Gene Action and Amino Acid Coding 



435 



be UUf/ (phenylalanine), or UC/C (serine?), 

 and still other times UCC (proline?). Thus, 

 nucleotide-sharing may be partially respon- 

 sible for actual incorporation rate deviating 

 from that expected.) 



You should be able to work out that a 

 polymer synthesized from ribotides of U, A, 

 and C in the relative amounts of 6, 1, and 1, 

 respectively, has the triplet code letters, UUU, 

 UUA, AAU, UAC, AAA, CCC, in a given 

 sequence in the relative frequencies 216 : 36 : 

 6:6:1 : 1, respectively. 



Using synthetic polyribotides, the triplet 

 code letters have been determined for 19 

 amino acids ^^ and predicted for one amino 

 acid, and it is these which are given in Figure 

 47-3. All meaningful triplets in the experi- 

 mentally detected code contain U. This does 

 not seem to be dependent solely upon the 

 fact that, for technical reasons, each mixed 

 polynucleotide contained U. For no amino 

 acid was incorporated following treatment 

 with pure polyribotides of A or of C. It 

 seems reasonably certain that all meaningful 

 triplets in the messenger RNA code for amino 

 acids normally contain uracil. (This does not 

 preclude the occurrence of mistakes, as the 

 result of which a given amino acid is encoded 

 by another triplet which may or may not 

 contain U.) 



Of the 64 unidirectionally read triplets 

 possible using A U G C, 37 have one or more 

 U's and 27 have none. (The chance a triplet 

 has no U is % X Ya X %, or %.) It is 

 reasonable, therefore, to expect that triplets 

 which do not contain U do not appear in 

 messenger RNA, except possibly to inter- 

 rupt the message in order to end a polypep- 

 tide chain. If the presence of U in each trip- 

 let is characteristic of messenger RNA, then 

 this RNA must have been the complement 

 of a DNA strand which characteristically 

 has triplets containing A. Moreover, the 

 complementary DNA chain would be charac- 

 terized by the usual presence of T in its trip- 

 le By S. Ochoa and coworkers. 



FIGURE 47-3. Messenger RNA code for amino 

 acids. (After Speyer, J. F. , et al. , Proc. Nat. Acad. 

 Sci., U.S., 48:441-448, 1962.) 



lets and the template RNA made from it 

 should contain A in almost every triplet. 

 This RNA would not be used as messenger. 

 We have already presumed (see Figure 47-2 

 and its discussion in the text) that part of 

 this RNA is used to make sRNA. This is 

 supported by the fact that although only one 

 triplet of sRNA is known, it is ACC. We 

 are led to suppose that sRNA has a different 

 triplet which pairs with a triplet of messenger 

 RNA, and that it contains at least one A. It 

 is also supposed that this is the same triplet 

 which directs a particular activating enzyme 

 to join a particular kind of amino acid to 

 a specific sRNA. 

 Therefore, we expect that (1) if a portion 



