Translational control of globin synthesis by haemin in Xenopus oocytes

Codon recognition mechanisms in plant chloroplasts

In chloroplasts, all 61 sense codons are found in chloroplast (cp) DNA sequences coding for proteins. However among the sequenced cp tRNAs or tRNA genes, tRNAs with anticodons complementary to codons CUU/C (Leu), CCU/C (Pro), GCU/C (Ala) and CGC/A/G (Arg) [or CGC/A (Arg) in Marchantia] have not been found. In this paper we show that cp tRNA(Ala)(U*GC) cp tRNA(Pro)(U*GG) and cp tRNA(Arg)(ICG) are able to decode the corresponding four-codon family. In the case of leucine codons CUU/C, we show that 'U:U and U:C wobble' mechanisms can operate to allow the reading of these codons by cp tRNA(Leu)(UAm7G).

Effect of isoniazid, a haem inhibitor, on globin chain synthesis in reticulocytes from non-thalassaemic and beta thalassaemic subjects

The effect of isonicotinic acid hydrazide (INH), a potent haem inhibitor, on globin chain synthesis was studied in reticulocytes from the following groups of patients: four non-thalassaemic patients (group i); five beta thalassaemia heterozygotes (group ii); three Hb S/beta thalassaemia heterozygotes (group iii); and two additional patients--one with homozygous beta thalassaemia and the other with thalassaemia intermedia (group iv). This was done to determine whether haem inhibitors depress alpha globin chain synthesis. The progressive increase of INH concentration (10-40 mmol l-1) in reticulocytes from a beta thalassaemia heterozygote resulted in a remarkable decrease of the alpha and beta chain synthesis, ranging from 80% to 97% and from 74% to 96% of control values, respectively, and in a gradual drop of alpha:beta ratio from 1.87 to 1.38. Furthermore, in the samples incubated with 40 mmol l-1 INH, a pronounced inhibition of globin chain synthesis 77 (19%) for alpha chain and 67 (27%) for beta or beta S chain) and a substantial drop of the alpha:beta or beta S ratio in samples with INH (median 1.16) compared with that in samples without INH (median 1.70) were observed. The inhibitory effect of INH was significantly or completely corrected by adding exogenous haem. It is suggested that haem inhibition and the resulting preferential diminution of alpha chain synthesis could provide a new approach to the treatment of homozygous beta thalassaemia with an excess of detrimental free alpha chain in erythroid cells.

Enzymatic amplification of translation inhibition of rabbit beta-globin mRNA mediated by anti-messenger oligodeoxynucleotides covalently linked to intercalating agents

The effects of anti-messenger oligodeoxynucleotides, covalently linked to an intercalating agent, on translation of rabbit beta-globin mRNA, were investigated both in wheat germ extract and in microinjected Xenopus oocytes. A specific inhibition of beta-globin synthesis was observed in both expression systems with a modified 11-mer covalently linked to an acridine derivative. In injected oocytes a more efficient block was observed with this modified oligonucleotide than with its unsubstituted homolog. This was ascribed to stacking interactions of the intercalating agent with base pairs which provide an additional stabilization of the [mRNA/DNA] hybrid. We demonstrated that in wheat germ extract, the modified and unmodified oligonucleotides behaved similarly due to the presence of a high RNaseH activity. RNaseH was also present, although to a lesser extent, in the oocyte cytoplasm. This anti-messenger DNA-induced degradation of target mRNA resulted in amplified efficiency of hybrid-arrested translation. This additional mechanism might provide anti-sense DNAs with an advantage over anti-sense RNAs.

Anti-messenger oligodeoxynucleotides: specific inhibition of rabbit beta-globin synthesis in wheat germ extracts and Xenopus oocytes

Oligodeoxyribonucleotides complementary to the initiation region of rabbit beta-globin messenger RNA were used to selectively inhibit translation in a wheat germ extract and in injected Xenopus oocytes. The oligonucleotides interacted specifically with their RNA target as shown by thermal denaturation studies of hybrids on nitrocellulose filters. The longest oligonucleotide used (17-mer) efficiently blocked translation both in vitro and in vivo. In contrast the shortest one (8-mer) exhibited only a limited effect. The translation block was specific. The synthesis of endogenous proteins in oocytes and that of alpha-globin in the in vitro system were not affected by anti-beta-globin oligonucleotides. A non-complementary oligonucleotide had no inhibitory effect.

