Specific Mechanisms of Protein Synthesis in the Developing Chick Embryo 131 



These intermediates are not free amino acids, as evidenced by their relatively 

 low incorporation rates. They may be small peptides or activated forms of 

 amino acids, formed readily and reversibly from protein precursors, but not 

 identical and not in equilibrium with the pool of added low-molecular weight 

 precursors. This view would be in accord with the findings of Francis and 

 WiNNiCK (12), although not with their interpretation. The occurrence of 

 pools of modified amino acids, incapable of equilibrating with those in the 

 medium, has been demonstrated in micro-organisms. Thus Gale, working 

 with Staphylococcus aureus, found that added glutamic acid could be so trans- 

 formed, and the modified fonn used for protein synthesis (24). Similarly 

 CowiE and Walton (25) have presented evidence that the pools of amino 

 acids formed metabolically in Torulopsis utilis and utilized as effective precur- 

 sors in protein synthesis, are present in some modified form, possibly as com- 

 plexes adsorbed onto macromolecules, and do not equilibrate freely with 

 added amino acids in the medium. In all the cases presented, this metaboli- 

 cally active form of the amino acids may be formed by a variety of pathways 

 as indicated below. 



Proteins 



1 



[Peptide Intermediates] 



y 



>' 



1 



Free peptides ^'Amino Acids'-^ Free amino acids 



(modified) 



Recent investigations, especially by Zamecnik and his collaborators, (26) 

 have disclosed that free amino acids are first 'activated' by enzymes in the 

 soluble portion of the cytoplasm (27), probably through mixed anhydride 

 formation with adenylic acid (27, 29, 30) prior to their incorporation into a 

 protein-bound form (30, 31), which takes place in RNA-rich granules associated 

 with the microsomal fraction of homogenates (32, 33, 34). Whether or not 

 the metabolically active form of amino acids alluded to above can be equated 

 with these aminoacyl adenylates has not yet been established. 



An alternative explanation, which has been invoked to account for apparent 

 preferential utilization of proteins over amino acid precursors in the formation 

 of specific proteins, postulates proteolysis and protein synthesis sites in such 

 close spatial juxtaposition as to permit ready transfer of intermediates from 

 breakdown to synthesis site at the expense of penetration of the latter by 

 added amino acids. This has been suggested by Loftfield and Harris (10) 

 as the mechanism operative in ferritin synthesis, and by Walter et al. (20) 

 in the transformation of serum into organ proteins. Purely spatial factors 

 of this sort are probably not the determining ones in the present instance, 

 since it can be demonstrated that the bulk of the proteolytic activity is centred 

 in the yolk (23), and thus remote from the synthetic activity which is, presum- 

 ably, occurring in the embryo itself. It is hoped that critical experiments 

 now in progress will permit a choice to be made between the various alter- 

 natives suggested. 



