Transcription polymerase–catalyzed emergence of novel RNA replicons

Revisiting replicating RNAs DNA-dependent RNA polymerases are well known for their ability to produce RNA from DNA templates. Much less is known about their noncanonical activity: the generation and replication of RNA from RNA templates. Deeper insights into this process could shed light on questions relating to the origin of life, molecular evolution, and the replication of certain RNA pathogens such as hepatitis delta virus and plant viroids. Jain et al. explore in detail how the bacteriophage T7 RNA polymerase generates and amplifies diverse RNA sequences in vitro. Using sequencing, microfluidics, and bioinformatics, they chart the emergence and evolution of replicating RNA motifs and identify mechanisms that explain their selection and structures. Science , this issue p. eaay0688