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Abstract
Transposable genetic elements (TEs) are substantial components of eukaryotic and prokaryotic genomes1. In humans, for example, ~45% of the genome consists of TEs, and, owing to their contribution to genomic instability through de novo insertion and recombination events, TEs are responsible for a number of genetic disorders and cancers2,3. LINE-1 (L1) elements are nonlong terminal repeat (non-LTR) retrotransposons that duplicate through a reverse- transcribed RNA intermediate and integrate at new genomic loci. Active, full-length L1 elementsthe only autonomous non-LTR retrotransposons in primate genomesare ~6 kb long and contain a 5 UTR with an RNA polymerase II promoter, two open reading frames (ORF1 and ORF2, encoding an RNA-binding protein and a reverse transcriptase with endonuclease activity, respectively), and a 3 UTR that harbors a guanine-rich sequence together with a polyadenylation signal and terminates with an adenine-rich tail4 (Fig. 1a). Unlike LINE elements in plants5 and in some metazoans6,7, in which conserved stem-loop secondary structures at the 3 tail control de novo insertions by recruiting the ORF2-encoded protein8, mammalian 3 UTRs of L1 elements are thought to lack such cis-regulating elements. Indeed, early studies could not confirm a functional role of the 3 UTRs of the L1 elements found in human and primate genomes9. Although the 3 UTRs of primate L1 elements are devoid of canonical secondary structures, they contain conserved guanine-rich sequences (Fig. 1a) with the ability to fold in vitro into G4 structures10,11. G4s are noncanonical secondary structures formed by guanine-rich nucleic acids and stabilized by the stacking of guanine tetrads held together by Hoogsteen base pairing12. Recent works showed that putative G4 sequences are present and conserved in specific parts of TEs in plants and humans13,14. Interestingly, young L1 remnants were found to be enriched in detectable G4-forming sequences11. On the basis of these observations, we hypothesized a functional role for G4-forming sequences within L1 elements and carried out a study to evaluate the contribution of the L1 3 UTR to retrotransposition activity.