Hemoglobin synthesis was studied in the rabbit reticulocyte cell-free system of R. Schweet, H. Lamfrom, and E. Allen (1958, Proc. Natl. Acad. Sci. U.S. 44, 2163). In this system, amino acid incorporation proceeds at about 1 % the whole cell rate of 100 amino acids per minute per ribosome. The amino acids are added stepwise to incomplete polypeptide precursors found in the ribosomes; chain growth proceeds from the amino to the carboxyl terminus. Finished chains are released from the ribosomes without any appreciable delay. However, the initiation of new chains is essentially absent. One complete hemoglobin chain is released per three to four ribosomes in the most active preparations. Thus, about half of the ribosomes which were participating in protein synthesis in the whole cell can complete and release their polypeptide chains during the cell-free incubation. On the average, 50% of each chain released is composed of amino acids added during the cell-free incubation. Hence, the chains which are completed are selected, without regard to their initial length, from the total polypeptide chain population present on ribosomes prepared from the whole cell.
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