RETROTRANSPOSITION OF A MARKED DROSOPHILA LINE-LIKE ELEMENT IN CELLS IN CULTURE

We have marked a Drosophila transposable element-the LINE-like I eleme nt-with an intron-containing indicator gene inserted in place of a lar ge deletion in the I element second ORF encompassing the reverse trans criptase domain, and this marked element was placed downstream to a po tent actin promoter. An expression vector for the I element ORFs was a lso constructed, under the same heterologous promoter. The indicator g ene contains a lacZ reporter gene the expression of which is condition ed by retrotransposition of the marked element, thus allowing detectio n of transposition events by testing for either beta-galactosidase exp ression or occurrence of spliced DNA molecules. The marked I element w as introduced into Drosophila melanogaster cells in culture by transfe ction. Spliced DNA copies of the marked element and specifically stain ed beta-galactosidase-expressing cells were detected only upon co-tran sfection with the I expression vector, thus indicating that an ORF2-de leted element can be complemented in trans for transposition. This sim ple assay for retrotransposition in Drosophila cells in culture provid es a tool for the rapid analysis of the mechanism of I transposition i n its cis and trans sequence requirements.