Evidence for Nontranslated Messenger Ribonucleic Acid in Membrane-bound and Free Polysomes of Rabbit Liver

Abstract

Abstract In previous publications (Shafritz, D. A., Drysdale, J. W., and Isselbacher, K. J. (1973) J. Biol. Chem. 248, 3220–3227 and Shafritz, D. A. (1974) J. Biol. Chem. 249, 81–88) we reported the isolation and translation of RNA fractions with messenger activity from membrane-bound and free polysomes of rabbit liver with evidence for specific synthesis of albumin and ferritin in a reticulocyte cell-free system. Using both immunological and chemical purification methods, the synthesis of these proteins has now been compared with homologous membrane-bound and free liver polysomes versus the respective RNA extracts from these polysomes, translated in the heterologous reticulocyte cell-free system. Under the conditions used, there was a clear distinction in the synthesis of albumin and ferritin by isolated membrane-bound versus free liver polysomes. However, in the reticulocyte system, as a result of large increases in the relative percentage of ferritin synthesis by RNA from membrane-bound polysomes and albumin synthesis by RNA from free polysomes, this distinction was almost totally abolished. These results are consistent with an interpretation that, under conditions generally used for protein synthesis, considerable portions of ferritin messenger RNA in membrane-bound polysomes and albumin messenger RNA in free polysomes are not translated in cell-free systems derived from rabbit liver.