37. NUCLEIC ACID AND PROTEIN SYNTHESIS 403 



implicitly states that the adaptor molecules, and the enzymic reactions 

 catalyzing attachment of amino acids thereto, need have no species speci- 

 ficity; they could be common to all organisms having the same twenty 

 amino acids to deal with. Indeed, the reactions by which the amino acids 

 are attached to the adaptors and those by which the amino acid adaptors 

 are conveyed to the template need not differ from species to species. Only 

 in the template itself — in the ordering of the specific hydrogen bonding, 

 adaptor-reacting sites — would the genetic constitution of the organism be 

 manifest. (6) Finally, the theory states that the association of adaptor and 

 ribosome is transitory; the adaptor constantly cycling through the ribo- 

 some bearing its charge of amino acid. 



It will already be clear that the discovery that two kinds of RNA are 

 involved in protein synthesis made necessary a revision of conventional 

 concepts and found the adaptor hypothesis a valuable hypothetical frame- 

 work. Let us look at the data in the light of the concept. 



It seems highly probable that the RNA of the ribosomes is the cytoplas- 

 mic template for protein synthesis. A priori, one would expect that a tem- 

 plate might have to be particulate, thus permitting a reasonably rigid 

 spatial arrangement of the RNA. Experimentally, we have seen that the 

 ribosome is the chief site of peptide bond condensation. There are no dis- 

 cernible stages in the synthetic process prior to the admission of the amino 

 acids to the particles in which they could be arranged in sequence: i.e., 

 peptides do not appear to be formed as intermediates in protein synthesis 

 (cf. Loftfield 7 ). 



We have seen that amino acids are first activated by formation of amino 

 acyl adenylate compounds. Nature could conceivably have arranged that 

 this step be catalyzed by a single species of enzyme but we find that a 

 specific enzyme is required for each amino acid. An explanation of this be- 

 comes clear when we observe that the same enzyme which activates the 

 amino acid must also transfer it to a specific RNA molecule. Thus twenty 

 amino acids use twenty enzymes to convey them to twenty specific RNA 

 molecules. 



The versatility of the amino acid activating enzymes is impressive. They 

 have in common a site able to recognize ATP, each one has a specific site 

 for an amino acid R-group, and each is further able precisely to discern 

 differences in base sequence of twenty closely related RNA molecules; this 

 base sequence in each case being removed, by at least three nucleotides, 

 from the site at which the enzyme deposits its amino acid. 



We have seen that the transfer RNA is uniquely able to serve as the 

 source of new amino acid appearing in protein in the particles. We have 

 seen further that during the course of this reaction the RNA itself, or at 

 least part of it, accompanies the amino acid into the particle where it be- 



