The Effect of Estrogen on Gene Expression in Avian Liver

About four years ago, we turned our attention from regulation of gene expression in prokaryotes and began to search for a system that would tell us something about regulation in eukaryotic cells. We were guided in this search by the following considerations. First, we did not want to work with a system in which cellular differentiation was itself an integral part of the regulatory response. We felt that we could obtain more clear-cut information if we could study a response in a tissue that was already fully differentiated and in which the regulatory response did not require DNA synthesis. We recognized that a hormonally responsive system would be the easiest to manipulate, and for practical reasons we wanted a system in which the hormonal response was of very great magnitude. And as long as were asking, we thought we might as well put in a request that the tissue we study also contain a whole group of genes that are hormonally responsive, so that we would be able to study not only the effect of hormone on a specific gene but also the mechanisms that coordinate the expression of the hormone-responsive domain of the tissue. In addition, we wanted our hypothetical tissue to have a gene expressed at a high level that is not responsive to hormonal stimulation. Such a gene would provide an ideal control for in vitro transcription studies, where one must show that the specificity of hormonal responsiveness reflects that of the tissue in vivo.