Liver cells contain constitutive DNase I-hypersensitive sites at the xenobiotic response elements 1 and 2 (XRE1 and -2) of the rat cytochrome P-450IA1 gene and a constitutive, nuclear XRE-binding factor that is distinct from the dioxin receptor.

Dioxin stimulates transcription from the cytochrome P-450IA1 promoter by interaction with the intracellular dioxin receptor. Upon binding of ligand, the receptor is converted to a form which specifically interacts in vitro with two dioxin-responsive positive control elements located in close proximity to each other about 1 kb upstream of the rat cytochrome P-450IA1 gene transcription start point. In rat liver, the cytochrome P-450IA1 gene is marked at the chromatin level by two DNase I-hypersensitive sites that map to the location of the response elements and exist prior to induction of transcription by the dioxin receptor ligand beta-naphthoflavone. In addition, a DNase I-hypersensitive site is detected near the transcription initiation site and is altered in nuclease sensitivity by induction. The presence of the constitutive DNase I-hypersensitive sites at the dioxin response elements correlates with the presence of a constitutive, labile factor which specifically recognizes these elements in vitro. This factor appears to be distinct from the dioxin receptor, which is observed only in nuclear extract from treated cells. In conclusion, these data suggest that a certain protein-DNA architecture may be maintained at the response elements at different stages of gene expression.