Bacteriophage and spliceosomal proteins function as position-dependent cis/trans repressors of mRNA translation in vitro.

The translational regulation of ferritin expression currently represents the only well characterized example for eukaryotic translational control by high affinity interactions between a specific cytoplasmic protein, iron regulatory factor [IRF], and an mRNA-binding site, the iron-responsive element [IRE], located in the 5' untranslated region [UTR] of ferritin mRNAs. To elucidate whether IRE/IRF may represent the first physiological example of a more general mechanism for mRNA-specific translational control, high affinity RNA-binding sites for the bacteriophage MS2 coat protein or the spliceosomal protein U1A were introduced into the 5' UTR of capped chloramphenicol acetyltransferase [CAT] transcripts. In the absence of these RNA-binding proteins, CAT mRNA was efficiently translated. Addition of purified MS2 coat protein or U1A caused a specific, dose-dependent repression of CAT biosynthesis in rabbit reticulocyte and wheat germ in vitro translation systems. The translational blockage imposed by the RNA/protein complex was reversible and did not alter the stability of the repressed mRNAs. Translational repression caused by binding of U1A or MS2 proteins to their target mRNAs is shown to be position-dependent in vitro. Thus, mRNA/protein complexes without an a priori role in eukaryotic mRNA translation function as translational effectors with characteristics resembling those of IRE/IRF.