we detect three groups: (i) a group of about 10 sequences that are relatively abundant in both fractions; (ii) a group that occurs mostly in the free fraction; (iii) a group of about 12 sequences, 6 or 7 of them encoding secretory polypeptides. The members of this group are largely or entirely confined to the ER fraction. Secretory proteins are almost entirely absent from the products of th free mRNA fraction. Quantitation of individual mRNA sequences by hybridization with cDNA clones shows that a range of sequence concentrations exists within the abundant class of ER mRNA.