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Livers of egg-laying species contain abundant mRNAs encoded by both estrogen-responsive and constitutively expressed genes. We have recently constructed cDNA clones from three members of the abundant mRNA class of hen liver. One of these mRNA species was identified as serum albumin mRNA, and another as vitellogenin mRNA. In this study we have identified the third member of the group as apoVLDLII mRNA. Hybridization analyses using cloned cDNA probes indicate that expression of the apoVLDLII gene in rooster liver, like that of the vitellogenin gene, is completely dependent upon the administration of estrogen. The apoVLDLII and vitellogenin genes appear to be the only genes capable of high rates of expression in the liver that exhibit such an exceptional response to the hormone. Administration of estrogen resulted in the appearance of both mRNA species within 30 min, followed by a rapid accumulation to several thousand copies per cell. Removal of the hormone caused a marked destabilization of both vitellogenin mRNA and apoVLDLII mRNA. In contrast, the absolute levels of serum albumin mRNA were unaffected by the hormone. Comparative studies on the structure and organization of these three genes may reveal elements involved in determining their rates of expression in the presence and absence of estrogen. Avian egg yolk proteins are a complex group of vitamin-and mineral-binding proteins, lipoproteins, and phosphoproteins. They are synthesized in the liver of the laying hen, under the regulation of estrogen (1). There is a wide variation in the rates at which individual proteins within the group are synthesized and also in the extent to which their synthesis is regulated by estrogen. Thus, it appears that the group is composed of some genes that may be regarded as "estrogen-dependent" and other genes whose expression appears to be modulated by, but not dependent upon, the hormone.it is possible to induce the synthesis of a complete spectrum of egg yolk proteins in mature males in response to a single injection of estrogen. Despite a considerable commitment to yolk protein synthesis, the liver of the estrogen-treated rooster continues to synthesize serum albumin throughout the vitellogenic response, at a rate that is relatively unaffected by the hormone. The constitutive expression of the albumin gene provides a useful control for distinguishing the specific effects of the hormone from its more general effects on the overall transcriptional and translational activities of the tissue.We are interested in comparing the structure and organization of genes that are estrogen-responsive in the liver with those that are constitutively expressed. In order to facilitate these studies, we recently constructed cDNA clones derived from chicken serum albumin mRNA (2) and two estrogeninducible mRNA species (3). One of these was vitellogenin mRNA which specifies the precursor of the egg yolk phos. phoproteins (4, 5). The other was a much smaller mRNA speciesThe publication costs of this article were defrayed in part by page ...

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