Autonomous transposition of the tobacco retrotransposon Tto1 in rice

The complete nucleotide sequence of the tobacco retrotransposon Tto1, one of the few active retrotransposons of plants, was determined. The sequence analysis suggests that Tto1 carries all functions required for autonomous transposition through reverse transcription. Gene organization and the nature of the transcription product suggest that Tto1 uses a gene expression mechanism different from those employed by retroviruses and most retrotransposons to regulate Gag and Pol stoichiometry. Tto1 was introduced into rice to study its autonomous transposition in heterologous hosts. Transcription and transposition of Tto1 were observed in rice cells. To probe the autonomous transposition through reverse transcription, a modified Tto1 retrotransposon in which part of a reverse transcriptase gene was replaced with an intron-containing hygromycin resistance gene was constructed and introduced into rice cells. Loss of the intron was observed only when intact Tto1 was cotransfected. These results indicate that Tto1 can transpose autonomously through reverse transcription and that the host factors required for transposition are conserved among monocots (class Magnoliopsida; rice) and dicots (class Liliopsida; tobacco), which diverged approximately 200 million years ago. These findings are discussed in relation to the regulation and evolution of retrotransposons and the possible use of Tto1 as a molecular genetic tool.