Cryo-EM structure and functional landscape of an RNA polymerase ribozyme

The emergence of an RNA molecule capable of replicating itself and other RNA sequences is a central pillar of hypotheses regarding the origin of life[1][1],[2][2]. In vitro evolution has yielded polymerase ribozymes (PR) that can copy a range of RNA templates using nucleotide[3][3]-[10][4] or trinucleotide triphosphates (triplets)[11][5] as substrates and may give rise to a replicase activity. However, our understanding of PR function is encumbered by a lack of structural information beyond the progenitor class I ligase (cIL) ribozyme[12][6]-[14][7]. Here, we report the structure of the complete 5TU+t1 triplet polymerase ribozyme (TPR) apoenzyme and map its structure / function landscape. The TPR is an RNA heterodimer, comprising a catalytic (5TU) and a catalytically inactive (t1) subunit held together by two kissing loop interactions and its overall structure resembles a left hand with thumb and fingers at a 70° angle. While the 5TU subunit shows partial structural homology to the cIL, the t1 accessory subunit - despite sharing the same progenitor - exhibits a dramatically reorganized secondary and tertiary structure. Our combined structural and functional data suggest a model for templated RNA synthesis by the TPR holoenzyme and provide a foundation for a better understanding of RNA’s potential for self-replication. ### Competing Interest Statement The authors have declared no competing interest. [1]: #ref-1 [2]: #ref-2 [3]: #ref-3 [4]: #ref-10 [5]: #ref-11 [6]: #ref-12 [7]: #ref-14