Translation in vivo and in vitro of mRNAs coding for vitellogenin, serum albumin and very-low-density lipoprotein II from chicken liver. A difference in translational efficiency

Regulation of gene expression for translation initiation factor eIF-2 alpha: importance of the 3' untranslated region

Gene expression of the alpha-subunit of eukaryotic initiation factor-2 (eIF-2 alpha), involves transcriptional and post-transcriptional mechanisms. eIF-2 alpha is a single-copy gene expressing two mRNAs, 1.6 and 4.2 kb in size. Cloning and sequencing of the cDNA for the 4.2 kb mRNA revealed that it is the result of alternative polyadenylation site selection. Four polyadenylation sites were identified within the 3' untranslated region (UTR) of eIF-2 alpha, only two of which are normally utilized in human and mouse tissues. A functional role for the extended 3' UTR was assessed by comparing the translatability and stability of the 1.6 and 4.2 kb mRNAs. Both the 1.6 and 4.2 kb transcripts could be translated in vitro and were identified in vivo as being distributed on large polyribosomes. This indicates that both mRNAs are efficiently translated. Stability studies showed that in activated T-cells the 4.2 kb mRNA was more stable than the 1.6 kb mRNA. Polyadenylation site selection and mRNA stability differ for the two mRNAs of eIF-2 alpha. These activities might be modulated by sequence elements contained within the untranslated regions of the eIF-2 alpha gene.

A new repetitive element of the CR1 family downstream of the chicken vitellogenin gene

We have analyzed a repetitive DNA sequence found in the 3'-flanking region of the chicken vitellogenin gene. By its sequence, the repetitive DNA has been identified as a hitherto unreported member of the chicken CR1 family of repetitive elements. The CR1 sequence displays the structural characteristics of a long terminal repeat located at the 3' end of an avian retrovirus. The CR1 element lies 2.2 kb downstream of the vitellogenin gene and 'points' away from the gene rather than toward it. In this respect, this element differs from other CR1 repeats. The CR1 element is embedded in a region showing changes in chromatin structure implying a potential role for this sequence in determining the structural state of the local chromatin.

Translational regulation of myelin basic protein synthesis

Synthesis of the mouse myelin basic proteins (MBPs) was studied in reticulocyte lysates programmed with brain mRNA in the presence or absence of brain factors. Addition of brain factors to the lysates increased the incorporation of [35S]methionine into total TCA-precipitable protein by a factor of 6-9, and the majority of this stimulation was found to be due to initiation factors. Although brain factors increased total protein synthesis, the percentage of MBP synthesis was reduced from 4.3% of the total counts incorporated (in the absence of brain factors) to 1.4% (in their presence). Increasing the concentration of brain mRNA in the lysates also reduced the relative levels of MBP synthesis. These results suggested the MBP mRNAs, as a group, were less efficiently initiated than most brain mRNAs. An analysis of the nucleotide sequence flanking the initiator codon of the MBP mRNAs indicates the presence of a second AUG codon 5 bases upstream, immediately followed by a termination codon, which may provide a structural explanation for the poorer initiation efficiency of the MBP mRNAs. Further analysis of the synthesis of the individual MBPs in the presence or absence of inhibitors of initiation (7-methylguanosine triphosphate and aurintricarboxylic acid) and elongation (anisomycin and emetine) indicated than the 14-kD MBP mRNA was less efficiently translated than the other MBP mRNAs. Synthesis of the 14-kD MBP was more strongly inhibited by the initiation inhibitors than the other MBP mRNAs, and synthesis of the 14-kD MBP was increased relative to the other MBPs in the presence of elongation inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)

Conserved sequence motifs upstream from the co-ordinately expressed vitellogenin and apoVLDLII genes of chicken

The vitellogenin and apoVLDLII yolk protein genes of chicken are transcribed in the liver upon estrogenization. To get information on putative regulatory elements, we compared more than 2 kb of their 5' flanking DNA sequences. Common sequence motifs were found in regions exhibiting estrogen-induced changes in chromatin structure. Stretches of alternating pyrimidines and purines of about 30-nucleotides long are present at roughly similar positions. A distinct box of sequence homology in the chicken genes also appears to be present at a similar position in front of the vitellogenin genes of Xenopus laevis, but is absent from the estrogen-responsive egg-white protein genes expressed in the oviduct. In front of the vitellogenin (position -595) and the VLDLII gene (position -548), a DNA element of about 300 base-pairs was found, which possesses structural characteristics of a mobile genetic element and bears homology to the transposon-like Vi element of Xenopus laevis.

