The genetic code is the cellular translation table for the conversion of nucleotide sequences into amino acid sequences. Changes to the meaning of sense codons would introduce errors into almost every translated message and are expected to be highly detrimental. However, reassignment of single or multiple codons in mitochondria and nuclear genomes, although extremely rare, demonstrates that the code can evolve. Several models for the mechanism of alteration of nuclear genetic codes have been proposed (including “codon capture,” “genome streamlining,” and “ambiguous intermediate” theories), but with little resolution. Here, we report a novel sense codon reassignment in Pachysolen tannophilus, a yeast related to the Pichiaceae. By generating proteomics data and using tRNA sequence comparisons, we show that Pachysolen translates CUG codons as alanine and not as the more usual leucine. The Pachysolen tRNACAG is an anticodon-mutated tRNAAla containing all major alanine tRNA recognition sites. The polyphyly of the CUG-decoding tRNAs in yeasts is best explained by a tRNA loss driven codon reassignment mechanism. Loss of the CUG-tRNA in the ancient yeast is followed by gradual decrease of respective codons and subsequent codon capture by tRNAs whose anticodon is not part of the aminoacyl-tRNA synthetase recognition region. Our hypothesis applies to all nuclear genetic code alterations and provides several testable predictions. We anticipate more codon reassignments to be uncovered in existing and upcoming genome projects.
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机译:遗传密码是用于将核苷酸序列转换为氨基酸序列的细胞翻译表。改变有义密码子的含义将在几乎每条翻译的信息中引入错误,并且被认为是非常有害的。然而,线粒体和核基因组中单个或多个密码子的重新分配,尽管极为罕见,但表明该代码可以进化。已经提出了几种用于改变核遗传密码的机制的模型(包括“密码子捕获”,“基因组精简”和“模棱两可的中间”理论),但解决方案很少。在这里,我们报告了一种新的有义密码子在Pachysolen tannophilus(一种与毕赤酵母相关的酵母)中的重新分配。通过生成蛋白质组学数据并使用tRNA序列比较,我们显示Pachysolen将CUG密码子翻译为丙氨酸而不是更常见的亮氨酸。 Pachysolen tRNACAG是反密码子突变的tRNA Ala sup>,其中包含所有主要的丙氨酸tRNA识别位点。酵母中CUG编码tRNA的多重性最好用tRNA丢失驱动的密码子重分配机制来解释。在古酵母中,CUG-tRNA的丢失是相应密码子逐渐减少,随后其反密码子不是氨酰基-tRNA合成酶识别区域的一部分的tRNA捕获了密码子。我们的假设适用于所有核遗传密码变更,并提供了一些可检验的预测。我们预计在现有和即将到来的基因组项目中将发现更多的密码子重分配。
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