Nascent Globin Chains from Rabbit Reticulocyte Ribosomes

Evidence is presented to show that rabbit reticulocyte ribosomes contain a significant component of completed α-globin which is still attached to tRNA (α-globyl-tRNA). Additional data are presented to show that contamination by labeled supernatant hemoglobin or labeled α-globin from the free α-globin pool present in reticulocytes is not a significant factor in these results. Some 4.6% of the nascent α-globin chains are present as α-globin-tRNA, instead of 0.71% as predicted on the basis of the assumption that the size distribution of nascent globin chains is uniform. On the other hand β-globyl-tRNA comprises 0.69% of the nascent β-globin chains. This value coincides closely with the predicted value for nascent β-globin chains uniformly distributed in size along the polysome. Further evidence is presented to show that both α-globyl-tRNA and β-globyl-tRNA exhibit the kinetic properties expected for normal intermediates of soluble hemoglobin biosynthesis following inhibition of the initiation of protein synthesis by pactamycin.