Schön A, Kannangara C G, Gough S, Söll D
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06511.
Nature. 1988 Jan 14;331(6152):187-90. doi: 10.1038/331187a0.
In the course of our studies on transfer RNA involvement in chlorophyll biosynthesis, we have determined the structure of chloroplast glutamate tRNA species. Barley chloroplasts contain in addition to a tRNA(Glu) species at least two other glutamate-accepting tRNAs. We now show that the sequences of these tRNAs differ significantly: they are differentially modified forms of tRNA(Gln) (as judged by their UUG anticodon). These mischarged Glu-tRNA(Gln) species can be converted in crude chloroplast extracts to Gln-tRNA(Gln). This reaction requires a specific amidotransferase and glutamine or asparagine as amide donors. Aminoacylation studies show that chloroplasts, plant and animal mitochondria, as well as cyanobacteria, lack any detectable glutaminyl-tRNA synthetase activity. Therefore, the requirement for glutamine in protein synthesis in these cells and organelles is provided by the conversion of glutamate attached to an 'incorrectly' charged tRNA. A similar situation has been described for several species of Gram-positive bacteria. Thus, it appears that the occurrence of this pathway of Gln-tRNA(Gln) formation is widespread among organisms and is a function conserved during evolution. These findings raise questions about the origin of organelles and about the evolution of the mechanisms maintaining accuracy in protein biosynthesis.
在我们对参与叶绿素生物合成的转运RNA的研究过程中，我们确定了叶绿体谷氨酸转运RNA种类的结构。大麦叶绿体中除了一种tRNA(Glu)种类外，至少还含有另外两种能接受谷氨酸的转运RNA。我们现在表明，这些转运RNA的序列有显著差异：它们是tRNA(Gln)的不同修饰形式（根据其UUG反密码子判断）。这些错误负载的Glu-tRNA(Gln)种类在粗叶绿体提取物中可转化为Gln-tRNA(Gln)。该反应需要一种特定的酰胺转移酶以及谷氨酰胺或天冬酰胺作为酰胺供体。氨酰化研究表明，叶绿体、植物和动物线粒体以及蓝细菌缺乏任何可检测到的谷氨酰胺-tRNA合成酶活性。因此，这些细胞和细胞器中蛋白质合成对谷氨酰胺的需求是由附着在“错误”负载转运RNA上的谷氨酸的转化来提供的。对于几种革兰氏阳性细菌也描述了类似情况。因此，看来这种Gln-tRNA(Gln)形成途径在生物中广泛存在，并且是进化过程中保守的功能。这些发现引发了关于细胞器起源以及维持蛋白质生物合成准确性机制进化的问题。
