Role of reverse transcription in the generation of extrachromosomal copia mobile genetic elements

The Drosophila genetic element copia is one of the best studied eukaryotic transposable sequences. Copia shares structural features with a wide variety of mobile elements in Drosophila, Lepidoptera, yeast and vertebrates, the last class being the retrovirus proviruses. Furthermore, retrovirus-like particles containing copia RNA have been isolated from Drosophila cells and extrachromosomal circular copias with structures closely resembling circular retrovirus proviruses have been isolated and cloned. Therefore, copia-like elements and retroviruses may be members of a class of mobile genetic elements existing throughout the eukaryotic kingdom. Consequently, there has been speculation that retroviruses evolved from transposable elements and, conversely, that copia-like elements transpose as retroviruses or retrovirus-like particles. To date, however, there has been no demonstration that copia RNA is reverse transcribed into copia DNA. The present report describes the isolation of linear extrachromosomal copias whose structure closely resembles the analogous retrovirus provirus linears and whose synthesis is unaffected by inhibitors of the cellular DNA polymerase responsible for chromosomal DNA replication.