Sequences mediating the translation of mouse S16 ribosomal protein mRNA during myoblast differentiation and in vitro and possible control points for the in vitro translation.

The translation of ribosomal protein (r-protein) mRNAs is generally inefficient and regulated during the differentiation of mouse myoblasts into fibers. In this discussion we show that the first 31 nucleotides of the S16 r-protein mRNA, when located at the 5' end of the mRNA, are sufficient to impart the translational properties of an r-protein mRNA to the SV-GALK mRNA, which is normally translated efficiently in both myoblasts and fibers. If the same S16 sequences are located within the interior of the 5'-untranslated region of the SV-GALK mRNA, however, they do not impart the translational properties of an r-protein mRNA to the SV-GALK mRNA. The translation of mouse r-protein mRNAs was examined in vitro to help elucidate the mechanisms controlling their translation. Mouse r-protein mRNAs are inefficiently translated in rabbit reticulocyte extracts, and the same sequences that mediate their inefficient and regulated translation during myoblast differentiation also mediate their inefficient translation in a position-dependent manner in reticulocyte extracts. To determine whether the subpolysomal r-protein mRNAs that are not actively translated in vivo are capable of translation, subpolysomal RNA was translated in reticulocyte extracts. The subpolysomal r-protein mRNAs are just as capable of translation as are polysomal mRNAs. To help identify the initiation factors and/or the steps in the initiation pathway that mediate the inefficient translation of r-protein mRNAs, reticulocyte extracts were supplemented with purified initiation factors. Only eIF-4F, the cap-binding complex, and eIF-3, which is involved in subunit dissociation and interacts with eIF-4F during initiation, stimulated the translation of r-protein mRNA. These experiments, along with m7GDP inhibition studies, suggest that eIF-4F and/or eIF-3, or the steps mediated by these factors, mediate the inefficient translation in reticulocyte extracts and raise the possibility that these steps also control the regulated translation of r-protein mRNAs during myoblast differentiation.