Differential estrogen responsiveness of the vitellogenin and apo very low density lipoprotein II genes in the rooster liver

The primary transcript of the chicken apo Very Low Density Lipoprotein II (apoVLDL-II) gene is formed almost immediately after a first estrogen administration, contrary to the appearance of the vitellogenin primary transcript which has a lag of at least 4 h. However, after a second estrogen administration the vitellogenin gene transcription shows no detectable lag (memory effect). After estrogen withdrawal, the primary transcripts of both genes rapidly decline to undetectably low levels. In the presence of estrogen, the half-lives of the mRNAs of apoVLDL-II and vitellogenin are 15 and at least 70 h, respectively, whereas in the absence of hormone they are only 3.5 and 5.5 h, respectively. Thus estrogen not only controls the transcription of both genes, but also the turnover of their mRNAs. In short, there appears to be a quantitative difference in the response of both genes.

Cross-linking of mRNA to initiation factor eIF-3, 24 kDa cap binding protein and ribosomal proteins S1, S3/3a, S6 and S11 within the 48S pre-initiation complex

Native small ribosomal subunits from rabbit reticulocytes contain all initiation factors necessary for the formation of the mRNA-containing 48S pre-initiation complex. The complex formed in the presence of Met-tRNAf and 125I-labelled globin mRNA was cross-linked with diepoxybutane, and the covalent mRNA-protein complexes were isolated under denaturating conditions. The proteins of the covalent complex were identified as the 110, 95 and 66/64 kDa subunits of eIF-3. In addition, the 24 kDa cap binding protein and the ribosomal proteins S1, S3/3a, S6 and S11 were found covalently linked to the mRNA. Ribosomal proteins S3/3a and S6 were also involved in the ribosomal mRNA-binding domain of reticulocyte polysomes.

Storage protein precursor polypeptides in cotyledons of Pisum sativum L. Identification of, and isolation of a cDNA clone for, an 80000-Mr legumin-related polypeptide

An 80 000-Mr polypeptide, which bound to anti-legumin IgG, was detected among labelled polypeptides from cotyledons at late stages of development. When poly(A)-containing RNA from similar cotyledons was translated in a cell-free protein-synthesizing system, an 80 000-Mr polypeptide was also detected. Immunoprecipitation of translation products with anti-legumin IgG showed that, in addition to the major legumin precursor polypeptides of Mr approximately 60 000, the 80 000-Mr polypeptide was specifically immunoprecipitated. A cDNA clone, pCD32, was found to select an RNA coding for an 80 000-Mr polypeptide in hybrid-selection experiments. Additional minor polypeptides of Mr 63 000 and 65 000 were present in translation products of RNA selected by pCD32; all three polypeptides were immunoprecipitated by anti-legumin IgG. Thermal elution of RNAs bound to pCD32 showed that the affinity of pCD32 to the RNA coding for the 80 000-Mr polypeptide was greater than to the RNAs coding for the 63 000-Mr and 65 000-Mr polypeptides. In similar hybrid-selection experiments, another cDNA clone, pCD40, selected RNAs coding predominantly for polypeptides of Mr 63 000 and 65 000. A minor polypeptide of Mr 80 000 was also detected among these products; again all three polypeptides were immunoprecipitated by anti-legumin IgG. Peptide mapping revealed close similarities between the 80000-Mr polypeptide and the 63 000-Mr/65 000-Mr polypeptides obtained by translation of RNAs selected by pCD32. There were similarities also between maps obtained from translation products of RNA selected by pCD32 and those obtained from anti-legumin IgG immunoprecipitates of total translation products of poly(A)-containing RNA.

Androgen induction of ornithine decarboxylase mRNA in mouse kidney as studied by complementary DNA

To investigate the mechanisms by which androgens regulate ornithine decarboxylase (OrnDCase; L-ornithine carboxy-lyase, EC 4.1.1.17) in mouse kidney, a cDNA clone encoding OrnDCase mRNA was prepared. Purification of OrnDCase mRNA from kidneys of androgen-treated mice was accomplished by immunoadsorption of renal polysomes to a protein A-Sepharose column and enrichment for poly(A)-containing RNA by oligo(dT)-cellulose. Double-stranded cDNA synthesized from this mRNA was inserted into the Pst I site of plasmid pBR322 by using oligo(dG . dC)-tailing and was propagated in Escherichia coli. Plasmids containing cDNA sequences coding for OrnDCase were identified by differential colony hybridization, by radioimmunological detection of OrnDCase-like antigens in bacterial cultures, and by cell-free translation of hybrid-selected mRNA followed by immunoprecipitation with monospecific OrnDCase antiserum. A restriction endonuclease fragment of the selected plasmid DNA (pODC54) was labeled by nick-translation and used to study changes in OrnDCase mRNA concentration. After a single dose of testosterone, renal OrnDCase mRNA concentration increased as soon as 6 hr and peaked 24 hr after steroid injection, as measured by RNA blot hybridization. Continuous androgen treatment for 4 days resulted in a 10- to 20-fold increase in OrnDCase mRNA concentration in normal animals, but no induction of this mRNA was detected in mice that have an inherent defect of the androgen receptor (testicular feminization). These results indicate that androgens regulate OrnD-Case synthesis in mouse kidney, at least in part, by increasing OrnDCase mRNA accumulation.

