Myosin heavy chain (MHC) mRNP particles have been purified from 13-day chick embryonic skeletal muscle by a combination of sucrose density gradient centrifugation and metrizamide buoyant density centrifugation. Associated with the mRNPs are at least three distinct low molecular weight RNA molecules including translational-control RNA (tcRNA). This particular RNA contains 102 nucleotides and is uridine and guanine rich, and its nucleotide sequence has been determined. tcRNA102 is capable of inhibiting the translation of the mRNAs with which it is associated upon preincubation in stoichiometric amounts. Under these conditions, endogenous reticulocyte mRNA is not inhibited. Under appropriate salt and temperature conditions, tcRNA102 is capable of reassociating with myosin heavy chain (MHC) mRNA, thus altering the sedimentation characteristics of the mRNA. This suggests that the mRNA-tcRNA102 interactions alter the secondary structure of the mRNA. In addition, tcRNA102 does not associate with ribosomal RNA or globin mRNA, suggesting that some degree of specificity is involved with the RNA-RNA interactions.

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Primary chick myoblast cultures demonstrate the ability to take up exogenously supplied polyadenylated RNA and express the encoded information in a specific manner . This expression is shown to exhibit tissue specificity . Analysis of creatine kinase activity monitored at various times of incubation in the presence of either polyadenylated or nonpolyadenylated RNA indicates that only the poly(A) + mRNA is capable of being actively translated . Radioactively labled poly(A)+ mRNA is taken up by the cell cultures in a time-dependent manner and subsequently shown to be associated with polysomes. This association with polysomes does not occur in the presence of puromycin and is unaffected by actinomycin D. Thus, nonspecific interaction with polysomes and induction of new RNA synthesis are ruled out and the association of the exogenously supplied poly(A) + mRNA with polysomes is indicative of its translation in the recipient cells. When heterologous mRNA (globin) is supplied to the myoblasts, it is also taken up and properly translated . In addition, exogenously supplied myosin heavy chain mRNA is found associated with polysomes consisting of 4-10 ribosomes in myoblast cell cultures while in myotubes it is associated with very large polysomes, thus reflecting the different translational efficiencies that this message exhibits at two very different stages of myogenesis .The results indicate that muscle cell cultures can serve as an in vitro system to study translational controls and their roles in development .A number ofstudies has suggested that not only purified DNA but also purified RNA can be taken up by cells (reviewed in references 3, 14, and 38) . The first direct evidence arose from repeated observations of the infectivity of purified viral RNA and viroids (21,35). The aminoacylation ofbacterial tRNA by mouse fibroblasts in culture (37) and the ability of RNA extracted from human fibroblasts treated with poly(I) " poly(C) to elicit production of human interferon in chick fibroblasts as well as the production of mouse interferon by avian and simian cells incubated with mRNA extracted from mouse cells (26) serve to further demonstrate the uptake of functional RNA by cells.The cellular uptake of RNA has been reported to result in specific immune responses (19), the synthesis of specific proteins (30), as well as the induction ofmembrane differentiation (27,31). In most of the cases in which RNA has been demonstrated to enter cells, there is a lack of information concerning the manner by which it penetrates the cells and subsequently exerts its effects, and often the RNA itself has been poorly characterized. On the other hand, specific mRNA transcripts and their activities have been studied after entry into Xenopus

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