TRANSLATIONAL EFFICIENCY OF CYTOPLASMIC NON-POLYSOMAL MESSENGER RIBONUCLEIC-ACID FROM SEA-URCHIN EMBRYOS

Messenger ribonucleic acid (mRNA) has been isolated from polysomes and the free cytoplasmic nonpolysomal ribonucleoprotein particles (free RNPs or informosomes) of sea urchin embryos. These RNA populations were examined for their capacities to direct protein synthesis in both the rabbit reticulocyte and wheat germ cell-free amino acid incorporation systems, pretreated with micrococcal nuclease to reduce endogenous incorporation. At equivalent concentrations of each RNA the rate of protein synthesis was greater with the templates from polysomes, and the maximal rate (Vmax) at saturating levels of RNA was about 2-3 times greater for polysomal mRNA compared with the free RNP mRNA. Experiments using mixtures of the two poly(A+) mRNA populations at nonsaturating levels show that the difference in amino acid incorporation is not due to the presence of inhibitors in the free RNP RNA. The ability of each class of RNA to form 80S initiation complexes was measured through ribosome-binding studies. The free RNP mRNA has a 20% lower capacity to bind ribosomes, suggesting that these nonpolysomal mRNA molecules are enriched for inefficient initiators of protein synthesis relative to the templates present in polysomes. However, a specific subset of mRNAs (the 9S histone mRNA) isolated from either the polysomes or free RNPs was found to be equivalent in its ability to form 80S initiation complexes. The difference in efficiency to be translated between the polysomal and nonpolysomal poly(A+) mRNA does not appear to lie in the structures of the 5′ termini since the translation of both classes of mRNA is inhibited to the same extent with the cap analogue m7G5′p. Gel analysis of in vitro translation products reveals that the free RNPs and polysomes contain a similar highly complex array of messages. However, several major quantitative differences in the pattern of translation products exist. Differences in the inherent abilities of various messages to be translated are suggested as an important means of regulating the array of proteins synthesized in an embryo during development. © 1979, American Chemical Society. All rights reserved.