In light of present progress in the understanding of how messenger RNA is constructed in eukaryotic cells, the levels of gene control are discussed. Transcriptional control, assessed by the rate of synthesis of specific nuclear RNA, is strongly indicated to be the most frequent level of control. Definition of eukaryotic transcription units as simple (encoding one protein} and complex (encoding two or more proteins) affords a framework in which to discuss control at the level of RNA processing for which several examples also are known. Finally, differential stability of cytoplasmic mRNAs and differential translation, both well established, are briefly described.

Variety in the level of gene control in eukaryotic cells

Publisher Summary This chapter describes the different aspects of vitellogenesis and oocyte assembly The major components of fish oocytes are derived from the blood-borne high-molecular-weight compound, vitellogenin, which is synthesized in the liver of oviparous vertebrates The classification of vitellogenin as a phospholipoglycoprotein indicates the crucial functional groups that are carried on the protein backbone of the molecule—namely, lipids, some carbohydrates, and phosphate groups In the rainbow trout, estrone administration leads to the induction of vitellogenin synthesis in the liver and its release into the bloodstream, but estrone displays only 5%–12% of the potency of estradiol The biochemical information concerning vitellogenin clearly indicates that a great deal of posttranslational modification must occur in the liver cell to reach the finished product seen in the serum An integral part of estradiol action is the observed hypercalcemia in vitellogenic fish, which can largely be ascribed to the calcium-binding properties of phosphorylated and highly charged, components of the native vitellogenin molecule

5 Vitellogenesis and Oocyte Assembly

Liver-specific mRNA sequences were examined in primary cultures of mouse hepatocytes. After cell disaggregation by collagenase treatment and for at least 24 h in culture, little change in liver-specific mRNA concentrations was noted. Gradually over a period of 140 h, liver-specific mRNAs declined. In contrast, transcriptional assays in which liver cell nuclei were used to produce 32P-labeled nuclear RNA showed that liver-specific gene transcription was greatly diminished within 24 h, while polymerase II transcription of "common" genes and transcription of tRNA and rRNA did not decline. Thus, a prompt differential transcriptional effect seems to underlie the gradual loss of tissue specificity of the primary cultures.

Changes in liver-specific compared to common gene transcription during primary culture of mouse hepatocytes.

The evolution of egg-yolk proteins

https://www.cell.com/cell/pdf/0092-8674(83)90335-5.pdf

Temporal order of chromatin structural changes associated with activation of the major chicken vitellogenin gene

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 apo VLDLII mRNA. Hybridization analyses using cloned cDNA probes indicate that expression of the apo VLDLII gene in rooster liver, like that of the vitellogenin gene, is completely dependent upon the administration of estrogen. The apo VLDLII 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 apo VLDLII 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.

W. Dower

Papers

Coordinate regulation of two estrogen-dependent genes in avian liver.