308 CELL HEREDITY 



shall confine our discussion to lines of evidence concerning the control 

 of nucleic acid and protein syntheses. 



PROTEIN SYNTHESIS 



A major pathway of protein synthesis has been discovered within 

 the last few years, and is being intensively investigated now in many 

 cell systems, including mammalian liver and pancreas, Ehrlich ascites 

 tumor cells, pea seedlings, yeast, and bacteria. This pathway was 

 initially demonstrated by means of in vitro incorporation of amino acids 

 into protein in mammalian cell systems by Zamecnik and collaborators, and 

 subsequently confirmed by a number of other investigators. It must be 

 borne in mind, however, that in all these systems, the yield of protein 

 is very low, necessitating caution in the interpretation of results. 



(1) Amino acidi + ATP ^ amino acidj^ — ^AMP + pyrophosphate 



E 



(2) Amino acidi~AMP + SRNA ^ amino acidi~SRNA + AMP 



(3) Ammo acidi~-SRNA + ribosomes ^ ammo acid j ^^ ribosome + SRNA 



FIGURE 11.2. 



As currently understood, and shown in Figure 11.2 this pathway is as 

 follows: 



1. Free amino acids are activated by reaction with ATP to form amino 

 acid-adenylate complexes. This reaction is catalyzed by a set of acti- 

 vating enzymes, each one specific for a particular amino acid. These 

 enzymes seem not to be species-specific in activity, since the cell frac- 

 tion which contains them can be provided from a variety of different cell 

 types, in in vitro experiments. 



2. The activated amino acid is transferred from this complex to an 

 RNA molecule. There are specific RNA's for each of the amino acids; to- 

 gether they are referred to as the soluble-RNA (s-RNA) or transfer-RNA 

 fraction. In E. coli, these RNA molecules have an average molecular 

 weight of about 25,000, thus containing some 100 nucleotides. The 

 base ratios so far reported indicate the same kind of complementarity as 

 in DNA, with the molar per cent of adenine equal to that of uracil, and 



