For the first time, scientists have produced a system of self-replicating molecules with a symbiotic relationship. The discovery, reported in today's Nature, illustrates how nonliving molecules could have organized into self-sustaining networks, a precondition for the emergence of life. Surprisingly, the molecules that do it are not nucleic acids--the basis of the cellular reproduction machinery in all living things--but proteins, which are more often thought of as helper molecules than as autonomous agents.
Last year, chemist Reza Ghadiri of The Scripps Research Institute in La Jolla, California, announced the discovery of the first protein that could reproduce itself. The presence of the protein, which is made of two fragments, sped up the self-assembly of other fragments. But, says Ghadiri, "having a replicating system doesn't make it alive." Any living system must consist not of a single type of molecule, but what Ghadiri calls a "molecular ecosystem," in which several molecules interact to promote each other's survival.
To construct such a system, Ghadiri first needed more kinds of replicating proteins; to date, his laboratory has identified about eight of them. Two of the replicators, called R-one and R-two, had one piece (called peptide E) in common, while using slightly different versions of the second piece. Because both replicators needed E to reproduce, one might expect that R-two, the more efficient reproducer, would crowd out R-one, by "survival of the fittest."
That didn't happen. In fact, each one sped up production of the other replicator more than it sped up production of its own kind, although the researchers aren't sure why. Thus, when they were together, both replicators grew as much as five times more rapidly than they would have by themselves.
"What's interesting is that this is accomplished with peptides rather than nucleic acids," says theoretical biologist Peter Wills of the University of Auckland in New Zealand, currently on sabbatical at the Santa Fe Institute in New Mexico. Although such symbiotic "hypercycles" were proposed as early as 1971 by Nobel Prize-winning chemist Manfred Eigen, says Wills, "it's been harder to find them than anyone imagined. Now that we have an example, it's not just a theoretical possibility anymore."