Synthesis of Rabbit Globin by Reticulocyte Postribosomal Supernatant and Heterologous Ribosomes

Cell‐free protein synthesis has been studied in heterologous systems using rabbit reticulocyte postribosomal supernatant and ribosomes obtained from HeLa or mouse myeloma cells. These cell‐free systems are quite active and incorporate labelled amino acids linearly for more than 30 min. The proteins synthesized have been analyzed by acrylamide gel electrophoresis, column chromatography and paper electrophoresis of the tryptic peptides. More than 70% of the protein synthesized with either type of ribosomes is α chain of rabbit hemoglobin; a small amount of β chain is also synthesized. This result can only be explained by the presence in the postribosomal supernatant of reticulocytes of messenger RNA for hemoglobin. This mRNA is translated more efficiently than the endogenous mRNA present on HeLa or myeloma ribosomes; previous measurements of the mRNA of reticulocyte postribosomal supernatant indicate that it is less on a weight basis than 1/20 of the mRNA associated with the corresponding amount of ribosomes used in an incubation. The preferential translation of mRNA present in the supernatant, presumably in a ribonucleoprotein particle, is discussed.