作者:Jonathan T. Sczepanski、Gerald F. Joyce
DOI:10.1038/nature13900
日期:2014.11.20
Here, a cross-chiral RNA polymerase is developedâan RNA enzyme that can catalyse the templated polymerization of activated mononucleotides that are of the opposite handednessâshedding light on how RNA-based life could have emerged. It is widely assumed that homochirality is a requirement for life and that biological macromolecules must be of the same stereochemical 'handedness' to interact efficiently. Working with Leslie Orgel and others, Gerald Joyce extended this idea in 1984 to suggest that homochirality may also be essential for the origins of life, as templated polymerization of RNA occurs readily in a homochiral system but is impaired in racemic mixtures. Now Joyce and co-author Jonathan Sczepanski show that RNAs of opposing chirality can work together. They devised a D-RNA enzyme that catalyses the polymerization of L-RNA on a L-RNA template â and vice versa. The catalytic potency of this ribozyme is sufficient for it to synthesize its own enantiomer by joining 11 component oligonucleotides. The ribozyme is thought to interact with its substrates via tertiary contacts rather than WatsonâCrick base pairing. This unexpected finding will add a new dimension to thoughts on how life could have emerged in an 'RNA world'. Thirty years ago it was shown that the non-enzymatic, template-directed polymerization of activated mononucleotides proceeds readily in a homochiral system, but is severely inhibited by the presence of the opposing enantiomer1. This finding poses a severe challenge for the spontaneous emergence of RNA-based life, and has led to the suggestion that either RNA was preceded by some other genetic polymer that is not subject to chiral inhibition2 or chiral symmetry was broken through chemical processes before the origin of RNA-based life3,4. Once an RNA enzyme arose that could catalyse the polymerization of RNA, it would have been possible to distinguish among the two enantiomers, enabling RNA replication and RNA-based evolution to occur. It is commonly thought that the earliest RNA polymerase and its substrates would have been of the same handedness, but this is not necessarily the case. Replicating d- and l-RNA molecules may have emerged together, based on the ability of structured RNAs of one handedness to catalyse the templated polymerization of activated mononucleotides of the opposite handedness. Here we develop such a cross-chiral RNA polymerase, using in vitro evolution starting from a population of random-sequence RNAs. The d-RNA enzyme, consisting of 83 nucleotides, catalyses the joining of l-mono- or oligonucleotide substrates on a complementary l-RNA template, and similar behaviour occurs for the l-enzyme with d-substrates and a d-template. Chiral inhibition is avoided because the 106-fold rate acceleration of the enzyme only pertains to cross-chiral substrates. The enzymeâs activity is sufficient to generate full-length copies of its enantiomer through the templated joining of 11 component oligonucleotides.
这里研制了一种交叉手性的RNA聚合酶(一种能够催化与自身手性相反的活化单核苷酸模板聚合的RNA酶),为基于RNA的生命如何起源提供了启示。普遍认为手性均一是生命的要求,生物大分子必须具有一样的立体化学"手性"才能有效相互作用。与 Leslie Orgel 等人一起,Gerald Joyce于1984年提出手性均一对于生命起源可能同样是必不可少,因为在手性均一系统中RNA的模板聚合反应能顺畅进行,而外消旋混合物中反应会受阻。现在Joyce和合作者Jonathan Sczepanski表明手性相反的RNAs可以在一起工作。他们设计出一种能够催化L-RNA在L-RNA模板上聚合的D-RNA酶,反之亦然。这种核酶的催化效率足以通过连接11个寡核苷酸组份来合成其对映体。人们认为这种核酶与底物间的相互作用是通过三级结构接触而非Watson-Crick碱基配对实现的。这一发现会为"RNA世界"中的生命如何起源开启新的思路。30年前就已经表明,非酶催化的模板调控活化单核苷酸聚合反应在手性均一系统中进行顺畅,但会严重抑制于相反手性的存在。这一发现使得RNA为基础的生命如何自发产生成为一个难题,由此带来RNA可能之前存在某种不受手性抑制的其他遗传聚合物,或是基于RNA的生命之前存在化学过程打破手性对称性两种猜想。一旦产生了一种能催化RNA聚合的RNA酶,就有可能分辨两种对映体,从而发生RNA复制和基于RNA的进化。普遍认为最原始的RNA聚合酶及底物手性一致,但不一定是这种情形。复制D手性及L手性的RNA分子可能会同时出现,基于一种手性的RNA结构可以催化相反手性的活化单核苷酸的模板聚合。这里我们用随机序列RNA群体为起始的体外进化研发出这样一种交叉手性的RNA聚合酶。由83个核苷酸组成的D-RNA酶催化L手性单核苷酸或寡核苷酸底物在互补的L手性模板上聚合,反之对L-酶及D-底物和D-模板也成立。因为这种酶的106倍的速率加速只对应于交叉手性底物,从而避开了手性抑制。这种酶活性足以通过模板调控连接11个寡核苷酸产生出其对映体的全长拷贝。