Translational repression of a viral messenger RNA by a host protein.

It is shown that factor i, a bacterial protein, specifically inhibits that step in the initiation of R17 bacteriophage RNA translation that involves the attachment of native R17 RNA to 30 S ribosomal subunits carrying fMet-tRNA. This inhibition by factor i is relieved by the addition of excess R17 RNA, but not by the addition of excess 30 S subunits. That R17 RNA is the only target of the inhibition is demonstrated further by the fact that in a cell-free extract containing all components for protein synthesis, factor i-mediated inhibition of exogenous R17 RNA translation can be overcome only by the addition of excess R17 RNA and not by excess cell-free extract. Upon relief of inhibition, phage coat protein synthesis is restored; enhancement of formation of other cistron products is not seen. While initiation of R17 RNA translation is blocked by factor i, chain elongation is not affected. Although foactor i inhibits the IF-3-dependent binding of R17 RNA to fMet-tRNA-30 S complexes, under conditions of initiation of protein synthesis formation of stable complexes between factor i and IF-3 could not be detected, and factor i did not interfere with the binding of IF-3 to free, native R17 RNA. Instead of affecting the function of IF-3 or ribosomes, factor i exerts its inhibition by binding to R17 RNA and acting as a translational repressor. Factor i prefers intact R17 RNA to fragments generated by autoradiolysis; its binding to R17 RNA is specific in that little competition is observed by transfer RNA, ribosomal RNA or poly(A). However, factor i has a high affinity for poly(U) sequences.