真核细胞中多重表达框mRNA的选择性翻译起始

扫描模型和遗漏扫描模型是真核生物mRNA翻译起始的两种主要机制，但其仍存在某些例外情况，如对具有多顺反子结构的mRNA，选择性翻译起始的发生机制目前仍不清楚.本研究基于GFP蛋白开放表达框(ORF)构建了一系列重组表达载体，用以转录在移码翻译顺序及同一翻译顺序下，AUG起始密码子处于不同序列背景，以及间隔不同距离的多顺反子结构mRNA.通过转染人Bel 7402细胞系，研究了这些多顺反子结构mRNA的翻译起始模式.结果表明，在移码翻译顺序下,多顺反子mRNA可翻译出对应的不同蛋白质，而在同一翻译顺序下,GFP蛋白表达框中的多个AUG密码子，仅有首位起始密码子可发挥作用，提示核糖体在从首位起始密码子开始翻译的同时，可能会有部分核糖体继续向下扫描并识别下游的起始密码子，而这种选择性的翻译起始效率，主要取决于密码子所处的序列背景及间隔距离等因素.

Alternative Translation Initiation of Multicistronic mRNA in Eukaryotic Cells

The scanning and leaky scanning paradigms of eukaryotic translation initiation are widely accepted, although numerous exceptions have been reported. The incidence of alternative translation initiation for eukaryotic multicistronic mRNAs is still unknown. In the present study, multiple recombinant vectors containing a GFP open reading frame (ORF) were constructed to transcribe multicistronic mRNAs, including out-of-frame and in frame downstream reading frames. Then, we assessed the translation initiation patterns of the multicistronic mRNAs, where the AUG triplets of reading forms were in different sequence contexts and varying distances apart from the authoritative initiation codon in eukaryotic cells. Our results demonstrated that a single multicistronic mRNA including out-of-frame downstream reading frame could be translated into different proteins, with the first AUG triplet serving as the unique initiation codon in the in frame multicistronic GFP mRNA. These findings suggested that some ribosomal subunits might keep moving forward and recognize the downstream AUG codons after recruitment by the authoritative translation initiation codon. The efficiency of the alternative translation initiation mainly depends on the sequence context and distance from the first initiation codon.