Jay G, Kaempfer R
J Biol Chem. 1975 Aug 10;250(15):5749-55.
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.
已表明，细菌蛋白因子i特异性抑制R17噬菌体RNA翻译起始过程中涉及天然R17 RNA与携带fMet - tRNA的30 S核糖体亚基结合的步骤。因子i的这种抑制作用可通过添加过量的R17 RNA而解除，但添加过量的30 S亚基则不能解除。R17 RNA是唯一的抑制靶点这一点进一步由以下事实证明：在含有蛋白质合成所有成分的无细胞提取物中，因子i介导的对外源R17 RNA翻译的抑制作用只能通过添加过量的R17 RNA而不能通过添加过量的无细胞提取物来克服。抑制解除后，噬菌体外壳蛋白合成得以恢复；未观察到其他顺反子产物形成的增强。虽然R17 RNA翻译的起始被因子i阻断，但链延伸不受影响。尽管因子i抑制R17 RNA与fMet - tRNA - 30 S复合物的IF - 3依赖性结合，但在蛋白质合成起始条件下，未检测到因子i与IF - 3之间形成稳定复合物，且因子i不干扰IF - 3与游离的天然R17 RNA的结合。因子i不是影响IF - 3或核糖体的功能，而是通过与R17 RNA结合并作为翻译阻遏物发挥其抑制作用。因子i更倾向于完整的R17 RNA而非自显影分解产生的片段；它与R17 RNA的结合具有特异性，因为在转运RNA、核糖体RNA或聚腺苷酸中几乎未观察到竞争。然而，因子i对聚尿苷酸序列具有高亲和力。
