Translation of a yeast mitochondrial tRNA synthetase initiated at redundant non-AUG codons
Translation of a yeast mitochondrial tRNA synthetase initiated at redundant non-AUG codons
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DOI：
10.1074/jbc.m408081200
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发表时间：
2004-11-26
影响因子：
4.8
通讯作者：
Wang, CC
中科院分区：
文献类型：
--
作者：
Tang, HL;Yeh, LS;Wang, CC
尽管在原核生物和高等真核生物中偶尔发生非AUG密码子的翻译起始，但直到最近才在酵母中报道。这里提出的证据表明，冗余ACG密码子被认为是替代翻译起始位点的ALA1，在酿酒酵母中唯一的基因编码丙氨酰-tRNA合成酶。ALA1被证明是一个双功能基因，提供细胞质和线粒体的活动。与大多数含有替代性框内AUG起始子的双功能基因不同，在靠近ALA1开放阅读框的5 '端处只有一个AUG密码子，称为AUG（1）。转录作图鉴定了三个重叠的转录物，其5 '端分别位于AUG（1）上游的54、105和117个核苷酸位置。位点特异性诱变证明，ALA1的细胞质和线粒体功能由具有不同氨基末端的两种蛋白质同种型提供;即，起始于AUG（1）的短细胞质形式和起始于两个上游符合读框的ACG密码子（即ACG（-25）和ACG（-24））的较长线粒体同种型。这两个ACG密码子在翻译起始中起冗余作用。任何一个密码子都可以在另一个密码子不存在的情况下发挥作用。短的转录本似乎作为细胞质形式的模板，而较长的转录本可能通过交替起始编码两种亚型。由于酵母核糖体通常不能有效地识别非AUG起始子，因此在单个mRNA中非AUG起始子冗余的这种独特特征本身可能代表了从不良起始子翻译起始的新范例。
Although initiation of translation at non-AUG codons occurs occasionally in prokaryotes and higher eukaryotes, it has not been reported in yeast until very recently. Evidence presented here shows that redundant ACG codons are recognized as alternative translation start sites for ALA1, the only gene in Saccharomyces cerevisiae coding for alanyl-tRNA synthetase. ALA1 is shown to be a bifunctional gene that provides both cytoplasmic and mitochondrial activities. Unlike most bifunctional genes that contain alternative in-frame AUG initiators, there is only one AUG codon, designated AUG(1), close to the 5'-end of the ALA1 open reading frame. Transcriptional mapping identified three overlapping transcripts, with 5'-ends at positions 54, 105, and 117 nucleotides upstream of AUG(1), respectively. Site-specific mutagenesis demonstrated that the cytoplasmic and mitochondrial functions of ALA1 are provided by two protein isoforms with distinct amino termini; that is, a short cytoplasmic form initiated at AUG(1) and a longer mitochondrial isoform initiated at two upstream in-frame ACG codons, i.e. ACG(-25) and ACG(-24). These two ACG codons function redundantly in initiation of translation. Either codon can function in the absence of the other. The short transcript appears to serve as the template for the cytoplasmic form, whereas the longer transcripts are likely to code for both isoforms via alternative initiation. Because yeast ribosomes in general cannot efficiently recognize a non-AUG initiator, this unique feature of redundancy of non-AUG initiators in a single mRNA may in itself represent a novel paradigm for translation initiation from poor initiators.