Di Giulio M
International Institute of Genetics and Biophysics, CNR, Via G. Marconi 10, 80125 Naples, Italy.
J Theor Biol. 1998 Mar 21;191(2):191-6. doi: 10.1006/jtbi.1997.0580.
The origin of the organisation of the genetic code reflects both the biosynthetic relationships between amino acids and the physicochemical interactions between these and anticodons; moreover, these two forces do not act independently. It must therefore be explained why it is simultaneously true that the anticodons of product amino acids were assigned prior to their biosynthetic appearance and that physicochemical correlations must exist between anticodons and amino acids. This gives rise to difficulties of interpretation, even of a more general nature, in the theories that have been proposed to explain the origin of the genetic code; a hypothesis is thus presented in this manuscript. In particular, the hypothesis suggests that RNA hairpin structures, the ancestors of tRNAs, housing anticodon-like nucleotides in the stem were charged with precursor amino acids. As the precursor amino acids gradually developed into product amino acids, a coevolution came into being between the development of amino acids and that of anticodons, with a concomitant formation of the complete tRNA molecule thought direct duplication of the hairpin structures. All this led to the definition of the genetic code organisation. Furthermore, this made it possible for the evolving anticodons to select the emerging product amino acids, which the hypothesis therefore considers to be unspecified in the initial phase of genetic code evolution but which were selected also because of their ability to interact with anticodons. In this way the main obstacle to interpretation is removed. Fossils of these events can now be observed in some amino acid-modified nucleosides specifically located in the tRNA anticodon loops. This is all presented in the framework of a general discussion of the ideas and data in favour of a late origin of the genetic code, as opposed to an early origin in the context of the theories proposed to explain the origin of the genetic code organisation.
遗传密码组织的起源既反映了氨基酸之间的生物合成关系，也反映了这些氨基酸与反密码子之间的物理化学相互作用；此外，这两种力量并非独立起作用。因此，必须解释为什么会同时出现这样的情况：产物氨基酸的反密码子在其生物合成出现之前就已被指定，而且反密码子与氨基酸之间必然存在物理化学相关性。这在已提出的用于解释遗传密码起源的理论中引发了诠释上的困难，甚至是更普遍性质的困难；因此，本文稿提出了一个假说。具体而言，该假说认为，作为tRNA祖先的RNA发夹结构，其茎中含有类似反密码子的核苷酸，被装载上了前体氨基酸。随着前体氨基酸逐渐发展为产物氨基酸，氨基酸的发展与反密码子的发展之间形成了共同进化，同时通过发夹结构的直接复制形成了完整的tRNA分子。所有这些导致了遗传密码组织的确定。此外，这使得不断进化的反密码子能够选择新出现的产物氨基酸，因此该假说认为这些氨基酸在遗传密码进化的初始阶段是未确定的，但它们也因其与反密码子相互作用的能力而被选择。通过这种方式，诠释的主要障碍得以消除。现在可以在一些特异性位于tRNA反密码子环中的氨基酸修饰核苷中观察到这些事件的化石痕迹。这一切都是在对支持遗传密码晚期起源的观点和数据进行一般性讨论的框架内呈现的，这与在为解释遗传密码组织起源而提出的理论背景下的早期起源形成对比。
