Cao J, Geballe A P
Department of Molecular Medicine, Fred Hutchinson Cancer Research Center, Seattle, Washington 98104.
J Virol. 1995 Feb;69(2):1030-6. doi: 10.1128/JVI.69.2.1030-1036.1995.
The second of three short upstream open reading frames (uORF2) in the transcript leader of the human cytomegalovirus gp48 (gpUL4) virion glycoprotein gene inhibits downstream translation approximately 10-fold. Remarkably, this inhibition depends on the amino acid coding information of uORF2. In the current studies we demonstrate that expression of the cistron downstream from uORF2 depends on ribosomes bypassing the uORF2 AUG codon (AUG2) by a leaky scanning mechanism. Replacing the nucleotides surrounding the wild-type AUG2 codon with those optimal for translation initiation reduces downstream translation approximately 10-fold. Analyses of mutants in which uORF2 either overlaps or is in frame with the downstream reading frame reveal that the initiation frequency at the wild-type AUG2 codon is surprisingly low; rather, the majority of ribosomal subunits bypass the wild-type AUG2 codon because of its suboptimal context. We propose a model to explain this unprecedented example of a paradoxically strong inhibitory effect of an upstream ORF despite inefficient utilization of its initiation codon.
人类巨细胞病毒糖蛋白48（gpUL4）病毒粒子糖蛋白基因转录前导序列中的三个短上游开放阅读框（uORF2）中的第二个，可使下游翻译受到约10倍的抑制。值得注意的是，这种抑制作用取决于uORF2的氨基酸编码信息。在当前研究中，我们证明uORF2下游顺反子的表达取决于核糖体通过渗漏扫描机制绕过uORF2的AUG密码子（AUG2）。用最适合翻译起始的核苷酸替换野生型AUG2密码子周围的核苷酸，可使下游翻译减少约10倍。对uORF2与下游阅读框重叠或框内的突变体分析表明，野生型AUG2密码子处的起始频率出奇地低；相反，由于其上下文不理想，大多数核糖体亚基绕过野生型AUG2密码子。我们提出了一个模型来解释这个前所未有的例子，即尽管上游开放阅读框的起始密码子利用效率低下，但却具有矛盾的强烈抑制作用。
