Reticulocyte polyribosomes

Waxman HS, Rabinowitz M. 1966. Control of reticulocyte polyribosome content and hemoglobin synthesis by heme. Biochim Biophys Acta 129 369-379. 

Hori, M., Fisher, J. M., and Rabinovitz, M., Tryptophan deficiency in rabbit reticulocytes Polyribosomes during interrupted growth of hemoglobin chains, Science, 155J758], 83, 1967. 

Huez, G., A. Burny, and G. Myrbaix. 1967. Release of messenger RNA from rabbit reticulocyte polyribosome at low concentration of divalent cations. Biochim. Biophys. Acta, 145 629-636. 

Temmerman, J., and B. Lebleu. 1969. Evidence for the detachment of a RNP m-RNA complex from EDTA-treated rabbit reticulocyte polyribosomes. Biochim. Biophys. Acta, 174 544-550. 

An RNA fraction derived from reticulocyte polyribosomes has been isolated by Marbaix and Burny (1964). This fraction was characterized by a sedimentation constant of 9 to 10 S, by high specific activity following labeling, and by extreme sensitivity to ribonuclease. Evidence for its capacity to code for the synthesis of globin remains to be provided. 

Ryazanov AG, Ashmarina LI, Muronetz VI Association of glyceraldehyde-3-phosphate dehydrogenase with mono- and polyribosomes of rabbit reticulocytes. Eur J Biochem 1988, 171(1—2) 301-305. 

Fig. 15. Polysomal mRNP from reticulocytes. A. Sucrose gradient sedimentation of the polyribosomal suspension after EDTA treatment (40 hours centrifugation). B.Sucrose gradient sedimentation of unlabeled, sodium dodecyl sulfate-extracted polyribosomal RNA supplemented with the high specific radioactivity RNA detached from labeled polyribosomes by EDTA treatment. C. CsCi equilibrium sedimentation of the messenger ribonucleoprotein complex. Ribosomal subparticles have been added as Internal markers. The centrifugation was carried out at 4°C for 18 hours at 36,000 rpm in an SW-39 rotor. (From Huez et al. 1967. Biochim. Biophys. Acta, 145 629-636 Burny et al. 1969. Biochim, Biophys. Acta, 190 228-231.)...

Fig. 3. Schematic representation of the distribution of nascent a- and /S-globin chains on the polyribosomes of a rabbit reticulocyte lysate. The ribosomes have been separated into 80 S monomers and polyribosomes by sucrose density gradient centrifugation. The numbers 2-6 denote the number of ribosomes per mRNA in each 260 nm optical density peak (continuous line). After Hunt et al. (1968) and Lodish and Jacobsen (1972).

The existence of selective inhibition of HbA synthesis in thalassemia was demonstrated by experiments to study comparative HbA and HbF synthesis by ribosomes of reticulocytes from patients with various forms of hemoglobinopathies (Burka and Marks, 1963). In fact, although the total concentration of ribosomes and polyribosomes in the reticulocytes is unchanged in thalassemia, the ability of these ribosomes to incorporate C -leucine was sharply reduced below the control level. Meanwhile, incorporation of isoleucine (present in HbF but absent from HbA) was increased in this case. Burka and Marks postulate that these anomalies are evidently associated with structural changes in messenger RNA for the a chain or with a decrease in the rate of synthesis of this mRNA. 

Marks, B. A., Rifkind, R. A., and Danon, D. (1963). Polyribosomes and protein synthesis during reticulocyte maturation in vitro. Froc. Natl. Acad. Sci. U.S. SO, 336-342. 

These experiments have led to the conclusion that the effect of hemin in promoting polyribosome formation is dependent on significant protein synthesis. When reticulocytes are pulse labeled with amino acids for less than 10 minutes in the presence of hemin, one observes increased radioactivity in the polypeptide chains attached to the polyribosomes. In addition, the time required to reach maximal radioactivity of the nascent polypeptide chains is much shorter in hemin-treated cells than in control cells. These findings suggest that the addition of hemin may promote more rapid translation of the mRNA. 

These findings of excess chains in the rabbit reticulocyte, the presence of large numbers of excess chains in / thalassemic reticulocytes (Fessas and Loukopoulos, 1964 Weatherall et al., 1965 Bank and Marks, 1966) and the finding that some 40% of the total hemoglobin in a thalassemia may exist as y8 chain tetramers (hemoglobin H), provide ample evidence that neither nor y8 chains are essential for the release of the other. This evidence is at variance with the model of Colombo and Baglioni (1966) in which yS chains are required for the release of completed chains from the polyribosomes. It is possible that an a or yS chain may facilitate the release of the complementary ehain but neither is essential for release of the other. 

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