Rate of hemoglobin synthesis controlled at the translational level in differentiating erythroid cells from adult mice

Polyribosomal structures inactive for protein synthesis present in erythroid cells during terminal stages of differentiation

During the terminal stages of differentiation nucleated erythroid cells from the fetal mouse synthesize hemoglobin at a lower rate because after the last cycle of cell division about half of their polyribosomal structures are rendered inactive for protien synthesis though they maintain their aggregated shape. Partially inactive polyribosomes are tested in comparison with normal polyribosomes for the capacity to support polypeptide chain synthesis in cell-free conditions. The following observations are made: a) no difference is found for the profile on sucrose density gradients; b) partially inactive polyribosomes carry growing polypeptide chains in reduced amounts in comparison with active polyribosomes; c) partially inactive polyribosomes are not capable to release "run off" 80 S ribosomal monomers and to dissociate to active ribosomal subunits. These data are interpreted as the evidence for a block of chain termination producing inactivation of polyribosomes during the late maturation of nucleated erythroid cells.

Rate of synthesis and half-life of globin messenger ribonucleic acid. Rate of synthesis of globin messenger ribonucleic acid calculated from data of cell haemoglobin content

By the use of the favoured models defining mRNA synthesis and half-life from the preceding paper (Hunt, 1974) and the known content of globin in a reticulocyte it is possible to estimate the absolute rate of mRNA and globin synthesis and the mRNA and globin content in each type of erythroid cell. The best model requires an mRNA-synthetic rate of 3000 molecules per h/cell. This rate compares favourably with the estimated chain-extension rate of 43 nucleotides/s in Escherichia coli (Manor et al., 1969) provided that the four alpha- and beta-chain cistrons per cell are transcribed by polymerases spaced 50 nucleotide base pairs apart. Similar calculations can be made for erythropoiesis in the chick embryo, where cell times and relative globin content at each mitosis have been measured (Campbell et al., 1971), but where no reliable estimates of mRNA half-life have been made. In this case it was estimated that a constant rate of mRNA synthesis at 10000 molecules per h/cell through six cell divisions is necessary if the mRNA half-life is 15h; after the sixth mitosis the mRNA synthesis would stop and its half-life would increase to approx. 20h. If an mRNA half-life of 4.5h is used, the synthesis rate through the six mitoses would be 21000 molecules per h/cell, ceasing at the sixth mitosis, when the half-life would need to increase to 25h. The chain-elongation rate for the four alpha- and beta-globin cistrons per cell would be 1-2 times higher than in E. coli and would either require a greater rate, polymerases spaced between 25 and 50 nucleotide base pairs apart on the DNA, or limited gene replication. These possibilities are discussed in the light of the low values found for globin cistron multiplicity in ducks and mice.