The RNA i-Motif in the Primordial RNA World.

The primordial RNA world is a hypothetical era prior to the appearance of protein and DNA, when RNA molecules were the sole building blocks for early forms of life on Earth. A critical concern with the RNA-world hypothesis is the instability of the cytosine nucleobase compared to the other three bases (adenine, guanine, and uracil). The author proposes that cytosine residues could have stably existed in the primordial world in the RNA i-motif, a four-stranded quadruplex structure formed by base-pairing of protonated and unprotonated cytosine residues under acidic conditions. The i-motif structure not only increases the lifetime of cytosine residues by slowing their deamination rate, but could also allow RNA polymers to bind to certain ligands (e.g., anions) to perform critical functions. Future studies focused on determining the rate of cytosine deamination in RNA i-motifs over a range of pH, temperature, and pressure conditions, and on interrogating the interactions between ligands and RNA i-motifs, could uncover new evidence of the origin of life on Earth.