Steroid hormones orchestrate a variety of biological events that are critical for the development and homeostasis of multicellular organisms. They are also essential for the growth of certain tumors. Regulation by steroids can be effected at a number of dif- ferent levels and, in the case of transcriptional regulation, DNA sequences have been, identified that act as steroid-dependent enhancers. An outstanding challenge is to understand how these elements are utilized to effect distinct tissue-specific programs of steroid-dependent gene expression. An illustrative example of this type of differential regulation is the laying hen, where elevated levels of estrogen induce the expression of the egg white protein genes in the oviduct and the egg yolk protein genes in the liver. We have identified two estrogen response elements (EREs) upstream of one of the yolk protein genes (VTGII). These ERE core sequences are homologous to EREs associated with frog vitellogenin genes but are not evident within the region that confers estrogen- responsiveness to the ovalbumin gene. This suggests that distinct strategies may have evolved to effect these programs. In addition, although the yolk protein genes fail to become activated in the oviduct we have evidence to suggest that an ERE associated with one of these genes (VTGII) may interact with receptors in both liver and oviduct. To move toward an understanding of these observations at the molecular level, we propose first to identify the VTGII cis- acting regulatory elements that, in addition to the EREs, are required to effect the transcription of this gene in the liver. Secondly, we will search for trans-acting factors that recognize these cis-acting regulatory sequences and address whether their representation in liver and oviduct could account for the tissue- specific expression of this gene. Thirdly, we will address whether the activity of any of these factors is stably altered in the liver in response to a primary injection of estradiol, and whether such a change might underlie the fact that the yolk protein genes are activated more rapidly following a secondary injection (i.e., memory). Fourthly, we will also search for factors that interact at sites within the VTGII gene region that exhibit precise tissue- specific and/or steroid-induced chromatin structural discontinuities. Finally, we will address whether common strategies are used to effect the coordinate expression of the three chicken vitellogenin genes.
