RESCUE OF DROSOPHILA ENGRAILED MUTANTS WITH A HIGHLY DIVERGENT MOSQUITO ENGRAILED CDNA USING A HOMING, ENHANCER-TRAPPING TRANSPOSON

Specific fragments of Drosophila regulatory DNA can alter the insertio nal specificity of transposable elements causing them to 'home' to the ir parent gene. We used this property to insert a transposon-encoded f unctional coding region near a defective one and rescue a null mutatio n. This approach differs from homologous recombination in that the end ogenous defective coding region is left in place and the genomic DNA i s altered by the addition of the therapeutic transposon. We constructe d a P-element-based transposon in which an engrailed cDNA from Anophel es gambiae (a mosquito) is expressed from a Drosophila engrailed minim al promoter. The promoter fragment used includes 2.6 kb of regulatory DNA that causes transposons to home to the endogenous Drosophila engra iled gene at high frequencies. We inserted this transposon onto a Dros ophila chromosome that produces no functional engrailed proteins. When this transposon integrated near the engrailed promoter, adult viabili ty was restored to engrailed mutant flies showing that the highly dive rgent mosquito engrailed protein can replace the Drosophila engrailed protein at all stages of development. Insertion of this transposon int o the adjacent invected gene, which is transcribed in a pattern simila r to engrailed, led to only embryonic rescue, suggesting an important difference in the regulation of these two genes.