Unorthodox mRNA start site to extend the highly structured leader of retrotransposon Tto1 mRNA increases transposition rate.

Retroelement RNAs serve as templates for both translation and reverse transcription into extrachromosomal DNA. DNA copies may be inserted into the host genome to multiply element sequences. This transpositional activity of retroelements is usually restricted to specific conditions, particularly to conditions that impose stress on the host organism. In this work, we examined how the mRNA initiation point, and features of primary and secondary structure, of tobacco retrotransposon Tto1 RNA influence its transpositional activity. We found that the most abundant Tto1 RNA is not a substrate for reverse transcription. It is poorly translated, and its 5'-end does not contain a region of redundancy with the most prominent 3'-end. In contrast, expression of an mRNA with the 5'-end extended by 28 nucleotides allows translation and gives rise to transposition events in the heterologous host, Arabidopsis thaliana. In addition, the presence of extended hairpins and of two short open reading frames in the 5'-leader sequence of Tto1 mRNA suggests that translation does not involve ribosome scanning from the mRNA 5'-end to the translation initiation site.