VIII. PROTEIN SYNTHESIS AND GENE ACTION 375 



results were obtained in the cell-free system from reticulocytes (Schweet 

 et al, 1958b). 



Since earlier studies had siiown that the labeled ribosomal material 

 could be "chased" directly into soluble hemoglobin when labeled ribo- 

 somes were incubated in the complete cell-free system (Schweet et al., 

 1961; ]\Iorris and Schweet, 1961), this system was used to study the 

 nature of the ribosomal intermediates. Ribosomes were labeled in tlie 

 intact reticulocyte with C^'* -valine, the N-terminal amino acid of 

 hemoglobin. These ribosomes were washed and then incubated with 

 C^--valine in the cell-free system. Comparison of the amount of C^'*- 

 valine in N-terminal positions and in internal positions in the soluble 

 hemoglobin formed, indicated that the ribosomes contained incomplete 

 chains synthesized sequentially from the N-terminal end (Bishop et al., 

 1960b). This was also demonstrated in intact reticulocytes by similar 

 methods (Schweet et al., 1961). 



Dintzis (1961) studied the labeling in peptides formed by the tryptic 

 digestion of the separated chains of hemoglobin labeled in the intact 

 cell. He showed that tryptic peptides isolated from hemoglobin after 

 short periods of incubation with labeled leucine could be arranged in a 

 sequence of increasing specific activity relative to the specific activities 

 of the same peptides from hemoglobin isolated after 6 hours of incuba- 

 tion. The peptide with the highest specific activity after short periods of 

 labeling was near the C-terminus. These results are also suggestive of 

 synthesis from the N-terminus is sequential fashion, although other 

 interpretations of labeling data using intact cells are possible (see 

 Steinberg et al., 1956). The results of Goldstein and Brown (1961) indi- 

 cate that this mechanism may be general. A leucine-requiring strain of 

 E. coli was starved of leucine to reduce the possibility of interference by 

 unlabeled pools and then labeled for very short times with C"-leucine. 

 The shorter the labeling period, the more C^*-leucine was found near 

 the C-terminus. On the other hand, Shimura et al. (1956) have shown 

 that shortly after injection of C'^-glycine into silkworm larvae, the 

 N-terminal glycine of silk fibroin had the highest specific activity. A 

 sequential synthesis of many proteins from the N-terminal end appears 

 likely. 



The significance of polypeptide chain synthesis from the N-terminal 

 end, if any, is not known ; however, if one makes several reasonable 

 assumptions some interesting consequences follow. The two main assump- 

 tions are: 



1. In addition to functioning to position amino acids in the proper 

 sequence for the protein being made, the ribosome functions to position 

 amino acids in the proper steric relation so that the peptide bond can be 



