Lincoln synthesized in the laboratory a large population of variants of the RNA enzyme that would be challenged to do the job, and carried out a test-tube evolution procedure to obtain those variants that were most adept at joining together pieces of RNA.
Ultimately, this process enabled the team to isolate an evolved version of the original enzyme that is a very efficient replicator, something that many research groups, including Joyce's, had struggled for years to obtain. The improved enzyme fulfilled the primary goal of being able to undergo perpetual replication. "It kind of blew me away," says Lincoln.
Immortalizing Molecular Information
The replicating system actually involves two enzymes, each composed of two subunits and each functioning as a catalyst that assembles the other. The replication process is cyclic, in that the first enzyme binds the two subunits that comprise the second enzyme and joins them to make a new copy of the second enzyme; while the second enzyme similarly binds and joins the two subunits that comprise the first enzyme. In this way the two enzymes assemble each other what is termed cross-replication. To make the process proceed indefinitely requires only a small starting amount of the two enzymes and a steady supply of the subunits.
"This is the only case outside biology where molecular information has been immortalized," says Joyce.
Not content to stop there, the researchers generated a variety of enzyme pairs with similar capabilities. They mixed 12 different cross-replicating pairs, together with all of their constituent subunits, and allowed them to compete in a molecular test of survival of the fittest. Most of the time the replicating enzymes would breed true, but on occasion an enzyme would make a mistake by binding one of the subunits from one of the other replicating enzymes. When such "mutations" occurred, the resulting recombinant enzymes a
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Scripps Research Institute