284 ELLIOT VOI.KIN 



ri'action. Likewise, the syiitlu'sizcd l^NA, freed of the j)()lyinerase and 

 DNA, eould a('ti\-ate the iiicoiporatioii of amino acids sonic 3- to 5-fold. 

 Althouiih it could he siiown (Chanil)eiiain and Bcrfj;, 1962) that sinf^le- 

 stranded DNA from hcat-denatiired T2 DNA, or ^X174, could prime the 

 foi-niatit)!! of KNA by tlu' polynieiase with about v(\un\ facility as the 

 native, or double-stranded DNA, these KNA's are completely inactive 

 in the amino acid-incorporating system (Wood and Berg, 1962). In 

 view of the ability of the biologically unspecific |)olynucleotides synthe- 

 sizetl l)y polynucleotide phosphosphylase to activate amino acid incorj)o- 

 ration in the systems of Ochoa and Nirenberg, and especially the fact 

 that double-stranded synthetic polymers are ineffective (Nirenberg et al., 

 1962), an enigma seems to be created on the mechanism of activation by 

 RNA in these systems. 



C. CORREL.\TIONS WITH PLANT VIRI^S MUTANTS 



If there are suspicions about the validity of establishing a genetic 

 code on the basis of in vitro incorporation of very limited amounts of 

 radioactive amino acids into "acid-insoluble protein," it should be noted 

 that the code letters often are the same as those surmised from in vivo 

 experiments involving some plant virus mutants (see Chapter X). One 

 nitrous acid mutant contains leucine in place of a proline residue (Tsu- 

 gita and Fraenkel-Conrat, 1960). Nitrous acid acts by deaminating C 

 to U. Inspection of Table I reveals that such a conversion of C of the 

 proline code to U is in line with the specification of a leucine in place of 

 proline. In the same mutant, replacement of threonine by serine can be 

 explained in a similar way (Tsugita, 1961). In another nitrous acid 

 mutant (No. 167) serine is replaced by phenylalanine and glutamine by 

 valine (Tsugita, 1961). The former conversion is readily predictable 

 from the data in Table I (2U IC^UUU). Although no code letters 

 have been established as yet for glutamine, it is of interest that glutamic 

 acid (lU IC IG) fulfills the requirements for such a conversion to valine 

 (2U IG) by deamination. Also in line with the code as determined by 

 Nirenberg and Ochoa (Table I) is the observation that the proline- 

 phenylalanine ratio in the proteins of wild cucumber and tobacco mosaic 

 virus can be correlated with their respective RNA C:U ratios (Yama- 

 zaki and Kaesberg, 1961). 



VII. The Triplet Code 



The question of the transfer of information from nucleic acid to the 

 synthesis of protein involves the reading of a polymer with sequences of 

 only four different units (nucleotides) for the .sequential ordering of 

 some 20 different units (amino acids). The minimum coding unit then 



