It began with cooperation. When life first arose, teams of small molecules got together to perform tasks none could manage alone or so the theory goes. For the first time, networks like this have been built in the lab.
The earliest life may have been a primordial soup of RNA molecules, but the first crude self-replicating molecules in this “RNA world” would have faced a big problem. They had to grow to store more information, but that made copying errors more likely. Get big enough and these errors become almost certain, destroying the molecule’s information.
In theory, the first replicators could have avoided this “error catastrophe” by splitting their information between several cooperating molecules. Then the network could function as long as copies of each molecule survived.
Repair one for the team
To see if this strategy would work, Niles Lehman of Portland State University in Oregon and colleagues created three RNA molecules that could repair each other – A did B, B did C, and C did A.
When the team put these broken molecules together in a test tube, the collective network worked well. When they pitted the cooperative network against a selfish, self-repairing molecule, the cooperators won out.
Although earlier studies showed that pairs of molecules can cooperate, Lehman is the first to create a network of 3, opening the door to much larger networks. “If you can go from 2 to 3, you can go from 3 to infinity,” he says. Lehman repeated the study with 48 different fragments of an RNA molecule. Sure enough, they assembled into a network that eventually included all 48.
Such cooperation may have arisen early in the RNA world and helped to build complexity, says Gerald Joyce of the Scripps Research Institute in San Diego. “It’s an experimental demonstration that real molecules can do this,” he says.
Cooperating RNA networks might have an even greater advantage if the component molecules could cluster together in space.
To show this, Philip Bevilacqua and colleagues at Penn State University in University Park studied an RNA called a “hammerhead ribozyme” that cuts itself into pieces. They helped the RNAs to cluster by putting them into a solution containing both dextran and polyethylene glycol. These two compounds separate instead of mixing, causing the ribozyme, which is more soluble in the dextran portion, to become more concentrated.
They found this increased the RNA’s reaction rate about 70-fold (Nature Chemistry, doi.org/jjk). Something similar – a pore on a rock surface, say, or a slime layer – could have given prebiotic molecules a boost as life got started, says Joyce.
Journal reference: Nature, DOI: 10.1038/nature11549
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