Quantitative Measurement of Ovalbumin Messenger Ribonucleic Acid Activity

Insulin effects on protein synthesis and secretion in primary cultures of amphibian hepatocytes

The objective of the present study was to determine the effects of insulin on amphibian hepatocytes in primary culture. Hepatocytes were isolated from adult bullfrogs by collagenase perfusion and maintained as monolayers in serum-free medium. Cells cultured in the continuous presence of insulin exhibited a relatively constant rate of protein secretion over the first four to five days, whereas controls showed an almost three-fold decrease over the same time period. The decline in secreted proteins was equally represented in most exported proteins, except that serum albumin secretion showed twice as much of a decrease relative to the other proteins. The maintenance of protein secretion by insulin was the result of its effect on protein synthesis. The rate of protein synthesis was measured by the incorporation of (3H)-leucine into protein using culture medium containing 0.5 mM leucine, a condition where the specific radioactivity of leucyl-tRNA was shown to be equal to that of (3H)-leucine in the medium. Cultures maintained with insulin for 60 hours synthesized protein at two to three times the rate found in non-insulin treated controls whose rate of protein synthesis was first detectably decreased after nine hours of culture in the insulin-free medium. Sedimentation profiles of polyribosomes from hepatocytes maintained for 60 hours without insulin showed proportionately fewer ribosomes in large polysomes and more in monosomes and free ribosomal subunits than ribosomes from cells cultured with insulin. This result suggests that the decrease in protein synthesis found in the absence of insulin is due to a defect in initiation. Insulin does not exert its effect by regulating cellular levels of ATP; no change in ATP content was found in cells maintained with or without insulin. The results show that insulin maintains high levels of protein synthesis and secretion in amphibian hepatocytes. The hepatocytes in monlayer culture provide a system to study the molecular mechanisms involved in the translational control of protein synthesis by insulin.

Identification of potential ovomucoid mRNA precursors in chick oviduct nuclei

Denaturing gel electrophoresis of chick oviduct nuclear RNA reveals multiple species of RNA that are 1.5-5 times larger than ovomucoid mRNA. By analogy with ovalbumin RNA processing, these data suggest that ovomucoid mRNA is derived from a primary transcript that contains intervening sequences.

The chick ovomucoid gene contains at least six intervening sequences

A 15-kilobase pair EcoRI chick DNA fragment, containing both the termination codon UGA and the 5'-portion of the structural ovomucoid gene, has been cloned in lambda phage Charon 4A by in vitro packaging. Restriction mapping and electron microscopic analyses of this cloned DNA have revealed that the structural ovomucoid gene sequences are separated by at least six intervening sequences.

Synthesis of high molecular weight thyroglobulin peptides by thyroid polysome in vitro

Bovine thyroid polysomes were isolated under conditions which had yielded large polysomes in other systems. Between 25 and 40% of the protein synthesized by these polysomes could be precipitated by thyroglobulin antibody. When these immunoprecipitates were separated by dodecyl sulfate-polyacrylamide gel electrophoresis (4% running gel), over 50% of the radioactivity was located in the regions of polypeptides greater than 100,000 daltons. Between 11 and 13% of the total radioactivity was found as a single peak co-migrating with the main band of bovine thyroglobulin (Mr = 330,000). Peaks of radioactivity were also found in regions of molecular weights between 130,000 and 200,000. When the immunoprecipitates were separated in a 10% running gel, about 50% of the radioactivity was located in the top 8 mm of the gel. Most of the remaining radioactivity was distributed in regions corresponding to molecular weights greater than 68,000. No peak of radioactivity was seen corresponding to peptides of 15,000 daltons.

