The role of mRNA competition in regulating translation. III. Comparison of in vitro and in vivo results.

Competition of encephalomyocarditis virus, reovirus, and L-cell mRNAs for a message-discriminatory component was studied in vitro. The data were analyzed qualitatively to determine the relative initiation efficiencies among the various mRNAs. The effects of potassium chloride concentration, magnesium acetate concentration, and m7G methylation on mRNA competition in vitro were also studied. These results were correlated with translation rates in vivo for the same mRNAs, to determine if the sites of competition in vitro and in vivo are the same. It was found that under a particular set of magnesium acetate and potassium chloride concentrations, the order of mRNA initiation efficiencies was the same both in vivo and in vitro, suggesting that the same limiting message-discriminatory factor is regulating initiation rates in both cases. This can only be accomplished in a competitive situation when RNA is in molar excess relative to the discriminatory component.