​Drosophila and I: A Journal of My Time with Flies 

Expression systems that utilize separately encoded transactivators and reporters/targets are called binary expression systems. That means there is one fly stock carrying the transactivator LHG as a stable insertion, and another stable stock that has the LexAop target. They need to be crossed to each other to produce a genotype where both of them are present and the expression cof the target can take place. Why not making a so called “direct construct” and hook a strong enhancer/promoter combo directly to the target gene sequence? Several reasons: One needs to decide on what enhancer/promoter combo to use, do the cloning and make the transgene. Then, you have exactly one choice where and when to express the target. With a binary system, the experimentalist can choose from a wide variety of expression patterns to express the target of choice where end when it is desired. Binary systems are simply more flexible. This system also insulates against putative toxic effects of the target. That means that even if the combination of the LHG transactivator with the LexAop target is lethal, at least you have stocks with the single lines still alive to try something else. Also note that any LexAop construct generated can now be driven in the expression domain of any particular enhancer trap. Like constructs that ablate cells, inhibit their electric activity, etc. LexA - LexAop is the second binary expression system. The first binary expression system in Drosophila is the Gal4 – UAS system generated by Andrea Brand and Norbert Perrimon in 1993.

So why do we need a second one? The LexA-LexAop system and the Gal4-UAS system do not cross talk (influence each other. Gal4 does not bind to LexAop elements, and the LexA DNA binding domain does not bind to the UAS element). Science is moving into a direction where the interaction of tissues needs to be dissected by expressing something in tissue A and something else in tissue B. So, Science needs two binary systems.