Distribution of the messenger RNA coding for the common precursor of corticotropin and beta-lipotropin within the bovine pituitary

The distribution of the mRNA coding for the common precursor of corticotropin and beta-lipotropin among different parts of the bovine pituitary has been investigated by quantifying the mRNA activity with the use of a cell-free protein-synthesizing system. The results obtained have demonstrated that this mRNA activity is located both in the anterior lobe and in the intermediate lobe, while it is essentially not detectable in the neural lobe nor in the stalk. The structural identity of the translation products of corticotropin/beta-lipotropin mRNA from the anterior and from the intermediate lobe has been indicated by their molecular weight as well as by the electrophoretic patterns of the peptide fragments formed from them upon partial enzymatic proteolysis or upon cyanogen bromide cleavage. The specific activity of corticotropin/beta-lipotropin mRNA in the intermediate lobe is about 20-fold higher than that in the anterior lobe, and the total activity of this mRNA in the former is about 2-fold higher than that in the latter. In the intermediate lobe, the translation product of corticotropin/beta-lipotropin mRNA amounts to almost one-third of the products encoded by total translatable mRNA. These results indicate that corticotropin/beta-lipotropin mRNA represents a major mRNA species in intermediate lobe of the pituitary, thus suggesting that this lobe may perform a highly specialized function in producing a large amount of the common precursor of corticotropin and beta-lipotropin.

Co-translational modification of nascent immunoglobulin heavy and light chains

We have investigated the in vivo co-translational covalent modification of nascent immunoglobulin heavy and light chains. Nascent polypeptides were separated from completed polypeptides by ion-exchange chromatography of solubilized ribosomes on QAE-Sephadex. First, we have demonstrated that MPC 11 nascent heavy chains are quantitatively glycosylated very soon after the asparaginyl acceptor site passes through the membrane into the cisterna of the rough endoplasmic reticulum. Nonglycosylated completed heavy chains of various classes cannot be glycosylated after release from the ribosome, due either to rapid intramolecular folding and/or intermolecular assembly, which cause the acceptor site to become unavailable for the glycosylation enzyme. Second, we have shown that the formation of the correct intrachain disulfide loop within the first light chain domain occurs rapidly and quantitatively as soon as the appropriate cysteine residues of the nascent light chain pass through the membrane into the cisterna of the endoplasmic reticulum. The intrachain disulfide loop in the second or constant region domain of the light chain is not formed on nascent chains, because one of the cysteine residues involved in this disulfide bond does not pass through the endoplasmic reticulum membrane prior to chain completion and release from the ribosome. Third, we have demonstrated that some of the initial covalent assembly (formation of interchain disulfide bonds) occurs on nascent heavy chains prior to their release from the ribosome. The results are consistent with the pathway of covalent assembly of the cell line, in that completed light chains are assembled onto nascent heavy chains in MPC 11 cells (IgG2b), where a heavy-light half molecule is the major initial covalent intermediate; and completed heavy chains are assembled onto nascent heavy chains in MOPC 21 cells (IgG1), where a heavy chain dimer is the major initial disulfide linked intermediate.

Synthesis of an ovalbumin-like protein by Escherichia coli K12 harbouring a recombinant plasmid

A cloned DNA transcript of ovalbumin mRNA was cut a few nucleotides away from the initiator codon, and fused in phase to the beginning of the Escherichia coli beta-galactosidase gene. The hybrid gene has been cloned in E. coli where it produces large amounts of an ovalbumin-like protein.

Regulation of translation of ovalbumin messenger RNA by estrogens and progesterone in oviduct of withdrawn chicks

In the oviduct of chicks withdrawn from previous treatment with estrogens, no ovalbumin synthesis can be detected, although there are a limited number of ovalbumin mRNA sequences. These sequences are predominately associated with membrane-bound ribosomes. However, the size of the polysomes is small compared to those from the laying hen, suggesting that the inability to detect ovalbumin synthesis is the result of inefficient initiation of ovalbumin synthesis. When the rate of peptide chain elongation is reduced by treatment of chicks with cycloheximide, there is an increase in the average size of polysomes and a shift of ovalbumin mRNA sequences from small to large-sized polysomes. Readministration of estrogen to withdrawn chicks results in a time-dependent shift of monosomes to polysomes and a proportional shift of ovalbumin mRNA sequences between the two fractions, indicating that estrogen stimulates the rate of initiation of all mRNA species in the oviduct to essentially the same extent. In contrast, progesterone administration results in a preferential shift of ovalbumin mRNA relative to total RNA, suggesting a preferential effect of progesterone on initiation of protein synthesis with ovalbumin mRNA.

