37. NUCLEIC ACID AND PROTEIN SYNTHESIS 



401 



E 3 (AMP~aa 3 ) + 



Fig. 7. A schematic representation of the "adaptor hypothesis." [From M. B. 

 Hoagland, P. C. Zamecnik, and M. L. Stephenson, in "A Symposium on Molecular 

 Biology" (R. E. Zirkle, ed.), P- 105. Univ. of Chicago Press, Chicago, 111., 1959.] The 

 contours of the transfer RNA molecules represent a specific base sequence, for each 

 molecule (upper part) and a common terminal nucleotide sequence (lower part) to 

 which the specific amino acids (aai-aa?) are attached. Ribosomal RNA is depicted as 

 a helix but this is only for convenience of exposition. The complementary nature of 

 the interaction of transfer RNA and ribosomal RNA is suggested by the complemen- 

 taritv of their contours. 



"I don't think that anybody looking at DNA or RNA would think of them as tem- 

 plates for amino acids were it not for other indirect evidence. 



"What the DNA structure does show (and probably RNA will do the same) is a spe- 

 cific pattern of hydrogen bonds, and very little else. It seems to me, therefore, that we 

 should widen our thinking to embrace this obvious fact." . . . "Each amino acid 

 would combine chemically, at a special enzyme, with a small molecule which, having a 

 specific hydrogen-bonding surface, would combine specifically with the nucleic acid 

 template. This combination would also supply the energy necessary for polymeriza- 

 tion. In its simplest form there would be 20 different kinds of adaptor molecule, one 

 for each amino acid, and 20 different enzymes to join the amino acid to their adaptors. 

 Sydney Brenner, with whom I have discussed this idea, calls this the 'adaptor hy- 

 pothesis,' since each amino acid is fitted with an adaptor to go on to the template." 229 



These thoughts had value in focusing attention on the problem of the 

 specificity of sequence determination. However, we now know that it is not 

 the free amino acid which would in all probability be required to find its 

 place on the template, but the amino acyl adenylate enzyme complex. This 

 complex might well be expected to be much better able to "recognize" the 

 correct sequence of bases on the template. We suspect that it can recognize 



229 F. H. C. Crick, "A Note for the RNA Tie Club," 1955. 



