First developed by Andrea Brand and Norbert Perrimon in 1993, the GAL4-UAS system has since been considered a powerful biochemical method for studying genetic expression. There are two parts to the system, the GAL4 gene (containing the GAL4 transcription-activating protein), and the UAS (Upstream Activating Sequence). The UAS lies in the promoter region of the second chromosome. The GAL4 protein binds to this region.
For a bigger picture of a frequent application of the GAL4-UAS system, take this example (Wikipedia), "For example, suppose a scientist wants to visualize where a certain class of neurons extends to in the fly. He/she can then pick a fly from a GAL4 line that expresses GAL4 in the desired set of neurons, and cross it with a reporter line that express GFP. In the offspring, the desired subset of cells will make GAL4, and in these cells the GAL4 will bind to the UAS, and enable the production of GFP. So the desired subset of cells will now fluoresce green and can be followed with a microscope. Next suppose instead of looking at the cells, the experimenter wants to figure out what these cells do? One way is to express channelrhodopsin in each of these cells, by crossing the same GAL4 line with a channelrhodopsin reporter line. In the offspring the selected cells, and only those cells, will contain channelrhodopsin and can be triggered by a (bright) light. Now the scientist can trigger these particular cells at will, and perhaps find out what they do."
For a bigger picture of a frequent application of the GAL4-UAS system, take this example (Wikipedia), "For example, suppose a scientist wants to visualize where a certain class of neurons extends to in the fly. He/she can then pick a fly from a GAL4 line that expresses GAL4 in the desired set of neurons, and cross it with a reporter line that express GFP. In the offspring, the desired subset of cells will make GAL4, and in these cells the GAL4 will bind to the UAS, and enable the production of GFP. So the desired subset of cells will now fluoresce green and can be followed with a microscope. Next suppose instead of looking at the cells, the experimenter wants to figure out what these cells do? One way is to express channelrhodopsin in each of these cells, by crossing the same GAL4 line with a channelrhodopsin reporter line. In the offspring the selected cells, and only those cells, will contain channelrhodopsin and can be triggered by a (bright) light. Now the scientist can trigger these particular cells at will, and perhaps find out what they do."
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