Di Giulio M
International Institute of Genetics and Biophysics, CNR, Via G. Marconi 10, 80125 Naples, Italy.
J Theor Biol. 1999 Apr 7;197(3):403-14. doi: 10.1006/jtbi.1998.0882.
The hypothesis that the tRNA molecule may have originated from the assembly of two similar RNA hairpin structures is utilised to understand the evolutionary period in which this molecule originated. Consistent with the exon theory of genes is the observation that the introns in tRNA genes are found almost exclusively in the anticodon loop and "stitched together" the two halves of the molecule, which originally may have been simply two hairpin structures and which can still be observed in the three-dimensional structure of tRNAs. This theory therefore considers these hairpin structures as minigenes on which complex protein synthesis may have been achieved. This in turn leads to the belief that the organisation of the genetic code may have been determined by use of the hairpin structures but not the complete tRNA molecule. In view of this, it can be conjectured that tRNA molecules might have been assembled only after the establishment of the main phyletic lines. If this is all true, then the origin of the tRNA molecule might have been non-monophyletic, i.e. a tRNA specific for a certain amino acid might have been assembled in different phyletic lines with a second and different hairpin structure. This leads to the belief that tRNAs specific for different amino acids but belonging to the same phyletic line might have been more similar to one another than to tRNAs specific for the same amino acid but belonging to different phyletic lines. This prediction seems to be supported by phylogenetic analysis making major use of the bootstrap technique performed on the tRNA sequences and by analysis already existing in the literature which supports the non-monophyletic origin of the tRNA molecule. The main conclusion of this paper is that if the tRNA molecule was assembled in the main phyletic lines this would imply a still rapidly evolving translation apparatus which, in turn, seems to imply that the last universal common ancestor was a progenote.
tRNA分子可能起源于两个相似RNA发夹结构的组装这一假说，被用于理解该分子起源的进化时期。与基因外显子理论一致的是，观察发现tRNA基因中的内含子几乎只存在于反密码子环中，并将分子的两半“缝合”在一起，这两半最初可能只是两个发夹结构，并且在tRNA的三维结构中仍然可以观察到。因此，该理论将这些发夹结构视为可能实现复杂蛋白质合成的微型基因。这进而导致一种观点，即遗传密码的组织可能是由发夹结构而非完整的tRNA分子决定的。鉴于此，可以推测tRNA分子可能是在主要种系形成之后才组装的。如果这一切都是真的，那么tRNA分子的起源可能是非单系的，即特定氨基酸的tRNA可能是在不同种系中与第二个不同的发夹结构组装而成。这导致一种观点，即属于同一种系的不同氨基酸特异性tRNA可能彼此之间比属于不同种系的相同氨基酸特异性tRNA更为相似。这一预测似乎得到了主要利用对tRNA序列进行的自展技术的系统发育分析以及文献中已有分析的支持，这些分析支持tRNA分子的非单系起源。本文的主要结论是，如果tRNA分子是在主要种系中组装的，这将意味着翻译装置仍在快速进化，而这反过来似乎意味着最后的普遍共同祖先是一个原祖。
