Separation of specific initiation factors involved in the translation of myosin and myoglobin messenger RNAs and the isolation of a new RNA involved in translation

Messenger RNA translation in the presence of homologous and heterologous tRNA

The effect of tRNA distribution in the cell on the rate of specific protein synthesis has been investigated. A tRNA-dependent cell-free protein-synthesizing system derived from Krebs-II ascites cells has been worked out. In this system it is possible to synthesize specific proteins (egg-white proteins or globin) by the translation of oviduct or reticulocyte messenger RNA in the presence of tRNA from homologous or heterologous tissues. The rate of translation of a give messenger RNA is optimal in the presence of tRNA from the homologous tissue. The lower level of protein synthesis obtained with an excess of heterologous tRNA can be overcome by adding the homologous tRNA. Nevertheless homologous isoaccepting species partly purified by a reversed-phase chromatography or a benzoylated DEAE-cellulose column cannot fully restore the optimal synthesis, eliminating the hypothesis according to which a particular tRNA species is involved in this phenomenon. The correct distribution of tRNA is necessary for optimal translation of a messenger RNA.

Purification of myosin translational control RNA and its interaction with myosin messenger RNA

Myosin messenger ribonucleoprotein-translational control ribonucleic acid (mRNP-tcNA) from myosin mRNPs found in embryonic chick muscle has been further purified by Dowex chromatography and, from a number of controls, it is suggested that this small RNA is not an artifact produced through the degradation of RNA during its isolation. This highly purified myosin mRNP-tcRNA is shown to have a molecular weight of 10 000 on formamide-acrylamide gels, and reacts stoichometrically (on a 1:1 mole ratio) with myosin mRNA. The stoichiometric interaction between myosin mRNA and myosin mRNP-tcRNP is demonstrated by ists ability to increase the nuclease resistance of the messenger, as well as inhibit its translation in a cell-free amino acid incorporating system.

Antibiotics and compounds affecting tanslation by eukaryotic ribosomes. Specific enhancement of aminoacyl-tRNA binding by methylaxnthines

The mode and site of action of inhibitors of translation (initiation, elongation and termination of protein synthesis) in eukaryotic systems is reviewed. The isolation and characterization of a factor is described that binds Ac-Phe-tRNA to form a complex made up of binding factors, Ac-Phe-tRNA, and ribosome. The binding of Ac-Phe-tRNA probably occurs at the ribosomal site involved in the binding of the initiator substrate Met-tRNAF. The effect of inhibitors of the intitiation phase of protein synthesis on the nonenzymic Ac-Phe-tRNA binding to ribosomes is investigated. The two sites translocation model for translation in eukaryotic cells is presented and the effects of inhibitors on the various steps of protein synthesis are determined empirically. The site of action of inhibitors of peptide bond formation at the ribosomal peptidyl transferase center is elucidated. The action of inhibitors of translocation is sutdied in model cell-free systems from human cells. In addition, a number of methylxanthines are shown to enhance the elongation phase in polypeptide synthesis by stimulating the enzymic binding of aminoacyl-tRNA. The effect of caffeine, theophylline and its derivatives are shown to be fairly specific and dependent on the ribosome concentration. Aminophylline is shown to have a similar effect but also enhances aminoacyl-tRNA synthetase activity at low Mg++ concentrations, probably displacing the optimal concentration of Mg++ in the reaction. This second effect of aminophylline appears to be due to the ethylenediamine moiety of aminophylline since it is also observed in the presence of different polyamines but not in the presence of caffeine or theophylline.