Differential breakdown of phylogenetically diverse ribosomal RNA's inserted via liposomes into mammalian cells

Liposomes were used to deliver ribosomal RNA's from the different organisms into cultivated mouse plasmacytoma cells. Ribosomal RNA from Escherichia coli was degraded intracellularly within 1 hour, whereas mouse and yeast ribosomal RNA's were degraded more slowly. This indicates that cells can discriminated between different ribosomal RNA's.

Nonsense suppression in eukaryotes: the use of the Xenopus oocyte as an in vivo assay system

Amber, ochre, and opal nonsense suppressor tRNAs isolated from yeast were injected into Xenopus laevis oocytes together with purified mRNAs (globin mRNA from rabbit, tobacco mosaic virus-RNA). Yeast opal suppressor tRNA is able to read the UGA stop codon of the rabbit beta-globin mRNA, thus producing a readthrough protein. A large readthrough product is also obtained upon coinjection of yeast amber or ochre suppressor tRNA with TMV-RNA. The amount of readthrough product is dependent on the amount of injected suppressor tRNA. The suppression of the terminator codon of TMV-RNA is not susceptible to Mg++ concentration or polyamine addition. Therefore, the Xenopus laevis oocyte provides a simple, sensitive, and well buffered in vivo screening system for all three types of eukaryotic nonsense suppressor tRNAs.

Unequal alpha and beta globin mRNA in reticulocytes of normal and mutant f/f fetal mice

Messenger RNA (mRNA) was isolated from adult as well as normal and mutant f/f fetal mouse reticulocytes by poly-U affinity chromatography. mRNA from normal adult reticulocytes directed equal synthesis of alpha and beta globin chains in wheat germ cell free translational system. mRNA from either normal or mutant f/f fetal reticulocytes directed 40% more alpha globin synthesis than beta globin synthesis. These observations are suggestive that there is significantly more alpha mRNA than beta mRNA in normal and mutant f/f fetal mouse reticulocytes.

Free cytoplasmic messenger ribonucleoprotein complexes from rabbit reticulocytes

Free cytoplasmic globin mRNA containing mRNP-particles were isolated from rabbit reticulocytes by zonal sucrose gradient centrifugation and their properties were compared with mRNP particles isolated in the same way from EDTA-dissociated reticulocyte polyribosomes. The average poly(A)-length of 9S mRNA from free cytoplasmic mRNP was 17-20 nucleotides being about two times shorter than the average poly(A)-length of polysomal 9S mRNA. The protein composition of the free cytoplasmic mRNP particles disclosed the absence of the 76,000 dalton protein which is associated with the 3'poly(A)-segment of polysomal globin mRNA. It was concluded that free cytoplasmic mRNP-particles from rabbit reticulocytes can be classified as "old" mRNP in a post-translational phase. Free cytoplasmic mRNPs were translated in heterologous cell-free systems as well as in Xenopus laevis oocytes. Addition of hemin stimulated the synthesis of alpha-globin in all systems, while the presence of the cap analogue m7G(5')p inhibited translation of free cytoplasmic mRNA completely. The latter finding suggested that free cytoplasmic mRNA has a 5' terminal "cap". Shortening of the poly(A)-segment with concomitant loss of the 76,000 dalton protein may lead to less efficient translation of free cytoplasmic mRNP.

Messenger RNA competition in living Xenopus oocytes

When calf lens crystallin mRNA and rabbit globin mRNA are competing for factors limiting protein synthesis in living Xenopus oocytes, no mRNA species is preferentially selected for translation. Differences in the intrinsic translational efficiency of the mRNA species exist, but the relative efficiencies are the same at high and low mRNA concentrations. mRNAs already being translated, in particular endogenous oocyte mRNAs, are less sensitive to competitive inhibition by injected mRNAs. As injected mRNAs gradually become incorporated into the protein-synthesizing machinery of the oocyte, they acquire the same status as the oocyte's own active mRNAs. Exogenous mRNAs this become endogenous mRNAs. These results, together with previous estmates of the translational efficiency of injected heterologous mRNA species, are compatible with the assumption that a large proportion of the endogenous mRNAs is not competing for the translational apparatus of the oocyte and, therefore, probably is present in the temporarily inactivated form.

