Extensions
GAL4 expression can be made even more specific by means of intersectional strategies. These can combine two different GAL4 lines, say A and B, such that GAL4 is only expressed in the cells that are in line A but not line B, or those that are in both lines A and B. When combined with intrinsically sparse GAL4 lines, this offers very specific selection, often limited to a single cell type. The disadvantage is that at least 3 independent insertion sites are required, so the lines must use different and independent insertion sites, and creating the desired final organisms needs more than a single cross. This is a very active field of research, and there are many such intersectional strategies, of which two are discussed below.
One way to create GAL4 expression in the cells that are in line A but not line B, requires line A to be made to express GAL4, and line B made to express GAL80, which is a GAL4 inhibitor. Therefore only the cells that are in A but not B will have active GAL4, which can then drive the reporter gene.
To express GAL4 in only the cells contained in both A and B, a technique called split-GAL4 can be used. Line A is made to express half of the GAL4 protein, which is inactive by itself. Similarly, line B is made to express the other half of GAL4, also inactive by itself. Only the cells that are in both lines make both halves, which self-assemble by leucine zipper into GAL4 and activate the reporter gene.
Read more about this topic: GAL4/UAS System
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