Fig. 5. Identification of the promoter-tethering element. On the B-A-5 construct (top diagram), the IAB5 enhancer specifically bypasses the abd-A promoter (B) to activate expression from the Abd-B promoter (A). When the 255 bp region extending from -40 to -294 bp relative to the transcription start site is removed from the Abd-B promoter (B^PTE-A-5 construct; middle diagram), the IAB5 enhancer is now re-directed to the abd-A promoter, as no CAT expression is detected (C) and strong lacZ expression is detected (D) in the majority of embryos. Extremely weak CAT expression in an IAB5-directed pattern was detected in a few transgenic embryos (data not shown). The integrity of the Abd-B^PTE promoter was confirmed by insertion of the IAB2 enhancer 3' of the Abd-B^PTE-CAT reporter gene on the 2-B^PTE-A-5 construct (bottom diagram). This resulted in IAB2 activation of the Abd-B^PTE promoter indicated by an IAB2-driven CAT expression pattern in parasegments 7 (black arrowhead), 9, 11 and 13 in transgenic embryos (E), whereas lacZ is activated in an IAB5-driven expression pattern in the more posterior parasegments 10, 12 and 14 (F). Embryos exhibiting weaker staining are shown to clearly demonstrate the different expression patterns driven by the two IAB enhancers, although overall the expression in the 2-B^PTE-A-5 embryos was comparable to other transgenic lines analyzed in this study.