Synthesis of lens crystallins in Xenopus oocytes as determined by quantitative immunoprecipitation

Total poly(A)-containing calf lens mRNA was microinjected into Xenopus oocytes and synthesis of alpha, beta, and gamma-crystallins was demonstrated. By a method of quantitative immunoprecipitation the rate of translation of purified 14S alphaA2-crystallin mRNA was compared with translation of 9-S rabbit globin mRNA. Maximal response of oocytes was obtained with virtually the same molar amounts of mRNA, taking into account the larger size of the alphaA2-crystallin mRNA. Kinetics of translation were also very similar and both mRNAs were translated with similar rate and efficiency for at least two days. It was estimated that 20-30 polypeptide chains per hour per mRNA molecule were synthesized.

Maintenance of the ratio of alpha and beta globin synthesis in rabbit reticulocytes

The synthesis of rabbit alpha and beta globins under various conditions was studied using intact reticulocytes and reticulocyte cell-free systems. Raising salt concentration of media in which reticulocytes were incubated with radioactive amino acids reduced the total protein synthesis but did not affect the ratio of alpha to beta globins produced. Using a reticulocyte lysate which had been incubated with micrococcal nuclease to remove the endogenous globin messenger RNA activity, it was found that unlike the intact cell or the untreated lysate very little alpha globin was synthesised on adding purified globin mRNA. These results are discussed in terms of their compatibility with some proposed models of coordination of alpha and beta globin production.

The croonian lecture, 1976. Egg cytoplasm and gene control in development

This article is concerned with how a fertilized egg develops into a complete individual. In nearly all animal species (the main exceptions being mammals), ferti­lized eggs develop entirely independently of their mother. Commonly, eggs are surrounded by layers of materials such as membranes, jelly layers, or a shell, which isolate the egg and developing embryo from their environment. Embryos remain inside these coverings until they hatch as a mobile, free-living larva capable of feeding itself. Such a stage is usually reached only a few days or even hours after fertilization. During this time, development appears to take place without any chemical or physical instructions from the embryo’s environment. Development involves the formation of hundreds or thousands of cells from a single fertilized egg cell, as well as the conversion of yolk, a food-reserve, into the numerous different kinds of proteins which make up the cells of a complete larva. The question of how a superficially structureless egg converts itself, in a relatively short time, into a complex and highly organized structure has interested scientists since the time of Aristotle, 2000 years ago. However, specific concepts or explanations of early development were not well formulated until the eighteenth century. In 1779, for example, Bonnet made the explicit proposal that in each egg is a miniature embryo which itself contains an ovary with eggs, each of which themselves contain miniatures with ovaries, and eggs, and so on - the so-called doctrine of ‘emboitement’. Even Bonnet did not believe this doctrine in its strict sense, which would demand, as Bonnet’s own calculations showed, that Eve would have had 27 million embryos in her ovary. Throughout the nineteenth century there was extensive discussion of the relative merits of epigenesis and preformation. † In the later part of the nineteenth century, there arose the concept of neopreformation, according to which the preformed components of a fertilized egg were thought of as molecules and not morphological structures. For example, Lankester (1877) stated that: ‘Though the substance of a cell may appear homo­geneous under the most powerful microscope, excepting for the fine granular matter suspended in it, it is quite possible, indeed certain, that it may contain,already formed and individualised, various kinds of physiological molecules. The visible process of segregation is only the sequel of a differentiation already estab­lished, and not visible.’ This concept of the existence of determinant molecules in eggs may be taken as the point of departure for the present article. A concise account of early theories of development is included in Davidson’s (1968) book, and a history of embryology has been published by Needham (1934) and Oppenheimer (1955).

