Roles of dimeric intermediates in RNA-catalyzed rolling circle synthesis.

The RNA world hypothesis is supported by the discovery of RNA polymerase ribozymes that can perform RNA-catalyzed RNA replication processes on different RNA templates. Recently, RNA-catalyzed rolling circle synthesis (RCS) on small circular RNA (scRNA) templates has been demonstrated. However, the structural and dynamic properties of scRNA replication and its products and intermediates have not been explored. Here, we have used cryogenic electron microscopy (cryo-EM) to characterize products and intermediates relevant for RCS replication. We find that these form an unexpectedly diverse group of RNA nanostructures. The main structural motif observed is a fully hybridized dimeric complex composed of two scRNAs and their complement strands resolved to 5.3 Å. Cryo-EM also reveals higher-order dimer filaments and dimer assembly intermediates, suggesting an assembly mechanism for the observed complexes. We show that the dimer complexes are stable and inhibit RNA-catalyzed RCS but can be reactivated by addition of more scRNA templates. We propose dimer formation as a general property of RCS replication and speculate that dimers might have benefited a primordial RNA genetic system by providing a stable ''storage'' form for RNA replication products and by coordinated RNA replication on both scRNA template strands.