VIII. PROTEIN SYNTHESIS AND GENE ACTION 391 



tively. The code words assigned then were U2C for leucine and serine, 

 and UCo for proline (calculated ratio is 25). However, proline is also 

 stimulated by a UAC polymer. The phenylalanine/proline ratio here 

 was 29, expected UUU/UC2 in the polymer was 36. Thus, agreement was 

 satisfactory. However, a UCG polymer with the same UUU/UC2 ratio 

 did not stimulate proline incorporation. The results for most of the 

 amino acids do not contain discrepancies of this type. But not many 

 polymers of different types with similar calculated ratios of two trip- 

 lets have been tested. Another example of this type is tryptophan. The 

 code word assigned by Speyer et al. (1962) and Matthaei et al. (1962) 

 is UG2. The ratios of stimulated incorporation by UG and UCG poly- 

 mers agree, but stimulation by UAG was not observed and should have 

 been the same as UCG (Speyer et al, 1962). On the other hand, Matthaei 

 et al., for the same amino acid, but using polymers of different composi- 

 tion, confinii the assignment with UG and UGA, but not with UGC. In 

 any event, a number of amino acids are given the same code letters by 

 both laboratories, although some differences are noted. Certain amino 

 acids, such as glutamic acid, were stimulated only by UAG so this word 

 was assigned although the UUU/UAG ratio did not agree with in- 

 corporation ratios. 



Despite these problems, which may be explained in the future, some 

 remarkable conclusions can be made from the results, and also new 

 problems are opened for investigation. For example, some of the earlier 

 codes are eliminated. All of the code words contain uridylic acid (see 

 Chapter VI for code words). The question arises whether this is a real 

 phenomenon or results from the technique used. Since it is likely that 

 UUU is the code for phenylalanine, and poly-A and poly-C do not code 

 for any amino acid, it is clear that polymers containing large amounts 

 of U will give runs of phenylalanine, thus providing a matrix for other 

 amino acids to attach to and yield an insoluble peptide. Both labora- 

 tories have only looked for insoluble peptides. Therefore, code words 

 containing no U may exist, but would yield stimulation into small pep- 

 tides, soluble in trichloroacetic acid. Matthaei et al. (1962) have noted 

 that poly-AG did not stimulate incorporation, but small peptides were 

 not looked for. If this argument is true then even the failure of poly-A 

 or poly-C to stimulate incorporation may not be correct. There are 37 

 triplets containing U and all the amino acids are coded for by 24 trip- 

 lets. There are two words for asparagine, and threonine, and three 

 words for leucine, according to Speyer et al. (1962). Matthaei et al. 

 (1962) list serine as having two code words. Thus, 12 triplets with U 

 are not accounted for. Either these are "nonsense," or further degen- 

 eracies (more than one code word per amino acid) exist. If triplets 



