Translational repression of mRNA for eucaryotic elongation factors in Friend erythroleukemia cells

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Abstract
Poly(A)-containing mRNA was prepared from polyribosomes and postpolyribosomal messenger ribonucleoprotein particles (mRNP) from Friend erythroleukemic cells. Both mRNA types were translated in vitro and the 35S-labeled translation products examined by 2-dimensional gel electrophoresis. Among the most abundant untranslated mRNA species was the mRNA coding for eukaryotic elongation factor Tu (eEF-Tu). In addition, the mRNA for eukaryotic elongation factor Ts was also present in Friend cells in untranslated form. Calculations based on translation assays indicate that eEF-Tu represents about 15% of the translation products of RNP mRNA and that .apprx. 40% of the eEF-Tu synthesized in vitro is encoded by translationally repressed mRNA. This repressed mRNA can be activated by addition of cycloheximide to cell cultures. At the level of 0.1 .mu.g/ml, cycloheximide inhibited cellular protein synthesis by about 50% while augmenting the relative rate of eEF-Tu synthesis 1.6-fold. This result suggested that eEF-Tu mRNA might initiate poorly. However, addition of supersaturating levels of mRNA to a reticulocyte lysate augmented eEF-Tu synthesis about 2-fold, while generally depressing the synthesis of other proteins by about 40%. Thus the storage of large amounts of eEF-Tu mRNA in vivo is unlikely to be due directly to the ineffectiveness of the mRNA in competing for the initiation machinery of the cell. The supply of active eEF-Tu in erythroleukemic cells may be controlled, at least in part, by a translational mechanism.