Retrotransposition of a marked Drosophila line-like I element in cells in culture

We have marked a Drosophila transposable element- the LINE-like I element-with an intron-contalnlng indicator gene inserted in place of a large deletion in the I element second ORF encompassing the reverse transcriptase domain, and this marked element was placed downstream to a potent actin promoter. An expression vector for the I element ORFs was also constructed, under the same heterologous promoter. The indicator gene contains a lacZ reporter gene the expression of which is conditioned by retrotransposition of the marked element, thus allowing detection of transposition events by testing for either β-galacto-sidase expression or occurrence of spliced DNA molecules. The marked I element was introduced into Drosophila melanogaster cells in culture by transfectlon. Spliced DNA copies of the marked element and specifically stained β-galactosldase-expresslng cells were detected only upon co-transfectlon with the I expression vector, thus indicating that an ORF2-deleted element can be complemented in trans for transposition. This simple assay for retrotransposition in Drosophila cells in culture provides a tool for the rapid analysis of the mechanism of I transposition in its els and trans sequence requirements.