GAL4/UAS system

Overview
The GAL4-UAS ectopic expression system is widely used in the fruit fly, Drosophila melanogaster, for the overexpression of transgenes. This system operates under the assumption that the yeast transcription factor, GAL4, is inactive in D. melanogaster. This bipartite (two-part) expression system utilizes the yeast transcription factor GAL4 and its target sequence UAS (upstream activation sequence), to which GAL4 binds in order to activate gene transcription. GAL4 can be expressed in many different patterns by placing it under the control of various D. melanogaster promoter sequences. Since UAS promoter sequences (CGGAGTACTGTCCTCC) are not found in D. melanogaster, it is assumed that GAL4 is inactive. This is very useful for the overexpression of transgenes from a UAS promoter because GAL4 will drive expression of the transgene while not otherwise affecting the cells.

GAL4-UAS expression systems are used to artificially express genes in organisms carrying both constructs. This is achieved by crossing a driver line with a UAS line, with the gene fused to UAS being expressed in any time and location the driver gene would normally be expressed.