Extrachromosomal circular forms of the tobacco retrotransposon Tto1

Extrachromosomal DNA forms of Drosophila retrotransposons (RTn) and retroviruses have been extensively analyzed. However, no such analysis with plant RTn has been reported. Here, we report the analysis of extrachromosomal forms of the tobacco RTn Tto1. Tto1 is one of a few active RTn of plants and has been shown to be activated in tissue culture. Extrachromosomal circular DNA forms of Tto1, with one or two long terminal repeats (LTR), were found in cultured cells. Sequence analysis of the sites of circularization through joining two LTR showed that the junction between the LTR contains small deletions and/or insertions. The insertions are heterogeneous and do not show any homology to the Tto1 sequence. Similar insertions have been detected in the extrachromosomal circular forms of the copia element of Drosophila and suggested to be the result of excision of genomic copia. The structural features of the junctions found in Tto1 suggest that the insertions are produced by a mechanism other than excision. The potential mechanism of production of the extrachromosomal circular forms of Tto1 is discussed.