ENHANCER-TRAP TARGETING AT THE BROAD-COMPLEX LOCUS OF DROSOPHILA-MELANOGASTER

Here, we describe the exact replacement of a defective unmarked P elem ent by an enhancer trap transposon marked by the miniwhite gene and ca rrying lacZ as a reporter gene. The original defective P element was l ocated in an intron of the Broad-Complex (BRC), a key gene involved in metamorphosis. Replacement events resulted from conversions induced b y the P-element transposase from a donor enhancer-trap element located on another chromosome. Six independent conversion events were selecte d. In all converted chromosomes, the enhancer-trap transposon was in t he same orientation as the original P element. From the pattern of X-g al staining observed, lacZ expression likely reflects the regulatory i nfluence of BRC enhancers on the convertant transposon. Reversion to w ild type was achieved by excision of the enhancer-trap transposon. The six convertants were analyzed in detail at the nucleotide level. The occurrence of a polymorphism at position 33 of the P-element sequences led us to propose a conversion mechanism involving homologous P seque nces for repair. This is in contrast to previously analyzed P-element transposase-induced conversion events and proposed models relying on s equence identity between genomic Drosophila sequences. The lack of any homology requirement other than between P element sequences means tha t our findings can be easily generalized. Targeting a marked P-element derivative at a precise site without loss or addition of genetic info rmation makes it possible to exploit the hundreds of defective P eleme nts scattered throughout the Drosophila genome by replacing them with engineered P elements, already available.