mRNA leader length and initiation codon context determine alternative AUG selection for the yeast gene MOD5.

MOD5, a nuclear gene of Saccharomyces cerevisiae, encodes two isozymic forms of a tRNA-modification enzyme. These enzymes modify both cytoplasmic and mitochondrial tRNAs. Two inframe ATGs of the MOD5 gene are used for initiation of translation, and the form of the protein translated from the first AUG is imported into mitochondria. Protein translated from the second AUG functions in the cytoplasm. Since all transcripts contain both of these translational start sites and two proteins are made, the question arises as to the factors that influence the translation start-site choice. Extending the 5' ends of the MOD5 mRNA to include leader sequences of the ADH1 (alcohol dehydrogenase defective) transcript produces significant changes in the choice of AUGs. This suggests that for wild-type MOD5 transcripts, the length or structure of the leader sequence plays a role in AUG choice. The nucleotides surrounding the first ATG of MOD5 also have an effect on translation initiation. Altering these nucleotides changes initiation choice and suggests that ribosomal bypass of a suboptimal AUG is another mechanism controlling the alternate use of two initiation codons. Our data support the model that at least one MOD5 transcript is able to produce two proteins with different N-terminal sequences. IMAGES:

MOD5是酿酒酵母（Saccharomyces cerevisiae）的核基因，编码两种同工型的转运RNA（tRNA）修饰酶。该酶可对细胞质与线粒体来源的tRNA均进行修饰。MOD5基因的两个框内ATG位点被用作翻译起始位点：从首个AUG翻译得到的蛋白质会被靶向导入线粒体，而从第二个AUG翻译得到的蛋白质则在细胞质中发挥功能。由于所有转录本均包含这两个翻译起始位点且可合成两种蛋白质，因此引发了关键科学问题：究竟哪些因素会影响翻译起始位点的选择？将MOD5 mRNA的5'端延伸，使其纳入ADH1（乙醇脱氢酶缺陷型，alcohol dehydrogenase defective）转录本的前导序列，会显著改变AUG位点的选择偏好，这表明对于野生型MOD5转录本而言，前导序列的长度或结构在AUG选择过程中发挥了调控作用。MOD5首个ATG侧翼的核苷酸序列同样对翻译起始具有调控效应：改变这些侧翼核苷酸序列会改变翻译起始位点的选择偏好，这提示核糖体绕过次优AUG是控制两个起始密码子交替使用的另一分子机制。本研究数据支持如下模型：至少存在一种MOD5转录本能够合成两种具有不同N端序列的蛋白质。IMAGES: