RNA replication in prebiotic and synthetic biology
Dr. Hannes Mutschler, MRC Laboratory of Molecular Biology Cambridge
There is strong evidence for a primordial biology, in which RNA was the central biomolecule responsible information storage and catalysis. A key component of this ‘RNA world’ would have been ribozymes that were capable of catalysing their own replication as well as replication of a nascent RNA genome. The closest known analogue of such primordial replicases are RNA polymerase ribozymes (RPRs) identified by directed evolution. Some RPR variants are able to synthesize another ribozyme or RNA oligomers exceeding their own size (~200 nt). However, a general replication of longer RNAs by RPRs or non-enzymatic processes is impeded by the related problems of template secondary structure and the high stability of RNA duplexes. Moreover, non-enzymatic polymerisation of RNA from all four natural nucleotides yields oligomers barely exceeding ~20 nt, even under favourable conditions. Thus, it appears that early molecular evolution was limited to short RNA oligomers, which are seemingly unable to create the phenotypically complex ribozymes required for enzymatic self-replication.
To close this conceptual gap between the primitive RNA oligomers and complex ribozymes, we are currently exploring the ability of short ribozymes to synthesize longer RNAs with sophisticated functions. Working backwards from one of the most advanced RPRs, we show generation of polymerase function from mixtures of RNA oligomers no longer than 30 nt. We discover that a cyclic physicochemical process – freeze-thaw cycling – is a potent driver of this assembly reaction and critical to unlocking the functional potential of the short RNA modules.
In the light of a hypothesised “ribo-organism” before the advent of modern cells, I will also briefly discuss the potential of protein-catalysed RNA replication for the bottom-up synthesis of minimal evolving biomimetic systems.