RNA IN THE SYNTHESIS OF PROTEINS 



posurc, however, to either weak alkali or to hydroxylamine, treatments 

 known to break amino-acyl-bonds, the sRNA and nascent proteins move 

 separately. 



The significance of the reversible binding by non-active (no messenger) 

 ribosomcs is not known. Conceivably inside growing cells, all ribosomes 

 have attached messenger and synthesize protein. Under these conditions, 

 only those sRNA molecules corresponding to the specific messenger se- 

 quence can slip into the ribosomal cavities. But when most ribosomes lack 

 messenger templates, as in our in vitro extracts, then any sRNA molecule, 

 charged or uncharged, may fill the empty site. 



All evidence suggests that covalent bonds are not involved in holding 

 nascent chains to ribosome. Instead it seems probable that the point of firm 

 attachment involves the terminal sRNA residue, bound by Mg ++ dependent 



70S 



\ 



Messenger RNA 

 (poly U) 



Active complex 



100 S — 200 S 



S-RNA 



Polypeptide chain 

 + + 



\ 



S- RNA-aa. 



cCQXQ- 



SDS 



S-RNA 



poly <{>-ala 



Fig. 6. Diagrammatic summary of ribosome participation in protein synthesis. (The 

 active complex is pictured in Fig. 7.) 



s-127 



