P element homing to the Drosophila bithorax complex

P elements containing a 7 kb DNA fragment from the middle of the Drosophila bithorax complex insert preferentially into the bithorax complex or into t he adjacent chromosome regions. This 'homing' property is similar to that r eported for the engrailed promoter (Hama, C., All, Z. and Kornberg, T. B. ( 1990) Genes Dev. 4, 1079-1093). The 7 kb fragment does not contain any know n promoter, but it acts as a boundary element separating adjacent segmental domains. An enhancer-trap P element was constructed with the homing fragme nt and the selectable marker flanked by FRT sites. P insertions can be trim med down by Flp-mediated recombination to just the lacZ reporter, so that t he beta-galactosidase pattern is not influenced by sequences inside the P e lement. Twenty insertions into the bithorax complex express beta-galactosid ase in segmentally limited patterns, reflecting the segmental domains of th e bithorax complex where the elements reside. The mapping of segmental doma ins has now been revised, with enlargement of the abx/bx, bxd/pbx, and the iab-3 domains. The FRT sites in the P elements permit recombination between pairs of elements on opposite chromosomes, to generate duplications or del etions of the DNA between the two insertion sites. Using this technique, th e length of the Ultrabithorax transcription unit was varied from 37 to 138 kb, but there was surprisingly little effect on Ultrabithorax function.