Chiaruttini C, Milet M, Springer M
Unité Propre de Recherche 9073 du Centre National de la Recherche Scientifique, Institut de Biologie Physico-Chimique, 13 rue Pierre et Marie Curie, 75005 Paris, France.
Proc Natl Acad Sci U S A. 1997 Aug 19;94(17):9208-13. doi: 10.1073/pnas.94.17.9208.
A pseudoknot formed by a long-range interaction in the mRNA of the initiation factor 3 (IF3) operon is involved in the translational repression of the gene encoding ribosomal protein L35 by another ribosomal protein, L20. The nucleotides forming the 5' strand of the key stem of the pseudoknot are located within the gene for IF3, whereas those forming the 3' strand are located 280 nt downstream, immediately upstream of the Shine-Dalgarno sequence of the gene for L35. Here we show that premature termination of IF3 translation at a nonsense codon introduced upstream of the pseudoknot results in a substantial enhancement of L20-mediated repression of L35 expression. Conversely, an increase of IF3 translation decreases repression. These results, in addition to an analysis of the effect of mutations in sequences forming the pseudoknot, indicate that IF3 translation decreases L20-mediated repression of L35 expression. We propose that ribosomes translating IF3 disrupt the pseudoknot and thereby attenuate repression. The result is a novel type of translational coupling, where unfolding of the pseudoknot by ribosomes translating IF3 does not increase expression of L35 directly, but alleviates its repression by L20.
起始因子3（IF3）操纵子mRNA中的长程相互作用形成的假结，参与了核糖体蛋白L20对核糖体蛋白L35编码基因的翻译抑制。形成假结关键茎5'链的核苷酸位于IF3基因内，而形成3'链的核苷酸位于下游280个核苷酸处，即L35基因的Shine-Dalgarno序列紧上游。在此我们表明，在假结上游引入的无义密码子处IF3翻译的提前终止，会导致L20介导的L35表达抑制显著增强。相反，IF3翻译增加则会降低抑制作用。这些结果，连同对形成假结序列中突变效应的分析，表明IF3翻译会降低L20介导的L35表达抑制。我们提出，翻译IF3的核糖体破坏了假结，从而减弱了抑制作用。结果是一种新型的翻译偶联，即翻译IF3的核糖体使假结解折叠，这并不会直接增加L35的表达，而是减轻了L20对它的抑制。