Effect of estradiol-17 beta on preprolactin messenger ribonucleic acid activity in the rat pituitary gland

Rat pituitary RNA was translated in the wheat germ system. Preprolactin messenger RNA activity was estimated by adsorption of cell-free products to solid phase antiprolactin. When male rats were injected for 4 days with estradiol-17beta, pituitary preprolactin mRNA activity was increased 2.5- to 3.0-fold over controls. This increase was evident when either total RNA, poly(adenylic acid) RNA, or polysomal RNA was translated in the cell-free system. In male rats receiving daily injections of estradiol-17beta, preprolactin mRNA activity was increased to an apparent maximum of 300% of controls after 7 days of treatment. Our data also indicate that estradiol increases preprolactin mRNA activity per microgram of RNA as well as the pituitary content of RNA. After estradiol treatment was discontinued, preprolactin mRNA activity declined to 50% of the maximum stimulation after approximately 2 days. In ovariectomized retired breeder female rats, a 5-fold increase in preprolactin activity over ovariectomized controls was obtained. In other studies, a 2-fold increase in preprolactin mRNA activity was obtained in male rats 24 h after a single injection of pimozide, a dopamine blocking drug.

Hormonal control of milk protein synthesis in cultured mouse mammary explants

SDS-urea polyacrylamide gel electrophoresis of mouse milk proteins revealed the presence of three major phosphoproteins (caseins) of m.w. 44,000, 26,000 and 22,000 daltons. By using an antiserum against crude casein fraction, an immunoprecipitation method was developed for the quantitative measurement of the rate of milk protein synthesis in the mouse mammary tissue. Cultivation of mammary explants with insulin, cortisol and prolactin resulted in the induction of milk protein synthesis as evidenced by the incorporation of [3H]amino acid and [32P]orthophosphate into immune precipitable materials. The present immunoprecipitation method coupled with a simplified explant culture technique provides a suitable procedure for the study of mouse mammary gland differentiation.

Frequency distribution of messenger sequences within polysomal mRNA and nuclear RNA from rat liver

DNA complementary to polysomal poly(A)-containing mRNA (cDNA) of male rat liver was used to study the diversity of messenger sequences in the nucleus and in polysomes. 1. Hybridization of cDNA against an excess of its own polysomal mRNA template revealed that about 10,000 different mRNA species are expressed in the liver tissue. They are distributed in a wide frequency range derived from approximately 0.5% of the total genome. 2. Hybridization of the cDNA against total nuclear RNA shows that messenger sequences comprise less than 1% of the mass of total nuclear RNA. Messenger sequences have a different frequency distribution in nucleus and cytoplasm. 3. In hybridizations using cDNA, which had been fractionated into sequences representing abundant and scarce polysomal mRNA molecules, it was found that although abundant cytoplasmic messenger sequences are also abundant in the nucleus, they exist in a significantly lower frequency range in the nuclear compartment.

