12 January 2009
Knocking on the door of life: Self-replicating RNA synthesized
by Kate Melville
Scientists have long known that DNA carries the genetic sequence for advanced organisms and that RNA is dependent on DNA for performing roles such as building proteins. But one prominent theory about the origins of life, called the RNA World model, postulates that because RNA can function as both a gene and an enzyme, it might have arrived on the scene before DNA and acted as the ancestral precursor for all life.
A stumbling block for this theory, however, is that the process of copying a genetic molecule (considered a basic qualification for life) appears to be exceedingly complex, involving many proteins and other cellular components. For years, researchers have wondered whether there might be some simpler way to copy RNA, brought about by the RNA itself. Some tentative steps along this road have previously been taken, but no one has been able to demonstrate that RNA replication could be self-propagating, that is, result in new copies of RNA that also could copy themselves.
Now, however, two Scripps Research Institute scientists have taken a significant step toward confirming the viability of the RNA World model. For the first time, they have synthesized RNA enzymes that can replicate themselves indefinitely without the help of any proteins or other cellular components.
Reporting their work in Science, Scripps' Tracey Lincoln and Gerald Joyce explained how their breakthrough began with a method of forced adaptation known as in vitro evolution. The ultimate goal was to take one of the RNA enzymes already developed in the lab that could perform the basic chemistry of replication, and improve it to the point that it could drive efficient, perpetual self-replication.
This involved synthesizing a large population of variants of the RNA enzyme that then underwent 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 was a very efficient replicator. The improved enzyme fulfilled the primary goal of being able to undergo perpetual replication. "It kind of blew me away," says Lincoln.
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 (known as 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," notes Joyce.
The subunits in the enzymes the team constructed each contain many nucleotides, so they are relatively complex and not something that would have been found floating in the primordial ooze. But, while the building blocks likely would have been simpler, the work does finally show that a simpler form of RNA-based life is at least possible.
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 also were capable of sustained replication, with the most fit replicators growing in number to dominate the mixture. "To me that's actually the biggest result," says Joyce.
The research shows that the system can sustain molecular information, a form of heritability, and give rise to variations of itself in a way akin to Darwinian evolution. "What we have is non-living, but we've been able to show that it has some life-like properties, and that was extremely interesting," explains Lincoln.
The main value of the work, according to Joyce, is at the basic research level. "What we've found could be relevant to how life begins, at that key moment when Darwinian evolution starts." He is quick to point out that, while the self-replicating RNA enzyme systems share certain characteristics of life, they are not themselves a form of life. But, he adds, "it might tell us how you go about starting the process of understanding the emergence of life in the lab."
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Source: Scripps Research Institute