Fig. 1.

The GAL4/UAS system can be used to express foreign viral proteins in vivo in Drosophila melanogaster. A gene of interest is placed next to the upstream activating sequence (UAS), which allows activation of gene expression by the GAL4 transcription factor. Flies with a UAS-responsive transgene are crossed with other flies with a specific GAL4 driver gene. In this illustrated example, the offspring has a GAL4 gene under control of the actin promoter, which expresses GAL4 ubiquitously throughout development. GAL4 then binds to UAS and turns on the viral gene in all cells that express GAL4. If expression had to be targeted to a specific subset of cells, then a different GAL4 line would be used to restrict viral gene expression to specific tissues. For example, GAL4 expression could be targeted to the eye where the viral gene would only be expressed in those cells but not in the other tissues of the organism.