Fixation and accumulation of thermotolerant catalytic competence of a pair of ligase ribozymes through complex formation and cross ligation

In the early stages of the hypothetical RNA world, some primitive RNA catalysts (ribozymes) may have emerged through self-assembly of short RNA oligomers. Although they may be unstable against temperature fluctuations and other environmental changes, ligase ribozymes (ribozymes with RNA strand-joining activity) may resolve structural instability of self-assembling RNAs by converting them to the corresponding unimolecular formats. To investigate this possibility, we constructed a model system using a cross-ligation system composed of a pair of self-assembling ligase ribozymes. Their abilities to act as catalysts, substrates, and a cross-ligation system were analyzed with or without thermal pretreatment before the reactions. A pair of self-assembling ligase ribozymes, each of which can form multiple conformations, demonstrated that thermotolerance was acquired and accumulated through complex-formation that stabilized the active forms of the bimolecular ribozymes and also cross-ligation that produced the unimolecular ribozymes.