RNA synthesis in isolated nuclei of Xenopus laevis emproyos

Nuclei were isolated from tailbud embryos of Xenopus laevis and their RNA synthetic activity was studied. The activity was sensitive to actinomycin D, and required the presence of all four ribonucleotide triphosphates and a divalent cation (Mg2+ or Mn2+). The effect of the divalent cation varied depending on the presence of 0.4 M KCl: in its absence more RNA was synthesized with Mg2+ than with Mn2+, but in its presence the situation was reversed. About 50% of the RNA synthesized was sensitive to the presence of alpha-amanitin. Polyacrylamide gel electrophoresis of the labeled RNA revealed that the RNA synthesized had a very heterogeneous size distribution ranging from 40 to 4 S, with a broad peak at around 28--18 S. The RNA which was distributed at around 28--18 S and 4 S was resistant to the presence of a low dose of alpha-amanitin, suggesting its ribosomal and soluble RNA nature. The RNA which was sensitive to this drug and hence may be assumed to be messenger RNA was distributed mainly between 18 and 4 S. From these results it appears that the isolated embryonic nuclei retain their activity to synthesize all of the major classes of RNA.

Glucocorticoid receptor-steroid complex binding to DNA. Competition between DNA and DNA-cellulose

The binding of the glucocorticoid receptor-steroid complex from a line of rat hepatoma tissue culture (HTC) cells to DNA has been examined. An equilibrium competition assay involving a constant, low total amount of double-stranded DNA was developed to compare the complex binding ability of DNA free in solution and bound to cellulose. This binding ability is lowered by a factor of five when DNA is associated with cellulose. Similar studies with HTC cell, calf-thymus, and Escherichia coli DNA revealed no difference in the relative number or affinity of binding sites for receptor-steroid complex in each DNA. The synthetic DNA molecules poly[d(A-T)-d(A-T)] and poly[d(G-C)-d(G-C)] bound complexes equally well but less than the three "natural" DNA molecules. This appears to be due to differences in acceptor site affinity and suggests that nucleotide complexity and/or sequence influences the affinity of HTC cell receptor-glucocorticoid complexes for DNA.

Benzene inhibition of in vitro rabbit reticulocyte haem synthesis at delta aminolaevulinic acid synthetase: reversal of benzene toxicity by pyridoxine

Benzene (0.113 M) inhibited haem and protein synthesis in rabbit reticulocytes. This inhibition of haem synthesis was found when L-2-[14C]-glycine was used as the radioactive precursor. However, when 4-[14C]delta-aminolaevulinic acid (ALA) was used, there was no significant inhibition. Since ALA measures the haem synthetic pathway beyond the enzyme delta-aminolaevulinic acid synthetase (ALA synthetase), these results suggest that benzene inhibits haem synthesis at or before ALA synthetase. This was confirmed by demonstrating that 1 mM ALA both protected against and reversed the benzene inhibition of reticulocyte protein synthesis. In addition, 1 mM pyridoxine both protected against and reversed the benzene inhibition of reticulocyte protein synthesis. In addition, ImM pyridoxine both protected against and reversed the benzene inhibition of reticulocyte haem and protein synthesis. These results indicate that benzene (or a metabolite) either competes with pyridoxal phosphate at ALA synthetase or competes with pyridoxine for pyridoxal phosphokinase. These results are discussed in terms of their implications for the possible roles of ALA synthetase and the haemin-controlled repressor in benzene-induced aplastic anaemia.

Studies on the synthesis of casein messenger RNA during pregnancy in the rabbit

The quantity of casein mRNA in the rabbit mammary gland was assayed during the course of pregnancy, by translation of casein mRNA in a reticulocyte lysate and by hybridization to DNA complementary to purified casein mRNA. Both methods indicated that the major increases in the concentration of casein mRNA in both polysomal and total cellular RNA occurred between the 18th and 25th day of pregnancy. The change in casein mRNA concentration during this period coincided with a sharp rise in casein synthesis in mammary gland explants suggesting that the levels of casein mRNA determine the rate of casein synthesis in the mammary gland. The sensitivity of the hybridization assay made it possible to detect the presence of casein mRNA in virgin animals and during the very early stages of pregnancy. At day 5, casein mRNA was found associated with polysomes indication that there was probably some casein synthesis at this early stage of gestation. These results suggest that the hormones controlling lactogenesis in the rabbit may function by augmenting the rate of casein mRNA synthesis rather that initiating transcription of previously inactive genes.