Fig. 1. Molecular organization of cis-regulatory sequences at the Drosophila bithorax complex. (A) Regulation of promoter-enhancer interactions. The diagram depicts a common enhancer (orange rectangle) at a model gene complex. The enhancer is able to interact with any of the promoters A, B, C or D. An insulator DNA (black ellipse) located between the enhancer and gene A prevents the enhancer from interacting with gene A but leaves it free to activate other promoters. A silencer element (red rectangle) represses gene B in cell types where it would otherwise be activated by the enhancer. Promoter competition regulates the interaction of the enhancer with gene C and gene D. The enhancer can activate either gene, but prefers the core promoter region of gene D. (B) Summary of the Abd-B-abd-A region of the BX-C. The abd-A and Abd-B transcription start sites are indicated by leftward arrows. The intergenic region is 100 kb in length. The iab regions that control expression of the two homeotic genes are indicated: IAB2-IAB8 (shown with respect to the corresponding embryonic parasegment). IAB2, IAB3 and IAB4 (shown in blue) regulate expression of abd-A. IAB5, IAB6, IAB7 and IAB8 (shown in green) direct Abd-B expression. The insulator DNAs that separate the different iab regions are marked by red ellipses. Characterized enhancers within the iab regions are shown as orange rectangles. The IAB5 enhancer is located 55 kb 3' of the Abd-B promoter and 48 kb 5' of the abd-A promoter, but only interacts with Abd-B over the intervening insulator sequences. The IAB2 enhancer is located 18 kb 3' of the abd-A promoter and directs expression specifically from abd-A. (C) Core promoter sequences at abd-A and Abd-B. The consensus sites for initiator (INR) and downstream promoter elements (DPE) in Drosophila are shown (Butler and Kadonaga, 2002). The sequences from the Abd-B and abd-A promoters do not match these consensus sites (mismatches shown in red). In addition, neither of the homeotic promoters contains a recognizable TATA box, suggesting that the core promoter elements at Abd-B(m) and abd-A are not distinctive. Abd-B(m), morphogenetic transcript.