Genetic Cotitrol of Protein Synthesis 



65 



thought to carry amino acids to the ribosome-messenger complex 

 and to act as an "adapter" to position amino acids in the proper 

 place for their polymerization into specific proteins (12). The 

 transfer RNA presumably "recognizes" the code for a particular 

 amino acid in the messenger RNA in order to provide specific 

 positioning of the amino acid. 



The evidence that transfer RNA is an intermediate in protein 

 synthesis is very good, at least in in vitro systems, and there is 

 strong experimental support for the idea that ribosomes are the 

 site of protein synthesis from both in vivo and in vitro studies (12). 

 The question of whether there is a distinct messenger RNA loosely 

 attached to nonspecific ribosomes, or whether the genetically 

 specific RNA is built into ribosomes as they are synthesized, giving 

 specific ribosomes, is still a matter of some controversy. In the 

 case of virus infected E. co/i, there is strong evidence favoring the 

 loosely bound messenger view (14). At present the model described 

 (and shown schematically in Figure 3) is the most adequate to ex- 

 plain existing experimental results. 



. DNA 



i 



poly 



U 



T-RNH-AA 



\ 



Protem T-RNVAA 

 / ^ 



Fig. 3. Schematic representation of the normal protein synthesizing system (on 

 the left) and the synthetic system (on the right). DNA has been removed from 

 the synthetic system by the enzyme deo.xyribonuclease and the synthetic mes- 

 senger poly U replaces the normal messenger RNA. 



One can imagine a simple base-pairing mechanism by which 

 all of this can occur. The messenger RNA may be synthesized 

 with a DNA primer by a base-pairing, enzyme-catalyzed process 

 which produces a "complementary" copy or translation of the 



