In vitro evolution of distinct self-cleaving ribozymes in diverse environments

In vitro evolution experiments have long been used to evaluate the roles of RNA in both modern and ancient biology, and as a tool for biotechnology applications. The conditions under which these experiments have been conducted, however, do not reflect the range of cellular environments in modern biology or our understanding of chemical environments on the early earth, when the atmosphere and oceans were largely anoxic and soluble Fe(2+) was abundant. To test the impact of environmental factors relevant to RNA's potential role in the earliest forms of life, we evolved populations of self-cleaving ribozymes in an anoxic atmosphere with varying pH in the presence of either Fe(2+) or Mg(2+). Populations evolved under these different conditions are dominated by different sequences and secondary structures, demonstrating global differences in the underlying fitness landscapes. Comparisons between evolutionary outcomes and catalytic activities also indicate that Mg(2+) can readily take the place of Fe(2+) in supporting the catalysis of RNA cleavage at neutral pH, but not at lower pH. These results highlight the importance of considering the specific environments in which functional biopolymers evolve when evaluating their potential roles in the origin of life, extant biology, or biotechnology.