Template Activity for Globin and Non-Globin Protein Synthesis in Reticulocytes

Factors which control the rate of synthesis of two distinct classes of protein, globin and non-globin proteins, by free reticulocyte ribosomes have been investigated.Three different inhibitors of protein synthesis, puromycin, cycloheximide, and hydroxylamine, were more effective in inhibiting the synthesis of globin than of non-globin protein by free reticulocyte ribosomes in the cell-free system.In contrast to the different degree of response to puromycin by free ribosomes in synthesizing globin and non-globin proteins, template activity for both classes of protein decreased at similar rates during maturation of reticulocytes in oiuo and during storage of reticulocyte ribosomes at 0'.The synthesis of non-globin protein by either free or membrane-bound ribosomes was similar in that puromycin in- hibited both to the same degree.The findings indicate that template activity for globin and non-globin protein in reticulocytes is not directed by a contiguous polycistronic messenger RNA, and suggest that the two classes of protein are not synthesized on the same ribosomes.The mammalian reticulocyte cell-free system has provided a valuable model in which to study the mechanisms involved in the synthesis of specific proteins in the eukaryocytic cell.A particularly attractive feature of this system is the property to synthesize predominantly a single well characterized protein, globin.A lesser amount of non-globin protein, about 14% of total protein synthesized by the intact reticulocyte, is also produced (1).Further, it appears that while both globin and non-globin protein are synthesized on ribosomes which exist, free within the cell, membrane-bound ribosomes synthesized only non-globin protein (1).Similar selectivity in protein synthesis by free and membrane-bound ribosomes has been documented in other mammalian cell types (2-5).The synthesis of well defined amounts of two distinct classes