Turnover of polyadenylated messenger RNA in fission yeast. Evidence for the control of protein synthesis at the translational level

Polyadenylated RNA was isolated from fission yeast (Schizosaccharomyces pombe) total RNA using oligo(dT)-cellulose, and was studied as a model for messenger RNA. The half-life of poly adenylated RNA was measured by two independent methods. (a) The rate of labelling of polyadenylated RNA during incubation of cells with [5-3H]uridine was measured. A half-life of 40-45 min was found by comparing the experimental data with theoretical curves calculated for labelling of RNAs with various half-lives. The influence of precursor-pool specific activity on RNA labelling kinetics is considered. (b) Cells were labelled with [5-3H]uridine then further RNA synthesis was inhibited by addition of 8-hydroxyquinoline. The rate of loos of radioactivity from polyadenylated RNA indicated a half-life of 50 min. The half-life found by these two methods is about one-third of the cell doubling time, and is much longer than previous estimates by indirect methods of yeast messenger RNA half-life. Both experimental methods provided evidence for the existence of tas a half-life of 40-50 min; a much smaller population is probably turning over more rapidly. After inhibition of RNA synthesis by 8-hydroxyquinoline, the rate of total protein synthesis declined much more rapidly than the polyadenylated RNA content of the cells. However, 60 min after inhibition of RNA synthesis there was a small rise in the rate of portein synthesis. These data are interpreted as evidence for mechanisms controlling protein synthesis which operate at the level of messenger RNA translation.

Studies concerning the mechanism by which translational-control RNA regulates protein synthesis in embryonic muscle

Muscle translational-control RNA (tcRNA) has been separated into two classes, polysomal and messenger ribonuclear protein (mRNA - protein), which have different sizes as determined by acrylamide gel electrophoresis. While normally translation of mRNA - protein mRNA is inhibited by tcRNA derived from the same mRNA - proteins, this inhibition does not occur if the messenger is previously de-adenylated. This suggests that the poly(A) segment of mRNA is required for the tcRNA activity. Utilizing different mRNA - protein fractions from muscle, myosin mRNA - protein and small mRNA - proteins ( less than 30 S), we have been able to demonstrate that a degree of specificity exists in the interaction of tcRNA and mRNA derived from the same mRNA - proteins. This is illustrated by the facts that (a) each tcRNA only inhibits the translation of its respective mRNA and (b) the highest percentage of structural change occurs when each tcRNA is hybridized to its respective mRNA as measured by its resistance to T1 and T2 RNase.

Terminal-sequence analysis of bacterial ribosomal RNA. Correlation between the 3'-terminal-polypyrimidine sequence of 16-S RNA and translational specificity of the ribosome

The 3'-terminal sequences of 16-S ribosomal RNA from a number of bacteria have been determined by a stepwise degradation and 3'-terminal labelling procedure. The sequences obtained were: Bacillus stearothermophilus, -G(Z)approximately 5 Y-U-C-C-U-U-U-C-U (A); B. subtilis, -G(Z)approximately 7 Y-C-U-U-U-C-U; Caulobacter crescentus, -G(Z)3 Y-U-C-C-U-U-U-C-U; Pseudomonas aerugionosa, -G-Z-Z-Y-C-U-C-U-C-C-U-U(A), where Z is any nucleotide other than G. Thus, as previously found in Escherichia coli, all bacterial 16-S rRNAs contain a pyrimidine-rich tract at the 3'-terminus. In B. stearothermophilus and Ps. aeruginosa this region shows substantial heterogeneity involving the 3'-terminal adenylic acid. A low level of 3'-terminal heterogeneity cannot be excluded for the other bacterial 16-S rRNAs examined. The 3'-termini of bacterial 16-S rRNA can be divided into two groups on the basis of sequence homology. The first group comprises E. coli and Ps. aeruginosa; the second, B. stearothermophilus, B. subtilis and C. crescentus. This division correlates with a previous separation of bacterial ribosomes into two categories based on ability to translate different mRNA preparations [Stallcup, Sharrock & Rabinowitz (1974) Biochem. Biophys. Res. Commun. 58, 92-98]. We have previously proposed that the precise base sequence at the 3'-terminus of 16-S rRNA determines the intrinsic capacity of bacterial ribosomes to translate a particular cistron [Shine & Dalgarno (1975) Nature (Lond.) 254, 34-38]. No difference was found in the 3'-terminal heptanucleotide sequence of 16-S rRNA from bacteriophage T7-infected E. coli, as compared to that in uninfected cells. Thus, the T7-induced alteration in translational specificity of E. coli ribosomes is probably not mediated by modification of the terminal seven nucleotides of the smaller rRNA. The 3'-terminal sequences of the 23-S rRNA species were also determined. The sequences obtained were: B stearothermophilus and B. subtilis, -Y-C; C. crescentus, -Y-C-U; Ps. aeruginosa, -Y-C-A; E. coli, -G-Y-U-U-A-A-C-C-U-U. No evidence for 3'-terminal heterogeneity was found. The results obtained are discussed in relation to possible base-pairing roles for the 3'-end of 16-S rRNA in bacterial protein synthesis.

