Quadruplex-forming DNA sequences spread by retrotransposons may serve as genome regulators

Transposable elements (TEs) are ubiquitous genome inhabitants in eukaryotes. Increasing evidence shows that TEs are involved in regulatory networks of eukaryotic cells and contribute to genome evolution. Recently, we reported that many plant long-terminal repeat (LTR) retrotransposons contain DNA quadruplex-forming sequences at precise positions inside their LTRs and that quadruplexes are better preserved in evolutionary younger elements. As quadruplexes can modulate molecular processes, quadruplexes found at specific distances upstream and downstream from the endogenous TE promoter can affect transcription of the element. Moreover, quadruplexes found in solo LTRs, as well as in 3' ends of 5'-truncated copies of LINE-1 elements, can affect expression of neighboring genes. Here, we propose that this way retrotransposons can serve as vehicles for spread of DNA quadruplexes. Quadruplexes can thus fulfill a dual regulatory role-to influence both the retrotransposons carrying them and the neighboring host genes, e.g., by direct effect on transcription or by modifying the local chromatin state. Additionally, four-stranded DNA structures may serve as hotspots for recombination-based genome rearrangements.