Translation and stability of human globin mRNA in Xenopus oocytes

Human globin messenger RNA (mRNA) prepared from erythroid cells of patients with sickle cell anemia has been translated in Xenopus laevis oocytes. Addition of hemin to the injected mRNA causes total globin synthesis to increase and the ratio of alpha- to betas-globin synthesis (alpha/betas ratio) to approach unity. To determine the effect of the length of the poly-(A) segment on human globin mRNA stability, 10 S globin mRNA was fractionated into poly-(A)-poor fractions by oligo (dT)-cellulose column chromatography. When oocytes are injected with each of these fractions, translation of the poly-(A)-rich globin mRNA is sustained for a longer period than that of the poly-(A)-poor mRNA. Regardless of the mRNA fraction injected, the alpha/betas ratio of the synthesized globin decreases as the injected oocytes are incubated for longer periods. The results indicate that in frog oocytes poly-(A)-rich mRNA has greater translational stability than poly-(A)-poor mRNA, AND beta-mRNA has greater stability than alpha-mRNA with comparable poly-(A) content.

Rat b/b anemia: translation of normal and anemic globin mRNA in wheat-germ cell-free system

Globin mRNAs were isolated from circulating reticulocytes both from rats carrying a homozygous, recessive mutation causing a severe thalassemia-like syndrome and from normal rats. After first identifying the rat globin chains as alpha or beta chains, the translational products primed by both polysomal and nonpolysomal mRNAs in wheat germ 30000 x g supernatant were analyzed: the ratio of alpha to beta globin mRNAs found in polysomes isolated from mutant rats is identical to the ratio of their products synthesized in vivo while the ratio of these mRNAs is quite different in the nonpolysomal fraction, the latter being enriched in alpha globin mRNA. No difference is found in the ratio of alpha and beta globin mRNAs in the polysomal and nonpolysomal RNA isolated from normal rats, both being identical to the ratio of their products synthesized in vivo. One third of the total amount of mRNA found in mutant cells is not in polysomes as compared to only 6 percent for the mRNA from normal lysates. These results suggest that a translational control mechanism is involved although the decreased globin synthesis in b/b anemia can not be fully accounted for by its operation.

Interaction between the synthesis of alpha and beta globin

We have examined the relationship between alpha and beta globin chain syntheses by utilizing the distribution of isoleucyl residues in rabbit hemoglobin. The alpha globin chain contains three isoleucyl residues while the beta chain of certain rabbits contains no isoleucine. O-Methyl-L-threonine, an isoleucine isostere, inhibits incorporation of radiolabeled amino acids into alpha chains in rabbit reticulocytes. When alpha chain synthesis is inhibited by 50-85%, beta synthesis is stimulated by 15-50%. The excess labeled beta chains are not distinguishable from authentic beta chains by any of the following criteria: (a) carboxymethyl cellulose chromatography in sodium phosphate-urea buffers, (b) electrophoresis on polyacrylamide gels containing sodium dodecyl sulfate, and (c) electrophoresis of methionine-containing tryptic peptides. The stimulation of beta synthesis continues after the pool of excess alpha chains has been exhausted by preincubation with O-methyl-L-threonine. The stimulation does not occur, however, when 1 mM 2-mercaptoethanol is added to the incubation medium or when the cells are excessively diluted in the incubation mixture. The rates of beta chain initiation and elongation during stimulation have been compared to the rates during normal synthesis. Although both rates are increased, the rate of elongation increases more than initiation, suggesting that initiation is the rate-limiting step in increased beta chain production. The stimulation of beta synthesis when alpha synthesis is inhibited is interpreted as resulting from relief of competition between alpha and beta mRNAs for limiting components of the protein synthetic apparatus.

Absence of polyadenylate segment in globin messenger RNA accelerates its degradation in Xenopus oocytes

Rabbit globin poly(A)-free and native mRNA preparations were microinjected into Xenopus oocytes. The amount of globin message remaining after incubation of injected oocytes was determined by molecular hybridization with a radioactive complementary DNA probe, synthesized by copying globin mRNA with purified RNA-dependent DNA polymerase (reverse transcriptase) from avian myeloblastosis virus. The results show that 56 hr after injection about 85% of the poly(A)-free mRNA molecules are degraded, while native poly(A)-containing mRNA chains are almost completely preserved during the same period of time.