Saks M E, Sampson J R
Division of Biology 147-75, California Institute of Technology, Pasadena 91125, USA.
J Mol Evol. 1995 May;40(5):509-18. doi: 10.1007/BF00166619.
The aminoacylation of tRNAs by the aminoacyl-tRNA synthetases recapitulates the genetic code by dictating the association between amino acids and tRNA anticodons. The sequences of tRNAs were analyzed to investigate the nature of primordial recognition systems and to make inferences about the evolution of tRNA gene sequences and the evolution of the genetic code. Evidence is presented that primordial synthetases recognized acceptor stem nucleotides prior to the establishment of the three major phylogenetic lineages. However, acceptor stem sequences probably did not achieve a level of sequence diversity sufficient to faithfully specify the anticodon assignments of all 20 amino acids. This putative bottleneck in the evolution of the genetic code may have been alleviated by the advent of anticodon recognition. A phylogenetic analysis of tRNA gene sequences from the deep Archaea revealed groups that are united by sequence motifs which are located within a region of the tRNA that is involved in determining its tertiary structure. An association between the third anticodon nucleotide (N36) and these sequence motifs suggests that a tRNA-like structure existed close to the time that amino acid-anticodon assignments were being established. The sequence analysis also revealed that tRNA genes may evolve by anticodon mutations that recruit tRNAs from one isoaccepting group to another. Thus tRNA gene evolution may not always be monophyletic with respect to each isoaccepting group.
氨酰 - tRNA合成酶对tRNA进行氨酰化，通过决定氨基酸与tRNA反密码子之间的关联，重现了遗传密码。对tRNA序列进行分析，以研究原始识别系统的性质，并推断tRNA基因序列的进化以及遗传密码的进化。有证据表明，在三大系统发育谱系建立之前，原始合成酶就已识别受体茎核苷酸。然而，受体茎序列可能并未达到足以准确指定所有20种氨基酸反密码子分配的序列多样性水平。遗传密码进化中的这一假定瓶颈可能已因反密码子识别的出现而得到缓解。对古菌深层tRNA基因序列的系统发育分析揭示了一些由序列基序联合起来的群体，这些基序位于tRNA中参与确定其三级结构的区域内。第三反密码子核苷酸（N36）与这些序列基序之间的关联表明，在氨基酸 - 反密码子分配确立之时，类似tRNA的结构就已存在。序列分析还表明，tRNA基因可能通过反密码子突变而进化，这些突变会将tRNA从一个同功受体组招募到另一个同功受体组。因此，就每个同功受体组而言，tRNA基因进化可能并不总是单系的。