Two classes of translational control RNA: their role in the regulation of protein synthesis

Two classes of translation control RNA (tcRNA) have been isolated from embryonic chick muscle. One of these classes, the tcRNA isolated from messenger ribonucleoprotein particles (mRNP-tcRNA), is effective in inhibiting the translation of mRNP-mRNA while having little if any effect on polysomal mRNA. The other class, polysome-tcRNA, has no effect on mRNP-mRNA while it stimulates the translation of polysomal mRNA. The mRNP tcRNA contains approximately 50 percent uridylate residues and forms small but stable hybrids with poly (A), while polysome-tcRNA contains fewer uridylate residues and is much less effective in forming a hybrid with poly (A). A proposed model concerning the role of these two classes of tcRNA in the regulation of protein synthesis is presented.

Different sensitivities of avian- and mammalian-haemoglobin synthesis to elevated temperatures

Avian- and mammalian-haemoglobin synthesis show different sensitivities to elevated temperatures. Temperature-dependent, reversible polyribosome disaggregation in avian cells occurs only at 45 degrees C, which is 3 degrees higher than the temperature for mammalian cells, and seems to be due to a block in the initiation of new polypeptide chains. The implications of these findings are discussed.

Non-specific stimulation of cell-free protein synthesis by a dialyzable factor isolated from reticulocyte initiation factors ("iRNA")

The specificity of a dialyzable component, isolated from rabbit reticulocyte initiation factors, in stimulating protein synthesis was examined. It appeard that his factor (hereafter designated as "iRNA") was able to restore the activity of initiation factor preparations that were inactivated by dialysis. The "iRNA" from reticulocytes stimulated polypeptide synthesis directed by the homologous globin mRNA as well as heterologous lens crystallin mRNAs. No selectivity in its stimulating action with regard to the type of mRNA studied could be observed. The source of ribosomes or supernatant enzymes did not influence the effect of "iRNA". However, in an ascites lysate that was dependent on the addition of initiation factors, "iRNA" increased polypeptide formation only marginally, suggesting that in this lysate a similar factor was already present in an active form. It is concluded that "iRNA" may regulate protein synthesis, but without exhibiting specificity towards mRNAs, at least those tested so far.

Messenger RNA metabolism of animal cells. Possible involvement of untranslated sequences and mRNA-associated proteins

The past several years have seen a virtual revolution in the study of eukaryotic mRNA. Among the notable recent achievements are the positive identification of mRNA precursors in HnRNA, the enumeration of the DNA sequences from which mRNA is transcribed, and the finding that mRNA in cultured cells is much more stable than was previously believed. One of most far-reaching discoveries has been the finding that mRNA in eukaryotes contains poly A. This discovery, aside from providing a powerful tool for mRNA isolation, has generated a large body of research into the properties and metabolism of poly A itself. In addition, the finding of a poly A-associated protein has given a renewed stimulus to the study of proteins associated with mRNA. This review is devoted to a discussion of these and related achievements, and some of their implications