Origins of Life ~ What RNA can do?

In the early 1990s, with the invention of in vitro evolution techniques that supported the “breeding” of RNAs in a test tube, the race was on to see what the molecule was capable of. Researchers directed the evolution of RNAs that could catalyze monomer synthesis, from the production of ribose to the attachment of the sugar to nucleobases. Others bred RNA enzymes, or ribozymes, that could conduct the steps of translation, phosphorylate other polymers, join molecules together, or break them apart. “Of course the big [function] is the one that Francis Crick talked about”—self-replication, says Gerald Joyce of the Scripps Research Institute in La Jolla, California, who helped pioneer in vitro evolution techniques. “Imagine if you had an RNA enzyme that had the function of producing copies of a parent RNA molecule, including itself.”

At the turn of the century, Joyce and Scripps colleague Natasha Paul hit the jackpot. In a 2002 PNAS paper, they described an RNA molecule that could, for all intents and purposes, make copies of itself.10 It wasn’t complicated: a ribozyme that the researchers dubbed “E” joined together two component RNA pieces, “A” and “B;” when ligated, “A” and “B” made “E.”

It didn’t do much other than self-replicate, Joyce admits, but it suggested the possibility that RNA could, without experimenter intervention, evolve. “We don’t have the smoking gun of some RNA-based life form out there . . . [and] we don’t have direct fossils of the RNA world,” he says. “So then you fall back on: What can we make in the laboratory to teach us about what RNA can do?”

Joyce’s lab went on to develop a cross-replicating ribozyme system, in which each of two different small RNA molecules made copies of the other. And with a bit more directed evolution, Joyce’s PhD student Tracey Lincoln was able to improve the system’s kinetic properties such that it began replicating exponentially. “There literally was a day when the thing went critical,” Joyce recalls.

“This system is unique in the sense that it’s currently the only RNA system that replicates exponentially,” says molecular biologist Michael Robertson, a staff scientist in Joyce’s lab. “So there’s all kinds of different evolutionary experiments that you could imagine doing.”

Most recently, Joyce and Robertson evolved what they call the super-replicator, which can undergo 10100-fold amplification in 36 hours, doubling every five minutes. The replicator Joyce developed with Lincoln doubles only once every 30 minutes. “So this thing really cranks,” Joyce says of the super-replicator. “And that’s letting us now do more powerful test-tube evolution.”

Of course, these artificial systems are unlikely to resemble the first RNAs to appear on the young planet, Joyce notes. “There was no Tracey [directing evolution] on the primitive Earth. This is not that kind of game.” Devising a self-sufficient RNA system could nevertheless be informative “of the later stages, perhaps, of some kind of RNA world scenario,” says Robertson, who is now trying to evolve the ribozyme system to do something besides replicate.

Joyce admits to another motivation for his research: he’s hoping to beat the astrobiologists to the discovery of a new kind of life. “I think there’s a pretty good shot at making RNA-based life from scratch, even if it never existed on Earth, just to make a second life-form.”

References

T.A. Lincoln, G.F. Joyce, “Self-sustained replication of an RNA enzyme,” Science, 323:1229-32, 2009.

M.P. Robertson, G.F. Joyce, “Highly efficient self-replicating RNA enzymes,” Chem Biol, 21:1-8, 2014.

#pathway #RNAworld #originsoflife