Upregulation of theDrosophila1731 Retrotransposon Long-Terminal Repeat by UV-B Irradiation Requires a Short Sequence in the U3 Region

A 1731 is aDrosophila melanogasterretrotransposon, the transcripts of which decrease inDrosophilacells after treatment by the 20-hydroxyecdysone (20-OH), the steroid-molting hormone of insects. In order to analyze the regulation of the long terminal repeat (LTR) directed transcription by UV-B, S2Drosophilacells were transfected with various chimeric constructs carrying the LTR of 1731 linked to the bacterial chloramphenicol acetyltransferase (CAT) reporter gene and then subjected to UV-B irradiation. The results demonstrate that the 1731 LTR is activated by UV-B irradiation in a dose- and time-dependent manner. Using constructions expressing the reporter gene under the control of either the entire or deleted LTRs of 1731, we established that a sequence located in the U3 region was required for the retrotransposon to respond to UV-B. Thecis-acting element is identical to the binding sequence of the dorsal transcription factor. In addition, the S2Drosophilacell produced extracellular factor(s) in response to UV-B irradiation which activated the 1731 LTR in nonirradiated cells. This factor(s) was detected when responding cells were cocultured with inducing cells and when conditioned medium from irradiated cultures was added to the cell cultures.