The origin and nature of the methionine residue initiating the synthesis of haemoglobin in vivo and in vitro

Genomic RNA of mengovirus. VI. Translation of its two cistrons in lysates of interferon-treated cells

The addition of low levels (40 ng/ml) of the synthetic double-stranded polyribonucleotide poly I:C to lysates of interferon-treated L-cells resulted in a strong inhibition (70 to 75%) of the in vitro translation of mengovirus RNA. Under these conditions, the rates of incorporation of [35S]methionine or formyl-[35S]methionine were depressed to a comparable extent. The sequences of mengovirus RNA recognized by ribosomes of interferon-treated cells at initiation of translation were compared with those present in initiation complexes formed by ribosomes of untreated controls. Fingerprint analysis revealed that the same sequences of mengovirus RNA were protected against nuclease attack by the 80S and the 40S initiation complexes formed in vitro in lysates of control or interferon-treated L-cells. Mengovirus RNA-coded proteins were labeled at their N-terminal end with formyl-[35S]methionine and digested to completion with trypsin. The resulting fragments were separated by high-voltage paper electrophoresis. Two different formyl-[35S]methionine-labeled N termini were resolved. Further analyses supported the notion that the two radioactive peaks originated in the initiation of translation at two different sites. This pattern did not change when mengovirus RNA was translated in lysates of interferon-treated cells.

The initiation of fetal hemoglobin biosynthesis

Biosynthesis of the alpha and beta chains of rabbit and human adult hemoglobin is initiated with a methionyl residue, which is removed during elongation of the peptide chain. To study the initiation of biosynthesis of the delta chain of human fetal hemoglobin, fresh placental blood was used for labeling experiments with radioactive amino acids. Labeled nascent peptide chains were purified from the polysomal fraction of placental blood reticulocytes. The number of amino acid residues in nascent gamma chain at the time of removal of its N-terminal methionine was estimated to be 40--60 from the relative yields of labeled tryptic peptides.

Inhibition of hemoglobin synthesis by pancreatic deoxyribonuclease in rabbit reticulocyte lysates

A pancreatic deoxyribonuclease preparation, shown to be free of significant ribonuclease activity, inhibits hemoglobin synthesis in a rabbit reticulocytelysate. Preincubation with DNAase (15 min at 25 degrees C) is required to obtain a marked inhibitory effect (nearly 70%). It has been shown that DNAase does not significantly interfere with the elongation or termination steps of translation, but it seems to prevent reinitiation of globin polypeptide chains allowing polysome run-off. In particular, the formation of the 40S/met-tRNAmetf complex is greatly reduced in the DNAase-treated lysate. At the moment the mechanism by which DNAase inhibits initiation is not clear.

Inhibition of Protein Synthesis in Cell-Free Systems from Interferon-Treated, Infected Cells: Further Characterization and Effect of Formylmethionyl-tRNA(F)

The translation of encephalomyocarditis virus (EMC) RNA is markedly inhibited in cell-free systems from interferon-treated, vaccinia virus-infected L-cells (10, 11). The polypeptide products synthesized in response to EMC RNA in cell-free systems from these and untreated infected cells have been analyzed by electrophoresis on polyacrylamide gels. Qualitatively, the same EMC-specific polypeptides were synthesized throughout. In experiments using preincubated microsomes from normal Krebs cells to assay cell sap from L-cells which had been exposed to interferon prior to infection, only the amount of the EMC-specific polypeptide products was reduced. This result suggests that there is an inhibition very early in translation in interferon-treated, infected cells. Initiation seems a priori the more attractive site for this inhibition, but an effect shortly after initiation cannot be excluded. With unfractionated cell-free systems from interferon-treated infected L-cells, however, there appeared to be an additional minor inhibitory effect on polypeptide chain elongation, in that the EMC-specific polypeptides synthesized showed not only a reduction in amount but also a bias towards lower molecular weight. The formylated methionyl initiator tRNA (Fmet-tRNA(F)) was used as a further probe into the apparent effect on intiation. With this reagent we have confirmed that there is one major initiation site for the translation of EMC RNA in these cell-free systems. In addition, the results have shown that EMC-specific polypeptide chains initiated with Fmet escape the major interferon-mediated inhibition at or shortly after initiation.

Delayed removal of N-terminal methionine from nascent globin chains in sickle-cell anemia reticulocytes

Synthetic alphaT1 and betaT1, the N-terminal tryptic peptides of alpha-chain and beta-chain of hemoglobin, and MetalphaT1 and MetbetaT1, peptides in which N-terminal methionyl residues are peptide-bonded to alphaT1 and betaT1, were prepared by the solid-state method of Merrifield. These synthetic peptides were used to establish conditions for chromatographic purification and analysis. When tryptic digests of nascent globin chains from rabbit and sickle-cell anemia reticulocytes incubated with (35)S- and (3)H-labeled amino acids were analyzed, radioactivity not present in tryptic digests of labeled hemoglobin appeared at the elution positions of MetalphaT1 and MetbetaT1. The fraction of nascent chains with N-terminal methionine was higher in sickle-cell anemia reticulocytes than in rabbit reticulocytes. If rate of peptide-chain elongation in polysomes is uniform, nascent human chains must attain a greater length before removal of the initial methionyl residue. Length of nascent chain at time of removal was calculated from two independent sets of data, one obtained from [(35)S]methionine incorporation into MetalphaT1, MetbetaT1, alphaT5, and betaT5, and the other obtained from [(3)H]lysine and [(3)H]valine incorporation into MetbetaT1 and betaT1.

Calf crystallin synthesis in frog cells: the translation of lens-cell 14S RNA in oocytes

14S RNA isolated from calf-lens polyribosomes was injected into oocytes of the frog Xenopus laevis. Oocytes injected with 14S RNA and buffer contained a protein resembling the A2 chain of calf alpha-crystallin; oocytes injected with buffer alone contained no crystallin-like material. alphaA2 crystallin polypeptides were identified by various criteria: urea-gel electrophoresis under acidic and basic conditions, gel electrophoresis in sodium dodecyl sulfate, N-terminal analysis, and paper chromatography of methionine-containing tryptic peptides. It is concluded that when it is injected into a living frog oocyte, the 14S RNA from lens tissue is reasonably stable and has the properties of an alphaA2 crystallin messenger. The messenger requires no lens cell-specific components for translation within the oocyte, and the translational machinery of the frog cell will accept messenger RNA from a totally different cell type from another species. The A2 chains of alpha-crystallin extracted from lens tissue possess an acetylated N-terminal methionine residue; the N-terminal methionine of alphaA2 chains derived from frog oocytes injected with 14S RNA was also acetylated.