Studies on the Mechanism of Protein Synthesis 167 



amino acids. In the schemes suggested by Bounce (1952) and 

 by Koningsberger and Overbeek (1953), the amino acids are 

 attacked by covalent bonding of the amino or carboxyl group 

 respectively of the amino acid with the phosphate of the 

 ribonucleic acid. Since, however, the amino acids appear 

 to be activated in the soluble enzyme fraction of the cell, it 

 would be reasonable to consider that activated aminoacyl 

 nucleotide compounds line up along a ribonucleoprotein 



amino acid + ATP-^ 



I 



aminoacyl to AMPp+ pp 



-rcraw 



../"■■■ 'o 



Activation Sequentialization Cross-linking 



and 

 (soluble enzymes) (ribonucleoprotein particle) potternization 



( elsewhere — 



Pendoplasmic 

 reticulum ) 



Fig. 2. Postulated steps in protein synthesis in rat liver cytoplasm. 



template, with their side-chain R groups determining the 

 sequence by their ability to fit into particular spaces occurring 

 on the ribonucleoprotein surface, rather than that there 

 occurs a formal triester linkage of amino acid to nucleic acid. 

 Van der Waal's forces and electrostatic charges would serve 

 as the binding forces of the side chains of the activated amino 

 acids to the ribonucleoprotein template. This scheme has 

 been drawn up in detail by our colleague Loftfield, and will 

 be published elsewhere. 



In the in vivo experiments of Littlefield and co-workers 

 (1955), the ribonucleoprotein fraction of the liver cell appears 

 to pass on its radioactive protein or large peptide chain to 



