Rolling-circle transposons in eukaryotes

All eukaryotic DNA transposons reported so far belong to a single category of elements transposed by the so-called "cut-and-paste" mechanism. Here, we report a previously unknown category of eukaryotic DNA transposons, Helitr on, which transpose by rolling-circle replication. Autonomous Helitrons enc ode a 5 ' -to-3 ' DNA helicase and nuclease/ligase similar to those encoded by known rolling-circle replicons. Helitron-like transposons have conserva tive 5 ' -TC and CTRR-3 ' termini and do not have terminal inverted repeats , They contain 16- to 20-bp hairpins separated by 10-12 nucleotides from th e 3 ' -end and transpose precisely between the 5 ' -A and T-3 ', with no mo difications of the AT target sites. Together with their multiple diverged n onautonomous descendants, Helitrons constitute approximate to2% of both the Arabidopsis thaliana and Caenorhabditis elegans genomes and also colonize the Oriza sativa genome. Sequence conservation suggests that Helitrons cont inue to be transposed.