Fig. 2. Enhancer-promoter interactions at the BX-C. Transgenic embryos were hybridized with digoxigenin-labeled antisense RNA probes (diagrams on left). The embryos shown on the right are at blastoderm stage and orientated with anterior to the left and dorsal up. (A) On the B-5 construct the IAB5 enhancer directs expression of the Abd-B-CAT reporter gene in three characteristic stripes in presumptive abdominal segments 5, 7 and 9. (B) IAB5 is also able to interact with the abd-A promoter in the absence of a competing promoter, as it activates transcription from the abd-A-lacZ reporter gene. Ectopic expression of the lacZ and CAT reporter genes is also detected in the anterior of some transgenic embryos, as previously described (Ohtsuki et al., 1998; Zhou et al., 1999). (C,D) IAB2 and IAB5 will simultaneously activate expression from the abd-A or Abd-B promoter, indicated by a broad band of expression extending from presumptive abdominal segment 2 (indicated by black arrowhead) towards the posterior in 2-A-5 (C) and 2-B-5 (D) transgenic embryos. These expression patterns indicate that, in the absence of competing promoters, both homeotic gene promoters are responsive to interaction with the enhancers from the BX-C.