Exobiology and Evolutionary Biology

Empirical Examination of Hypercycles in the RNA World

PI: Niles Lehman

The origins of life on the Earth required the establishment of self-replicating systems capable of maintaining and evolving biological information. Regarding the means by which chemical assemblages self-assembled into ordered such systems, both the RNA World hypothesis and the hypercycle concept have strongly influenced how we envision this happening some 4 billion years ago. The RNA World idea takes the observed catalytic properties of RNA and proposes that RNA or a similar molecule first displayed the self-replication needed for life as-we-know-it to emerge. The hypercycle was developed by Eigen and Shuster to explain how sets of molecules could cooperate during primordial evolution and thus overcome many theoretical barriers to the advent of self-replication. A hypercycle is a functional organization of molecules in which separate genotypes cooperatively interact in a cyclical network to produce an evolutionary stable coalition that has a large information capacity. They are believed to be a realistic manner in which naked molecules could have self-organized and evolved prior to encapsulation in cell-like structures that could successfully compete as selfish replicators.

While the hypercycle is a powerful idea, it has not been yet empirically observed in a molecular system composed solely of nucleic acids. In this proposal, we will examine the ability of pools of RNA molecules to establish hypercyclic networks without the aid of outside agents. First, we will set up a list of a priori conditions that would constitute a hypercyclic system. These include mutualistic interdependencies among components in a network of molecules, hyperbolic growth of the system as a whole, and an approach of the concentrations of each component to a stable value over time (measured, for example, in a simplex diagram). Then, we will adapt our set of self-assembling and self-replicating RNA fragments, developed in the previous NASA Exobiology granting period, to several experimental designs that can potentially demonstrate that hypercycles can indeed be established empirically.

The RNA system we will use involves the fragments of the Azoarcus group I intron ribozyme that we have shown can cooperate to create a self-replicating entity. The bulk of the proposed work will be to engineer combinations of these fragments so that they will create sets that cannot self-assemble independently, but will when combined with other sets. Specifically, we will attempt to construct hypercyclic arrangements of three sets that can create hypercyclic networks of the third order (n = 3; p = 3). We can test the ability of these arrangements to satisfy the criteria of hypercycles, by for example demonstrating that a simplex plot of concentrations converges on a fixed value with kinetics characteristic of hypercycles. Finally, we will undertake in vitro selection experiments designed to test the hypothesis that hypercyclic sets can out-compete selfish assemblages of RNA molecules, thereby examining the power and relevance of hypercycles to the origins of genetic information on the early Earth and elsewhere in the universe, a clear objective of NASA’s mission. We can accomplish these experiments inexpensively with graduate and undergraduate students at Portland State University in three years’ time.