Evolution of ribozymes in the presence of a mineral surface.

Mineral surfaces are often proposed as the sites of critical processes in the emergence of life. Clay minerals in particular are thought to play significant roles in the origin of life including polymerizing, concentrating, organizing, and protecting biopolymers. In these scenarios, the impact of minerals on biopolymer folding is expected to influence evolutionary processes. These processes include both the initial emergence of functional structures in the presence of the mineral and the subsequent transition away from the mineral-associated niche. The initial evolution of function depends upon the number and distribution of sequences capable of functioning in the presence of the mineral, and the transition to new environments depends upon the overlap between sequences that evolve on the mineral surface and sequences that can perform the same functions in the mineral's absence. To examine these processes, we evolved self-cleaving ribozymes in vitro in the presence or absence of Na-saturated montmorillonite clay mineral particles. Starting from a shared population of random sequences, RNA populations were evolved in parallel, along separate evolutionary trajectories. Comparative sequence analysis and activity assays show that the impact of this clay mineral on functional structure selection was minimal; it neither prevented common structures from emerging, nor did it promote the emergence of new structures. This suggests that montmorillonite does not improve RNA's ability to evolve functional structures; however, it also suggests that RNAs that do evolve in contact with montmorillonite retain the same structures in mineral-free environments, potentially facilitating an evolutionary transition away from a mineral-associated niche.