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We report initial studies on estrogen-mediated regulation of egg yolk protein synthesis in the rooster. Egg yolk proteins are normally synthesized as a large precursor, vitellogenin, in the liver o the laying hen; roosters synthesize vitellogenin only when treated with estrogen. Polysomal RNA from the liver of estrogen-treated roosters was translated in a reticulocyte cell-free system, and the newly synthesized proteins were identified by a highly specific and sensitive indirect immunoprecipitation reaction. The messenger RNA that specifies vitellogenin has been purified more than 800-fold from rooster liver polysomal RNA by a combination of methods, including immunoprecipitation of polysomes and chromatography of RNA on poly(U)Sepharose.In recent years an important approach has been developed for studying the molecular mechanisms by which hormones regulate gene expression in eukaryotic organisms. The goal of this approach is to study the regulatory effects of hormones in vitro, and involves the following steps: isolation of a specific messenger RNA (mRNA) from the target tissue after administration of hormone; identification of this mRNA by its ability to program the synthesis of a specific protein in vitro; purification of the specific mRNA to homogeneity; use of the pure mRNA as a template for synthesis of complementary DNA; and, finally, use of the complementary DNA as a probe for hybridization studies of in vitro transcripts from purified components with RNA polymerase and chromatin (1-14). Studies successfully utilizing this approach have been limited to specialized tissues in which thepotential for gene expression is severely limited. We have recently begun a study along the general lines of this approach but have chosen to study specific regulation of gene expression in liver, a tissue with diverse genetic potential. Our studies involve an investigation of the induction of vitellogenin synthesis by estrogen in rooster liver.Avian vitellogenin is a phosphoprotein that is normally synthesized in the liver of laying hens but is synthesized in roosters only after administration of estrogen. Vitellogenin was first identified in the plasma of estrogen-treated male Xenopus laevis (15, 16). After its synthesis in the liver, this phosphoprotein is transported through the plasma to the oviduct where it is deposited in the developing oocyte as lipovitellin and phosvitin, the two amphibian egg yolk proteins that are formed by cleavage of vitellogenin (16). Bergink et al. (17) have suggested that vitellogenin might also exist in the plasma of laying hens and estrogen-treated roosters.

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