Self-synthesising RNA strand offers clues to life’s origins

A strand of RNA just 45 nucleotides long has been discovered to perform two key reactions required for self-replication. The molecule, which is a kind of RNA enzyme, offers a tantalising clue towards understanding the origin of life, namely how non-living chemistry on prebiotic Earth started to self-replicate and evolve.

‘The two reactions don’t happen sequentially or in one pot so we are not yet at the stage of self-replication,’ explains lead researcher Eduoardo Gianni who conducted the work with colleagues in Philipp Holliger’s lab at the University of Cambridge in the UK. ‘What we describe is self-synthesis – the capacity of an RNA to copy both itself, and synthesise its encoding complementary strand. Neither of these reactions had been previously demonstrated by a polymerase ribozyme.’

Polymerase ribozymes, a kind of RNA enzyme, provide the essential catalytic machinery for the RNA world hypothesis. This theory proposes that RNA molecules emerged from a chemical soup and developed the ability to both encode genetic information and catalyse reactions for self-replication in protocells before DNA and proteins had evolved.

Polymerase ribozymes don’t exist in known lifeforms, but scientists have created them in the lab. They are known to catalyse the synthesis of other RNA molecules, suggesting their key role in the origin of life. However, ribozymes usually have long nucleotide sequences and complex folding structures, making it very unlikely they could copy themselves or emerge spontaneously on prebiotic Earth.

According to Gianni, studies on polymerase ribozymes have never been able to demonstrate self-replication. Scientists assumed that complex prebiotic chemistry and non-enzymatic replication must have enabled the emergence of large and complex ribozymes.

‘We decided to take a leap in the dark and see if we could obtain a new polymerase ribozyme lineage that might have a better chance at self-replication,’ says Gianni. Using directed evolution techniques on a collection of one trillion random RNA sequences, they arrived at a 45 nucleotide-long ribozyme, dubbed QT45.

Experiments in eutectic ice – a slushy mixture of water and salts that has been shown to promote polymerase activity and be a plausible environment on prebiotic Earth – revealed that QT45 could catalyse the synthesis of its complementary strand and also copy itself. However, for the copies, it took 72 days to get a yield of just 0.2%.

‘That is unbelievably slow,’ comments David Lilley, a molecular biologist who investigates ribozymes at the University of Dundee in the UK. ‘But this is a good step down the road to proof of principle, showing a critical element required for the plausibility of the RNA world hypothesis.’

In an RNA world scenario, shorter RNA strands would be much more abundant than longer strands. Meanwhile, previous lab experiments testing spontaneous RNA polymerisation have achieved lengths of around 20 nucleotides, which could then recombine to form RNAs the length of QT45, explains Gianni.

‘It substantially lowers the bar that needs to be achieved non-enzymatically before ribozyme-catalysed replication kicks in,’ says Gianni. ‘If this is the path needed for life to emerge, this in turn affects the overall probability of life spontaneously emerging from pure chemistry, which we can estimate to be higher than before.’

Moving forward, Gianni says the team wants to carry out the two reactions in one pot and close the cycle of self-replication. ‘We also want to improve yields to the point where the system can sustain itself, grow and begin to evolve,’ Gianni adds.