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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.
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