Organization and function of a dioxin-responsive enhancer.

The dioxin-responsive enhancer upstream of the CYP1A1 gene contains four copies of the recognition motif for the liganded Ah receptor. The results of deletion analyses, linker-scanning analyses, and the analysis of individual enhancer subdomains reveal that each copy of the motif contributes to the response of the enhancer to 2,3,7,8-tetrachlorodibenzo-p-dioxin. In the context of the dioxin-responsive enhancer, a GC box, representing the DNA binding site for Sp1 (or a related transcription factor), has no detectable intrinsic activity but enhances gene expression when linked to a recognition motif for the liganded Ah receptor, thereby producing a synergistic effect on enhancer function.