Alterations in chromatin structure are implicated in the activation of thesteroid hormone response unit of the ovalbumin gene

Hormone-responsive genes rely on complex regulatory elements known as hormo ne response units to integrate various regulatory signals. Characterization of the steroid-dependent regulatory element (SDRE) in the check ovalbumin gene (-892 to -796) suggests that it functions as a hormone response unit. Previous studies using gel mobility shift assays and several types of footp rinting analyses demonstrated that proteins bind to this entire element in vitro even in the absence of steroid hormones. However, the genomic footpri nting experiments described herein indicate that the binding of three diffe rent proteins or protein complexes to the SDRE requires estrogen and cortic osterone, suggesting that the chromatin structure of this site is restricte d in vivo. Transfection experiments using linker scanning and point mutatio ns support the contention that the binding of these three complexes is esse ntial for induction of the ovalbumin gene by steroid hormones. In addition, functional analyses suggest that a fourth complex is also necessary for ma ximal induction. These and other data suggest that the SDRE functions as a hormone response unit to coordinate signals generated by two steroid hormon es.