Translational barrier in central region of encephalomyocarditis virus genome. Modulation by elongation factor 2 (eEF-2)

A fractionated cell-free system from Krebs-2 cells was prepared which contained ribosomes and a high-speed supernatant. When this system was programmed with encephalomyocarditis virus RNA, the synthesis of a precursor of capsid proteins, polypeptide preA, proceeded at a rate not very different from that observed in unfractionated extracts, whereas the synthesis of more distally encoded proteins, in particular polypeptide F, was greatly retarded, if not abolished. A protein was purified from the cytoplasmic extracts of Krebs-2 cells which greatly enhanced production of polypeptide F as well as other noncapsid proteins in the fractionated system. By several criteria, this protein was identified as eukaryotic elongation factor 2 (eEF-2). By using the ADP-ribosylation assay, it was found that the fractionated system contained about 15% of the amount of eEF-2 present in the unfractionated extracts. The results suggest that changes in the eEF-2 content may affect the elongation rate differently at different regions of the RNA template.

Biosynthesis of rabbit serum albumin in a heterologous fractionated subcellular system

As demonstrated by a simple procedure based on indirect immunoprecipitation, proteins retained on heparin-Sepharose 4B from postmitochondrial supernatants of rat liver and Zajdela hepatoma catalyse the translation of rabbit serum albumin mRNA in the presence of ribosomal subunits from rat liver, Zajdela hepatoma or rabbit reticulocytes. The albumin synthesis shows an optimum at 1.5 mM MgCl2 and 25 mM KCl and requires ATP and GTP. It is significantly stimulated by tRNA and proceeds for more than 2 h, suggesting a high rate of reinitiation. At the optimum ribosomes:mRNA ratio of 13:1, the immunoprecipitable radioactivity exceeds 15-20-times the blank values. Fluorography of polyacrylamide slabs after electrophoresis of immunoprecipitates revealed the presence of only complete full-size serum albumin without any smaller peptides resulting from premature terminations of polypeptide chains, demonstrating faithful translation. In stained gels only, both heavy and light chains of immunoglobulin G were found, indicating that the assay procedure is highly specific and reliable. The fractionated heterologous protein-synthesizing system described in this paper may be generally useful for studies on the synthesis of specific proteins and factors affecting their rates since, unlike comparable translation assays, a precise calculation of the balance of newly synthesized proteins is possible.

Isolation and characterization of a cDNA clone specific for avian vitellogenin II

A clone for vitellogenin, a major avian, estrogen responsive egg yolk protein, was isolated from the cDNA library of estrogen-induced rooster liver. Two forms of plasma vitellogenin, vitellogenin I (VTG I) and vitellogenin II (VTG II), distinguishable on the basis of their unique partial proteolysis maps, have been characterized and their corresponding hepatic precursor forms identified. We have used this criterion to specifically characterize which vitellogenin protein had been cloned. Partial proteolysis maps of BTG I and VTG II standards, synthesized in vivo, were compared to maps of protein synthesized in vitro using RNA hybrid-selected by the vitellogenin plasmid. Eight major digest fragments were found common to the in vitro synthesized vitellogenin and the VTG II standard while no fragments were observed to correspond to the VTG I map. A restriction map of the VTG II cDNA clone permits comparison to previously described cDNA and genomic vitellogenin clones.

Estradiol-induced vitellogenin synthesis in duck liver

We have studied the induction of vitellogenin by estradiol in duck liver. From the accumulation of vitellogenin in blood plasma we calculated that the rate of vitellogenin synthesis increases linearly with time for about 4 days after estradiol administration. Vitellogenin from chicken and duck cross-react immunologically and their mRNAs show only 7% sequence divergence. We could therefore determine vitellogenin mRNA content of duck liver using chicken vitellogenin cDNA as a hybridization probe. The number of vitellogenin mRNA molecules per hepatocyte increases from less than one in normal duck liver to 18 000 at 4 days after estradiol injection. The rate of vitellogenin synthesis in vivo is roughly proportional to vitellogenin mRNA content, although the data suggest a somewhat enhanced translation of vitellogenin mRNA at later times after hormone administration. Vitellogenin mRNA levels had returned to control values after 4 weeks after hormone administration. In the first 11 h after secondary administration of hormone vitellogenin mRNA accumulates at an only slightly higher rate than is observed after primary hormonal stimulation.