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The long interspersed element-1 (LINE-1, hereafter L1) retrotransposon has generated nearly one-third of the human genome and serves as an active source of genetic diversity and human disease 1 . L1 spreads through a mechanism termed target-primed reverse transcription, in which the encoded enzyme (ORF2p) nicks the target DNA to prime reverse transcription of its own or non-self RNAs 2 . Here we purified full-length L1 ORF2p and biochemically reconstituted robust target-primed reverse transcription with template RNA and target-site DNA. We report cryo-electron microscopy structures of the complete human L1 ORF2p bound to structured template RNAs and initiating cDNA synthesis. The template polyadenosine tract is recognized in a sequence-specific manner by five distinct domains. Among them, an RNA-binding domain bends the template backbone to allow engagement of an RNA hairpin stem with the L1 ORF2p C-terminal segment. Moreover, structure and biochemical reconstitutions demonstrate an unexpected target-site requirement: L1 ORF2p relies on upstream single-stranded DNA to position the adjacent duplex in the endonuclease active site for nicking of the longer DNA strand, with a single nick generating a staggered DNA break. Our research provides insights into the mechanism of ongoing transposition in the human genome and informs the engineering of retrotransposon proteins for gene therapy., (© 2023. The Author(s).)

In plants, the biogenesis of small interfering RNA (siRNA) requires a family of RNA-dependent RNA polymerases that convert single-stranded RNA (ssRNA) into double-stranded RNA (dsRNA), which is subsequently cleaved into defined lengths by Dicer endonucleases. Here, we determined the structure of maize (Zea mays) RNA-DEPENDENT RNA POLYMERASE 2 (ZmRDR2) in the closed and open conformations. The core catalytic region of ZmRDR2 possesses the canonical DNA-dependent RNA polymerase (DdRP) catalytic sites, pointing to a shared RNA production mechanism between DdRPs and plant RDR-family proteins. Apo-ZmRDR2 adopts a highly compact structure, representing an inactive closed conformation. By contrast, adding RNA induced a significant conformational change in the ZmRDR2 Head domain that opened the RNA binding tunnel, suggesting this is an active elongation conformation of ZmRDR2. Overall, our structural studies trapped both the active and inactive conformations of ZmRDR2, providing insights into the molecular mechanism of dsRNA synthesis during plant siRNA production., (© American Society of Plant Biologists 2022. All rights reserved. For permissions, please email: journals.permissions@oup.com.)