If there is one theme uniting life-forms from the lowliest virus to the loftiest primate, it's that they all evolve. Now some lifeless strands of RNA are doing the same thing in a California laboratory. The animated molecules, described in today's issue of Science,* promise to be a powerful new tool in understanding the concept of evolution.
For several years, biochemists have been enticing ribozymes--RNA strands that catalyze chemical reactions--to "evolve" by creating a batch of different RNA molecules and selecting the best catalyzers among them. The fittest molecules are copied, mutated to create new variants, and so on through many cycles of selection. But the process requires human intervention and takes a long time. Besides, says Gerald Joyce, a biochemist at The Scripps Research Institute in La Jolla, California, it "has an artificial feel to it."
Joyce and Scripps colleague Martin Wright set out to eliminate the artificial selection step. The duo devised a replication "engine": a set of enzymes, including RNA polymerase, that makes copies of the ribozymes. For any ribozyme to access the engine, it must first attach to a string of nucleotides, which activate the RNA polymerase. The polymerase doesn't copy the ribozyme perfectly; it introduces variations, which are the raw material of evolution. Ribozymes that attach more tightly and quickly to the nucleotides are more likely to get replicated. "The ones with the best access to the machine have the most progeny," says Andrew Ellington, a biochemist at Indiana University in Bloomington. "It's survival of the fittest; the molecules are duking it out." Because ribozymes replicate in a matter of minutes, the population changes quickly over time, so long as it has a steady supply of nucleic acids.
"It's a neat system," says Ellington. "You feed the thing and it takes off." Indeed, the replicating ribozymes, which could be used to model evolutionary processes, bear an uncanny resemblance to reproducing life-forms. Says Ellington, they evolve in test tubes "just like bacteria" do.