GAL4/UAS System - Overview

Overview

The GAL4 system allows separation of the problems of defining which cells express a gene or protein and what the experimenter wants to do with this knowledge. Geneticists have created genetic varieties of fruit flies, called GAL4 lines, each of which expresses GAL4 in some subset of the fly's tissues. For example, some might express GAL4 only in muscle cells, or only in nerves, or only in the antennae, and so on. There are thousands of such lines, with the most useful expressing GAL4 in only a very specific subset of the fly - perhaps, for example, only those neurons that connect two specific compartments of the fly's brain. The presence of GAL4 in some cells has little or no effect, since GAL4 doesn't do anything if it cannot bind to a UAS region and these flies have no (or innocuous) UAS regions.

Likewise, scientists have created reporter lines, which are strains of flies with the special UAS region next to a desired gene. This is often GFP, a green fluorescent protein, or RFP, a red fluorescent protein, or channelrhodopsin, which allows light sensitive triggering of nerve cells. These genetic instructions occur in every cell of the fly, but nothing happens since these lines have no GAL4 and the proteins will only be made where GAL4 is present. The flies' behavior can be qualitatively examined, and each strain can be easily bred to keep them alive.

Scientists are able to visualize where a certain class of neurons extends to in the fly by choosing a fly from a GAL4 line that expresses GAL4 in the desired set of neurons, and crossing 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.