View Program - 63rd Annual Drosophila Research Conference

The invected (inv) and engrailed (en) genes are expressed throughout development in Drosophila melanogaster. Distinct enhancers drive the co-expression of inv and en in discrete parts of the embryos including in stripes, parts of the head, and the CNS. Inv/En are also co-expressed in the anterior/posterior compartment of larval imaginal discs. The inv and en promoters are separated by ∼54 kb and their expression is regulated by enhancers distributed across a ∼100 kb region. One enhancer that regulates expression in imaginal discs is located 40kb and 90kb upstream of the en and inv promoters. How does this imaginal disc enhancer find the en/inv promoters? Previous results from our lab suggested that there is a promoter-proximal tethering element (PTE) that facilitates this activation. We are interested in understanding how this PTE functions. Our lab isolated a transgenic line containing a 2 kb en regulatory fragment, including the PTE, fused to a Beta-galactosidase reporter gene, in a P-element vector including the mini-white reporter gene, inserted 250bp upstream of the endogenous en promoter. Flies heterozygous for the transgene show expression of Beta-galactosidase and en in the posterior compartment of the wing imaginal discs. However, homozygotes show a reduction in en expression in wing imaginal and die as pharate adults with no thorax. These results indicate that the imaginal disc enhancers are hijacked by the transgene, causing a tissue-specific loss-of-function phenotype. We are investigating which sequences are required to mediate enhancer hijacking by the transgene using the Cre-loxP recombination system to insert various transgenes 250bp upstream of the en promoter. We have identified a 181bp DNA fragment that, when combined with a minimal promoter, captures imaginal disc enhancers and disrupts en expression. Interestingly, this fragment also acts as a Polycomb response element (PRE) in which it can recruit Polycomb group proteins. In addition to identifying sequences necessary for enhancer-promoter communication, our study shows that interfering with promoter-enhancer communication is another way to generate a tissue-specific mutant phenotype.

Enhancer hijacking leads to flies with no thorax