R N A IN THE SYNTHESIS OF PROTEINS 



(a) AA +■ ATP — AMP ~ AA + PP 



(b) AMP~ AA + SRNA — AA~ SRNA 4- AMP 



(c) (AA ~ SRNA) n ■+- GTP 



Ri bo somes 



AA,-AA 2 AA n + GDP (GMP') 



Fig. 4. Enzymatic steps in protein peptide bond formation. Steps (a) and (b) are 

 catalyzed by single enzymes. The number of enzymes required in (c) is unknown. 



capable of selectively interacting with the hydrogen bonding surfaces pro- 

 vided by RNA's purine and pyrimidine bases. This scheme requires at least 

 twenty different adaptors, each specific for a given amino acid. These are 

 very neatly provided by the specific sRNA molecules. Soon after Hoag- 

 land's discovery of sRNA, many experiments, particularly by Hoagland and 

 Paul Berg 27 , established that the sRNA molecules are in fact specific for a 

 given amino acid. It thus became possible to imagine, following Crick's 

 reasoning, that the ribosomal template for protein synthesis combined not 

 with the amino acid side groups, but instead with a specific group of bases 

 on the soluble RNA portion of the amino-acyl-sRN A precursors. 



Participation of Active Ribosomcs in Protein Synthesis 



Very little protein synthesis occurred in the cell-free system developed by 

 the Massachusetts General Hospital Group. Only by using radioactive amino 

 acids could they convincingly demonstrate amino acid incorporation into 

 proteins. This fact, initially seemed trivial and there was much hope that 

 when better experimental conditions were found, significant net synthesis 

 would occur. But despite optimistic claims from several laboratories, no real 

 improvement in the efficiency of cell-free synthesis resulted. Some exper- 

 iments (1959) of Dr. Tissieres and Mr. Schlessinger 28 with E. coli extracts 

 illustrate well this point. At 30°C, cell-free synthesis occurs linearly for 5-10 

 minutes and then gradually stops. During this interval the newly synthesized 

 protein amounts to 1-3 y of protein per mg of ribosomcs. Of this about one 

 third was released from the ribosomes, the remainder being ribosomal bound. 

 Cell-free synthesis in E. coli extracts requires the high (~ io~ 2 M) Mg ++ 



s-119 



