Molecular trade-offs in RNA ligases affected the modular emergence of complex ribozymes at the origin of life.

In the RNA world hypothesis complex, self-replicating ribozymes were essential. For the emergence of an RNA world, less is known about the early processes that accounted for the formation of complex, long catalysts from small passively formed molecules. The functional role of small sequences has not been fully explored and, here, a possible role for smaller ligases is demonstrated. An established RNA polymerase model, the R18, was truncated from the 3' end to generate smaller molecules. All the molecules were investigated for self-ligation functions with a set of oligonucleotide substrates without predesigned base pairing. The smallest molecule that exhibited self-ligation activity was a 40-nucleotide RNA. It also demonstrated the greatest functional flexibility as it was more general in the kinds of substrates it ligated to itself although its catalytic efficiency was the lowest. The largest ribozyme (R18) ligated substrates more selectively and with greatest efficiency. With increase in size and predicted structural stability, self-ligation efficiency improved, while functional flexibility decreased. These findings reveal that molecular size could have increased from the activity of small ligases joining oligonucleotides to their own end. In addition, there is a size-associated molecular-level trade-off that could have impacted the evolution of RNA-based life.